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This study evaluated the spatial variability of trends in simulated snowpack properties across the Rio Grande headwaters of Colorado using the SnowModel snow evolution modeling system. SnowModel simulations were performed using a grid resolution of 100 m and 3-hourly time step over a 34-yr period (1984–2017). Atmospheric forcing was provided by phase 2 of the North American Land Data Assimilation System, and the simulations accounted for temporal changes in forest canopy from bark beetle and wildfire disturbances. Annual summary values of simulated snowpack properties [snow metrics; e.g., peak snow water equivalent (SWE), snowmelt rate and timing, and snow sublimation] were used to compute trends across the domain. Trends in simulated snow metrics varied depending on elevation, aspect, and land cover. Statistically significant trends did not occur evenly within the basin, and some areas were more sensitive than others. In addition, there were distinct trend differences between the different snow metrics. Upward trends in mean winter air temperature were 0.3°C decade−1, and downward trends in winter precipitation were −52 mm decade−1. Middle elevation zones, coincident with the greatest volumetric snow water storage, exhibited the greatest sensitivity to changes in peak SWE and snowmelt rate. Across the Rio Grande headwaters, snowmelt rates decreased by 20% decade−1, peak SWE decreased by 14% decade−1, and total snowmelt quantity decreased by 13% decade−1. These snow trends are in general agreement with widespread snow declines that have been reported for this region. This study further quantifies these snow declines and provides trend information for additional snow variables across a greater spatial coverage at finer spatial resolution.

USGS Water Availability and Use Science Program; USGS Climate and Land Use Change Program; NSF [DEB 1027341]

Frameworks for limiting ecosystem exposure to excess nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). While much progress has been made in developing total Nr deposition budgets for the U.S., current budgets remain limited by key data and knowledge gaps. Analysis of National Atmospheric Deposition Program Total Deposition (NADP/TDep) data illustrates several aspects of current Nr deposition that motivate additional research. Averaged across the continental U.S., dry deposition contributes slightly more (55%) to total deposition than wet deposition and is the dominant process (> 90%) over broad areas of the Southwest and other arid regions of the West. Lack of dry deposition measurements imposes a reliance on models, resulting in a much higher degree of uncertainty relative to wet deposition which is routinely measured. As nitrogen oxide (NO(x)) emissions continue to decline, reduced forms of inorganic nitrogen (NH(x) = NH(3) + NH(4)(+)) now contribute > 50% of total Nr deposition over large areas of the U.S. Expanded monitoring and additional process-level research are needed to better understand NH(x) deposition, its contribution to total Nr deposition budgets, and the processes by which reduced N deposits to ecosystems. Urban and suburban areas are hotspots where routine monitoring of oxidized and reduced Nr deposition is needed. Finally, deposition budgets have incomplete information about the speciation of atmospheric nitrogen; monitoring networks do not capture important forms of Nr such as organic nitrogen. Building on these themes, we detail the state of the science of Nr deposition budgets in the U.S. and highlight research priorities to improve deposition budgets in terms of monitoring and flux measurements, leaf- to regional-scale modeling, source apportionment, and characterization of deposition trends and patterns.

Recent studies have found insignificant or decreasing trends in time-series dissolved organic carbon (DOC) datasets, questioning the assumption that long-term DOC concentrations in surface waters are increasing in response to anthropogenic forcing, including climate change, land use, and atmospheric acid deposition. We used the weighted regressions on time, discharge, and season (WRTDS) model to estimate annual flow-normalized concentrations and fluxes to determine if changes in DOC quantity and quality signal anthropogenic forcing at 10 locations in the Mississippi River Basin. Despite increases in agriculture and urban development throughout the basin, net increases in DOC concentration and flux were significant at only 3 of 10 sites from 1997 to 2013 and ranged between −3.5% to +18% and −0.1 to 19%, respectively. Positive shifts in DOC quality, characterized by increasing specific ultraviolet absorbance at 254 nm, ranged between +8% and +45%, but only occurred at one of the sites with significant DOC quantity increases. Basinwide reductions in atmospheric sulfate deposition did not result in large increases in DOC either, likely because of the high buffering capacity of the soil. Hydroclimatic factors including annual discharge, precipitation, and temperature did not significantly change during the 17-year timespan of this study, which contrasts with results from previous studies showing significant increases in precipitation and discharge over a century time scale. Our study also contrasts with those from smaller catchments, which have shown stronger DOC responses to climate, land use, and acidic deposition. This temporal and spatial analysis indicated that there was a potential change in DOC sources in the Mississippi River Basin between 1997 and 2013. However, the overall magnitude of DOC trends was not large, and the pattern in quantity and quality increases for the 10 study sites was not consistent throughout the basin.

Snow sublimation can be an important component of the snow-cover mass balance, and there is considerable interest in quantifying the role of this process within the water and energy balance of snow-covered regions. In recent years, robust eddy covariance (EC) instrumentation has been used to quantify snow sublimation over snow-covered surfaces in complex mountainous terrain. However, EC can be challenging for monitoring turbulent fluxes in snow-covered environments because of intensive data, power, and fetch requirements, and alternative methods of estimating snow sublimation are often relied upon. To evaluate the relative merits of methods for quantifying surface sublimation, fluxes calculated by the EC, Bowen ratio–energy balance (BR), bulk aerodynamic flux (BF), and aerodynamic profile (AP) methods and their associated uncertainty were compared at two forested openings in the Colorado Rocky Mountains. Biases between methods are evaluated over a range of environmental conditions, and limitations of each method are discussed. Mean surface sublimation rates from both sites ranged from 0.33 to 0.36 mm day−1, 0.14 to 0.37 mm day−1, 0.10 to 0.17 mm day−1, and 0.03 to 0.10 mm day−1 for the EC, BR, BF and AP methods, respectively. The EC and/or BF methods are concluded to be superior for estimating surface sublimation in snow-covered forested openings. The surface sublimation rates quantified in this study are generally smaller in magnitude compared with previously published studies in this region and help to refine sublimation estimates for forested openings in the Colorado Rocky Mountains. Copyright © 2016 John Wiley & Sons, Ltd.

The effects of mountain pine beetle (MPB)-induced tree mortality on a headwater hydrologic system were investigated using an integrated physical modeling framework with a high-resolution computational grid. Simulations of MPB-affected and unaffected conditions, each with identical atmospheric forcing for a normal water year, were compared at multiple scales to evaluate the effects of scale on MPB-affected hydrologic systems. Individual locations within the larger model were shown to maintain hillslope-scale processes affecting snowpack dynamics, total evapotranspiration, and soil moisture that are comparable to several field-based studies and previous modeling work. Hillslope-scale analyses also highlight the influence of compensating changes in evapotranspiration and snow processes. Reduced transpiration in the Grey Phase of MPB-induced tree mortality was offset by increased late-summer evaporation, while overall snowpack dynamics were more dependent on elevation effects than MPB-induced tree mortality. At the watershed scale, unaffected areas obscured the magnitude of MPB effects. Annual water yield from the watershed increased during Grey Phase simulations by 11 percent; a difference that would be difficult to diagnose with long-term gage observations that are complicated by inter-annual climate variability. The effects on hydrology observed and simulated at the hillslope scale can be further damped at the watershed scale, which spans more life zones and a broader range of landscape properties. These scaling effects may change under extreme conditions, e.g., increased total MPB-affected area or a water year with above average snowpack.

The preferential elution of ions from melting snowpacks is a complex problem that has been linked to temporary acidification of water bodies. However, the understanding of these processes in snowpacks around the world, including the polar regions that are experiencing unprecedented warming and melting, remains limited despite being instrumental in supporting climate change adaptation. In this study, data collected from a snowmelt lysimeter and snowpits at meadow and forest-gap sites in a high elevation watershed in Colorado were combined with the PULSE multi-phase snowpack chemistry model to investigate the controls of meltwater chemistry and preferential elution. The snowdepth at the meadow site was 64% of that at the forest-gap site, and the snowmelt rate was greater there (meadow snowpit) due to higher solar irradiance. Cations such as Ca2+ and NH 4 + were deposited mostly within the upper layers of both the meadow and forest-gap snowpacks, and acid anions such as NO 3 − and SO 4 2 − were more evenly distributed. The snow ion concentrations were generally greater at the forest-gap snowpit, except for NH 4 + , which indicates that wind erosion of wet and dry deposited ions from the meadow may have reduced concentrations of residual snow. Furthermore, at the forest-gap site, snow interception and scavenging processes such as sublimation, ventilation, and throughfall led to particular ion enrichment of Ca2+, Mg2+, K+, Cl−, SO 4 2 − and NO 3 − . Model simulations and observations highlight that preferential elution is enhanced by low snowmelt rates, with the model indicating that this is due to lower dilution rates and increased contact time and area between the percolating meltwater and the snow. Results suggest that low snowmelt rates can cause multiple early meltwater ionic pulses for ions subject to lower ion exclusion. Ion exclusion rates at the grain-size level have been estimated for the first time.

Water quality of the Big Thompson River in the Front Range of Colorado was studied for 2 years following a high-elevation wildfire that started in October 2012 and burned 15% of the watershed. A combination of fixed-interval sampling and continuous water-quality monitors was used to examine the timing and magnitude of water-quality changes caused by the wildfire. Prefire water quality was well characterized because the site has been monitored at least monthly since the early 2000s. Major ions and nitrate showed the largest changes in concentrations; major ion increases were greatest in the first postfire snowmelt period, but nitrate increases were greatest in the second snowmelt period. The delay in nitrate release until the second snowmelt season likely reflected a combination of factors including fire timing, hydrologic regime, and rates of nitrogen transformations. Despite the small size of the fire, annual yields of dissolved constituents from the watershed increased 20–52% in the first 2 years following the fire. Turbidity data from the continuous sensor indicated high-intensity summer rain storms had a much greater effect on sediment transport compared to snowmelt. High-frequency sensor data also revealed that weekly sampling missed the concentration peak during snowmelt and short-duration spikes during rain events, underscoring the challenge of characterizing postfire water-quality response with fixed-interval sampling. Copyright © 2015 John Wiley & Sons, Ltd.

A better understanding is needed of how hydrological and biogeochemical processes control dissolved organic car- bon (DOC) concentrations and dissolved organic matter (DOM) composition from headwaters downstream to large rivers. We exam- ined a large DOM dataset from the National Water Information System of the US Geological Survey, which represents approximately 100 000 measurements of DOC concentration and DOM composition at many sites along rivers across the United States. Application of quantile regression revealed a tendency towards downstream spatial and temporal homogenization of DOC concentrations and a shift from dominance of aromatic DOM in headwaters to more aliphatic DOM downstream. The DOC concentration-discharge (C-Q) relationships at each site revealed a downstream tendency towards a slope of zero. We propose that despite complexities in river networks that have driven many revisions to the River Continuum Concept, rivers show a tendency towards chemostasis (C-Q slope of zero) because of a downstream shift from a dominance of hydrologic drivers that connect terrestrial DOM sources to streams in the headwaters towards a dominance of instream and near-stream biogeochemical processes that result in preferential losses of aromatic DOM and preferential gains of aliphatic DOM. Resume : Une meilleure comprehension de l'influence des processus hydrologiques et biogeochimiques sur les concentrations de carbone organique dissous (COD) et sur la composition des matieres organiques dissoutes (MOD) des cours d'eau d'amont jusqu'aux grands fleuves est necessaire. Nous avons examine un imposant ensemble de donnees sur la MOD du Systeme national d'information sur l'eau de la commission geologique des Etats-Unis (US Geological Survey), qui represente environ 100 000 mesures de concentration de COD et de composition de MOD en de nombreux endroits le long de rivieres ala grandeur des Etats-Unis. L'application de la regression quantile a revele une tendance al'homogeneisation spatiale et temporelle vers l'aval des concentrations de COD et le passage d'une predominance de MOD aromatiques dans les cours d'eau d'amont aplus de MOD aliphatiques en aval. Les relations concentrations de COD-debit (C-D) achaque site ont revele une tendance vers une pente nulle vers l'aval. Nous proposons que, malgre les complexites des reseaux hydrographiques qui ont mene ade nombreuses revisions du concept du continuum fluvial, les rivieres tendent vers la chimiostase (pente nulle de la relation C-D) en raison du passage vers l'aval d'une predominance de facteurs hy- drologiques qui relient les sources de MOD terrestres aux cours d'eau d'amont, aune predominance de processus biogeochimiques dans le cours d'eau ou aproximite qui se traduisent par des pertes preferentielles de MOD aromatiques et des gains preferentiels de MOD aliphatiques. (Traduit par la Redaction)

Organic carbon (OC) burial in lacustrine sediments represents an important sink in the global carbon cycle; however, large-scale OC burial rates are poorly constrained, primarily because of the sparseness of available data sets. Here we present an analysis of OC burial rates in water bodies of the conterminous U.S. (CONUS) that takes advantage of recently developed national-scale data sets on reservoir sedimentation rates, sediment OC concentrations, lake OC burial rates, and water body distributions. We relate these data to basin characteristics and land use in a geostatistical analysis to develop an empirical model of OC burial in water bodies of the CONUS. Our results indicate that CONUS water bodies sequester 20.8 (95% CI: 9.4-65.8) Tg C yr(-1), and spatial patterns in OC burial are strongly influenced by water body type, size, and abundance; land use; and soil and vegetation characteristics in surrounding areas. Carbon burial is greatest in the central and southeastern regions of the CONUS, where cultivation and an abundance of small water bodies enhance accumulation of sediment and OC in aquatic environments.

Inland waters are an important component of the global carbon cycle through transport, storage, and direct emissions of CO2 and CH4 to the atmosphere. Despite predictions of high physical gas exchange rates due to turbulent flows and ubiquitous supersaturation of CO2—and perhaps also CH4—patterns of gas emissions are essentially undocumented for high mountain ecosystems. Much like other headwater networks around the globe, we found that high-elevation streams in Rocky Mountain National Park, USA, were supersaturated with CO2 during the growing season and were net sources to the atmosphere. CO2 concentrations in lakes, on the other hand, tended to be less than atmospheric equilibrium during the open water season. CO2 and CH4 emissions from the aquatic conduit were relatively small compared to many parts of the globe. Irrespective of the physical template for high gas exchange (high k), we found evidence of CO2 source limitation to mountain streams during the growing season, which limits overall CO2 emissions. Our results suggest a reduced importance of aquatic ecosystems for carbon cycling in high-elevation landscapes having limited soil development and high CO2 consumption via mineral weathering.

Long-term patterns of stream nitrate export and atmospheric N deposition were evaluated over three decades in Loch Vale, a high-elevation watershed in the Colorado Front Range. Stream nitrate concentrations increased in the early 1990 s, peaked in the mid-2000s, and have since declined by over 40%, coincident with trends in nitrogen oxide emissions over the past decade. Similarities in the timing and magnitude of N deposition provide evidence that stream chemistry is responding to changes in atmospheric deposition. The response to deposition was complicated by a drought in the early 2000s that enhanced N export for several years. Other possible explanations, including forest disturbance, snow depth, or permafrost melting, could not explain patterns in N export. Our results show that stream chemistry responds rapidly to changes in N deposition in high-elevation watersheds, similar to the response observed to changes in sulfur deposition.

During 2010-2011, a study was conducted in Sequoia and Kings Canyon National Parks (SEKI) to evaluate the influence of pack animals (stock) and backpackers on water quality in wilderness lakes and streams. The study had three main components: (1) a synoptic survey of water quality in wilderness areas of the parks, (2) paired water quality sampling above and below several areas with differing types and amounts of visitor use, and (3) intensive monitoring at six sites to document temporal variations in water quality. Data from the synoptic water quality survey indicated that wilderness lakes and streams are dilute and have low nutrient and Escherichia coli concentrations. The synoptic survey sites were categorized as minimal use, backpacker-use, or mixed use (stock and backpackers), depending on the most prevalent type of use upstream from the sampling locations. Sites with mixed use tended to have higher concentrations of most constituents (including E. coli) than those categorized as minimal-use (P ≤ 0.05); concentrations at backpacker-use sites were intermediate. Data from paired-site sampling indicated that E. coli, total coliform, and particulate phosphorus concentrations were greater in streams downstream from mixed-use areas than upstream from those areas (P ≤ 0.05). Paired-site data also indicated few statistically significant differences in nutrient, E. coli, or total coliform concentrations in streams upstream and downstream from backpacker-use areas. The intensive-monitoring data indicated that nutrient and E. coli concentrations normally were low, except during storms, when notable increases in concentrations of E. coli, nutrients, dissolved organic carbon, and turbidity occurred. In summary, results from this study indicate that water quality in SEKI wilderness generally is good, except during storms; and visitor use appears to have a small, but statistically significant influence on stream water quality.

We investigated how visitor-use affects water quality in wilderness in Yosemite National Park. During the summers of 2012-2014, we collected and analyzed surface-water samples for water-quality indicators, including fecal indicator bacteria Escherichia coli, nutrients (nitrogen, phosphorus, carbon), suspended sediment concentration, pharmaceuticals, and hormones. Samples were collected upstream and downstream from different types of visitor use at weekly to biweekly intervals and during summer storms. We conducted a park-wide synoptic sampling campaign during summer 2014, and sampled upstream and downstream from meadows to evaluate the mitigating effect of meadows on water quality. At pack stock stream crossings, Escherichia coli concentrations were greater downstream from crossings than upstream (median downstream increase in Escherichia coli of three colony forming units 100 mL-1), with the greatest increases occurring during storms (median downstream increase in Escherichia coli of 32 CFU 100 mL-1). At backpacker use sites, hormones, and pharmaceuticals (e.g., insect repellent) were detected at downstream sites, and Escherichia coli concentrations were greater at downstream sites (median downstream increase in Escherichia coli of 1 CFU 100 mL-1). Differences in water quality downstream vs. upstream from meadows grazed by pack stock were not detectable for most water-quality indicators, however, Escherichia coli concentrations decreased downstream, suggesting entrapment and die-off of fecal indicator bacteria in meadows. Our results indicate that under current-use levels pack stock trail use and backpacker use are associated with detectable, but relatively minor, effects on water quality, which are most pronounced during storms.

The magnitude of Alaska (AK) inland waters carbon (C) fluxes is likely to change in the future due to amplified climate warming impacts on the hydrology and biogeochemical processes in high latitude regions. Although current estimates of major aquatic C fluxes represent an essential baseline against which future change can be compared, a comprehensive assessment for AK has not yet been completed. To address this gap, we combined available data sets and applied consistent methodologies to estimate river lateral C export to the coast, river and lake carbon dioxide (CO2 ) and methane (CH4 ) emissions, and C burial in lakes for the six major hydrologic regions in the state. Estimated total aquatic C flux for AK was 41 Tg C/yr. Major components of this total flux, in Tg C/yr, were 18 for river lateral export, 17 for river CO2 emissions, and 8 for lake CO2 emissions. Lake C burial offset these fluxes by 2 Tg C/yr. River and lake CH4 emissions were 0.03 and 0.10 Tg C/yr, respectively. The Southeast and South central regions had the highest temperature, precipitation, terrestrial net primary productivity (NPP), and C yields (fluxes normalized to land area) were 77 and 42 g C·m-2 ·yr-1 , respectively. Lake CO2 emissions represented over half of the total aquatic flux from the Southwest (37 g C·m-2 ·yr-1 ). The North Slope, Northwest, and Yukon regions had lesser yields (11, 15, and 17 g C·m2 ·yr-1 ), but these estimates may be the most vulnerable to future climate change, because of the heightened sensitivity of arctic and boreal ecosystems to intensified warming. Total aquatic C yield for AK was 27 g C·m-2 ·yr-1 , which represented 16% of the estimated terrestrial NPP. Freshwater ecosystems represent a significant conduit for C loss, and a more comprehensive view of land-water-atmosphere interactions is necessary to predict future climate change impacts on the Alaskan ecosystem C balance.

Current and historic atmospheric nitrogen (N) deposition has impacted aquatic ecosystems in the Greater Yellowstone Area (GYA). Understanding the spatial variation in total atmospheric deposition (wet + dry) of N is needed to estimate air pollution deposition critical loads for sensitive aquatic ecosystems. This is particularly important for areas that have an increasing contribution of ammonia dry deposition to total N (TN), such as the GYA. High resolution geostatistical models and maps of TN deposition (wet + dry) were developed using a variety of techniques including ordinary kriging in a geographic information system, to evaluate spatial variability and identify areas of elevated loading of pollutants for the GYA. TN deposition estimates in the GYA range from1.4 to 7.5 kg N ha

The National Weather Service's Snow Data Assimilation (SNODAS) program provides daily, gridded estimates of snow depth, snow water equivalent (SWE), and related snow parameters at a 1-km2 resolution for the conterminous USA. In this study, SNODAS snow depth and SWE estimates were compared with independent, ground-based snow survey data in the Colorado Rocky Mountains to assess SNODAS accuracy at the 1-km2 scale. Accuracy also was evaluated at the basin scale by comparing SNODAS model output to snowmelt runoff in 31 headwater basins with US Geological Survey stream gauges. Results from the snow surveys indicated that SNODAS performed well in forested areas, explaining 72% of the variance in snow depths and 77% of the variance in SWE. However, SNODAS showed poor agreement with measurements in alpine areas, explaining 16% of the variance in snow depth and 30% of the variance in SWE. At the basin scale, snowmelt runoff was moderately correlated (R2 = 0.52) with SNODAS model estimates. A simple method for adjusting SNODAS SWE estimates in alpine areas was developed that uses relations between prevailing wind direction, terrain, and vegetation to account for wind redistribution of snow in alpine terrain. The adjustments substantially improved agreement between measurements and SNODAS estimates, with the R2 of measured SWE values against SNODAS SWE estimates increasing from 0.42 to 0.63 and the root mean square error decreasing from 12 to 6 cm. Results from this study indicate that SNODAS can provide reliable data for input to moderate-scale to large-scale hydrologic models, which are essential for creating accurate runoff forecasts. Refinement of SNODAS SWE estimates for alpine areas to account for wind redistribution of snow could further improve model performance. Published 2012. This article is a US Government work and is in the public domain in the USA.

Pine forest in northern Colorado and southern Wyoming, USA, are experiencing the most severe mountain pine beetle epidemic in recorded history, and possible degradation of drinking-water quality is a major concern. The objective of this study was to investigate possible changes in soil and water chemistry in Grand County, Colorado in response to the epidemic, and to identify major controlling influences on stream-water nutrients and C in areas affected by the mountain pine beetle. Soil moisture and soil N increased in soils beneath trees killed by the mountain pine beetle, reflecting reduced evapotranspiration and litter accumulation and decay. No significant changes in stream-water NO 3 - or dissolved organic C were observed; however, total N and total P increased, possibly due to litter breakdown or increased productivity related to warming air temperatures. Multiple-regression analyses indicated that % of basin affected by mountain pine beetles had minimal influence on stream-water NO 3 - and dissolved organic C; instead, other basin characteristics, such as percent of the basin classified as forest, were much more important.

There is concern that visitor-use associated activities, such as bathing, dish washing, wastewater production, and stock animal use near lakes and streams, could cause degradation of water quality in Yosemite National Park. A study was conducted during 2004-2007 to assess patterns in nutrient and Escherichia coli (E. coli) concentrations in the Merced and Tuolumne Rivers and characterize natural background concentrations of nutrients in the park. Results indicated that nutrient and E. coli concentrations were low, even compared to other undeveloped sites in the United States. A multiple linear regression approach was used to model natural background concentrations of nutrients, with basin characteristics as explanatory variables. Modeled nitrogen concentrations increased with elevation, and modeled phosphorus concentrations increased with basin size. Observed concentrations (±uncertainty) were compared to modeled concentrations (±uncertainty) to identify sites that might be impacted by point sources of nutrients, as indicated by large model residuals. Statistically significant differences in observed and modeled concentrations were observed at only a few locations, indicating that most sites were representative of natural background conditions. The empirical modeling approach used in this study can be used to estimate natural background conditions at any point along a study reach in areas minimally impacted by development, and may be useful for setting water-quality standards in many national parks.

Critical loads of nitrogen (N) from atmospheric deposition were determined for alpine lake ecosystems in the western US using fossil diatom assemblages in lake sediment cores. Changes in diatom species over the last century were indicative of N enrichment in two areas, the eastern Sierra Nevada, starting between 1960 and 1965, and the Greater Yellowstone Ecosystem, starting in 1980. In contrast, no changes in diatom community structure were apparent in lakes of Glacier National Park. To determine critical N loads that elicited these community changes, we modeled wet nitrogen deposition rates for the period in which diatom shifts first occurred in each area using deposition data spanning from 1980 to 2007. We determined a critical load of 1.4 kg N ha−1 year−1 wet N deposition to elicit key nutrient enrichment effects on diatom communities in both the eastern Sierra Nevada and the Greater Yellowstone Ecosystem.

Alpine glaciers have receded substantially over the last century in many regions of the world. Resulting changes in glacial runoff not only affect the hydrological cycle, but can also alter the physical (i.e., turbidity from glacial flour) and biogeochemical properties of downstream ecosystems. Here we compare nutrient concentrations, transparency gradients, algal biomass, and fossil diatom species richness in two sets of high-elevation lakes: those fed by snowpack melt alone (SF lakes) and those fed by both glacial and snowpack meltwaters (GSF lakes). We found that nitrate (NO(3)(-)) concentrations in the GSF lakes were 1-2 orders of magnitude higher than in SF lakes. Although nitrogen (N) limitation is common in alpine lakes, algal biomass was lower in highly N-enriched GSF lakes than in the N-poor SF lakes. Contrary to expectations, GSF lakes were more transparent than SF lakes to ultraviolet and equally transparent to photosynthetically active radiation. Sediment diatom assemblages had lower taxonomic richness in the GSF lakes, a feature that has persisted over the last century. Our results demonstrate that the presence of glaciers on alpine watersheds more strongly influences NO(3)(-)concentrations in high-elevation lake ecosystems than any other geomorphic or biogeographic characteristic.

Trends in the timing of snowmelt and associated runoff in Colorado were evaluated for the 1978–2007 water years using the regional Kendall test (RKT) on daily snow-water equivalent (SWE) data from snowpack telemetry (SNOTEL) sites and daily streamflow data from headwater streams. The RKT is a robust, nonparametric test that provides an increased power of trend detection by grouping data from multiple sites within a given geographic region. The RKT analyses indicated strong, pervasive trends in snowmelt and streamflow timing, which have shifted toward earlier in the year by a median of 2–3 weeks over the 29-yr study period. In contrast, relatively few statistically significant trends were detected using simple linear regression. RKT analyses also indicated that November–May air temperatures increased by a median of 0.9°C decade−1, while 1 April SWE and maximum SWE declined by a median of 4.1 and 3.6 cm decade−1, respectively. Multiple linear regression models were created, using monthly air temperatures, snowfall, latitude, and elevation as explanatory variables to identify major controlling factors on snowmelt timing. The models accounted for 45% of the variance in snowmelt onset, and 78% of the variance in the snowmelt center of mass (when half the snowpack had melted). Variations in springtime air temperature and SWE explained most of the interannual variability in snowmelt timing. Regression coefficients for air temperature were negative, indicating that warm temperatures promote early melt. Regression coefficients for SWE, latitude, and elevation were positive, indicating that abundant snowfall tends to delay snowmelt, and snowmelt tends to occur later at northern latitudes and high elevations. Results from this study indicate that even the mountains of Colorado, with their high elevations and cold snowpacks, are experiencing substantial shifts in the timing of snowmelt and snowmelt runoff toward earlier in the year.

Trends in precipitation chemistry and hydrologic and climatic data were examined as drivers of long-term changes in the chemical composition of high-elevation lakes in three wilderness areas in Colorado during 1985–2008. Sulfate concentrations in precipitation decreased at a rate of −0.15 to −0.55 μeq/l/year at 10 high-elevation National Atmospheric Deposition Program stations in the state during 1987–2008 reflecting regional reductions in SO2 emissions. In lakes where sulfate is primarily derived from atmospheric inputs, sulfate concentrations also decreased although the rates generally were less, ranging from −0.12 to −0.27 μeq/l/year. The similarity in timing and sulfur isotopic data support the hypothesis that decreases in atmospheric deposition are driving the response of high-elevation lakes in some areas of the state. By contrast, in lakes where sulfate is derived primarily from watershed weathering sources, sulfate concentrations showed sharp increases during 1985–2008. Analysis of long-term climate records indicates that annual air temperatures have increased between 0.45 and 0.93°C per decade throughout most mountainous areas of Colorado, suggesting climate as a factor. Isotopic data reveal that sulfate in these lakes is largely derived from pyrite, which may indicate climate warming is preferentially affecting the rate of pyrite weathering.

a r t i c l e i n f o Article history: Concentrations of weathering products in streams often show relatively little variation compared to changes in discharge, both at event and annual scales. In this study, several hypothesized mechanisms for this "chemostatic behavior" were evaluated, and the potential for those mechanisms to in! uence relations between climate, weathering ! uxes, and CO2 consumption via mineral weathering was assessed. Data from Loch Vale, an alpine catchment in the Colorado Rocky Mountains, indicates that cation exchange and seasonal precipitation and dissolution of amorphous or poorly crystalline aluminosilicates are important processes that help regulate solute concentrations in the stream; however, those processes have no direct effect on CO2 consumption in catchments. Hydrograph separation analyses indicate that old water stored in the subsurface over the winter accounts for about one-quarter of annual stream! ow, and almost one-half of annual ! uxes of Na and SiO2 in the stream; thus, ! ushing of old water by new water (snowmelt) is an important component of chemostatic behavior. Hydrologic ! ushing of subsurface materials further induces chemostatic behavior by reducing mineral saturation indices and increasing reactive mineral surface area, which stimulate mineral weathering rates. CO2 consumption by carbonic acid mediated mineral weathering was quanti" ed using mass-balance calculations; results indicated that silicate mineral weathering was responsible for approximately two-thirds of annual CO2 consumption, and carbonate weathering was responsible for the remaining one-third. CO2 consumption was strongly dependent on annual precipitation and temperature; these relations were captured in a simple statistical model that accounted for 71% of the annual variation in CO2 consumption via mineral weathering in Loch Vale. Published by Elsevier B.V.

Concentration–discharge relationships have been widely used as clues to the hydrochemical processes that control runoff chemistry. Here we examine concentration–discharge relationships for solutes produced primarily by mineral weathering in 59 geochemically diverse US catchments. We show that these catchments exhibit nearly chemostatic behaviour; their stream concentrations of weathering products such as Ca, Mg, Na, and Si typically vary by factors of only 3 to 20 while discharge varies by several orders of magnitude. Similar patterns are observed at the inter-annual time scale. This behaviour implies that solute concentrations in stream water are not determined by simple dilution of a fixed solute flux by a variable flux of water, and that rates of solute production and/or mobilization must be nearly proportional to water fluxes, both on storm and inter-annual timescales. We compared these catchments' concentration–discharge relationships to the predictions of several simple hydrological and geochemical models. Most of these models can be forced to approximately fit the observed concentration–discharge relationships, but often only by assuming unrealistic or internally inconsistent parameter values. We propose a new model that also fits the data and may be more robust. We suggest possible tests of the new model for future studies. The relative stability of concentration under widely varying discharge may help make aquatic environments habitable. It also implies that fluxes of weathering solutes in streams, and thus fluxes of alkalinity to the oceans, are determined primarily by water fluxes. Thus, hydrology may be a major driver of the ocean-alkalinity feedback regulating climate change. Copyright © 2009 John Wiley & Sons, Ltd.

The sensitivity of high-elevation lakes to acidic deposition was evaluated in five national parks of the Rocky Mountains based on statistical relations between lake acid- neutralizing capacity concentrations and basin characteristics. Acid-neutralizing capacity (ANC) of 151 lakes sampled during synoptic surveys and basin-characteristic information derived from geographic information system (GIS) data sets were used to calibrate the statistical models. The explanatory basin variables that were considered included topographic parameters, bedrock type, and vegetation type. A logistic regression model was developed, and modeling results were cross-validated through lake sampling during fall 2004 at 58 lakes. The model was applied to lake basins greater than 1 ha in area in Glacier National Park (n ¼ 244 lakes), Grand Teton National Park (n ¼ 106 lakes), Great Sand Dunes National Park and Preserve (n ¼ 11 lakes), Rocky Mountain National Park (n ¼ 114 lakes), and Yellowstone National Park (n ¼ 294 lakes). Lakes that had a high probability of having an ANC concentration ,100 leq/L, and therefore sensitive to acidic deposition, are located in basins with elevations .3000 m, with ,30% of the catchment having northeast aspect and with .80% of the catchment bedrock having low buffering capacity. The modeling results indicate that the most sensitive lakes are located in Rocky Mountain National Park and Grand Teton National Park. This technique for evaluating the lake sensitivity to acidic deposition is useful for designing long-term monitoring plans and is potentially transferable to other remote mountain areas of the United States and the world.

Changes in stream chemistry were studied for 4 years following large wildfires that burned in Glacier National Park during the summer of 2003. Burned and unburned drainages were monitored from December 2003 through August 2007 for streamflow, major constituents, nutrients, and suspended sediment following the fires. Stream-water nitrate concentrations showed the greatest response to fire, increasing up to tenfold above those in the unburned drainage just prior to the first post-fire snowmelt season. Concentrations in winter base flow remained elevated during the entire study period, whereas concentrations during the growing season returned to background levels after two snowmelt seasons. Annual export of total nitrogen from the burned drainage ranged from 1Ð53 to 3Ð23 kg ha � 1 yr � 1 compared with 1Ð01 to 1Ð39 kg ha � 1 yr � 1 from the unburned drainage and exceeded atmospheric inputs for the first two post-fire water years. Fire appeared to have minimal long-term effects on other nutrients, dissolved organic carbon, and major constituents with the exception of sulfate and chloride, which showed increased concentrations for 2 years following the fire. There was little evidence that fire affected suspended-sediment concentrations in the burned drainage. Sediment yields in subalpine streams may be less affected by fire than in lower elevation streams because of the slow release rate of water during spring snowmelt. Published in 2008 by John Wiley & Sons, Ltd.

Seasonal snowpack chemistry data from the Rocky Mountain region of the US was examined to identify long-term trends in concentration and chemical deposition in snow and in snow-water equivalent. For the period 1993‐2004, comparisons of trends were made between 54 Rocky Mountain Snowpack sites and 16 National Atmospheric Deposition Program wetfall sites located nearby in the region. The region was divided into three subregions: Northern, Central, and Southern. A non-parametric correlation method known as the Regional Kendall Test was used. This technique collectively computed the slope, direction, and probability of trend for several sites at once in each of the Northern, Central, and Southern Rockies subregions. Seasonal Kendall tests were used to evaluate trends at individual sites. Significant trends occurred during the period in wetfall and snowpack concentrations and deposition, and in precipitation. For the comparison, trends in concentrations of ammonium, nitrate, and sulfate for the two networks were in fair agreement. In several cases, increases in ammonium and nitrate concentrations, and decreases in sulfate concentrations for both wetfall and snowpack were consistent in the three subregions. However, deposition patterns between wetfall and snowpack more often were opposite, particularly for ammonium and nitrate. Decreases in ammonium and nitrate deposition in wetfall in the central and southern rockies subregions mostly were moderately significant (po0.11) in constrast to highly significant increases in snowpack (po0.02). These opposite trends likely are explained by different rates of declining precipitation during the recent drought (1999‐2004) and increasing concentration. Furthermore, dry deposition was an important factor in total deposition of nitrogen in the region. Sulfate deposition decreased with moderate to high significance in all three subregions in both wetfall and snowpack. Precipitation trends consistently were downward and significant for wetfall, snowpack, and snow-telemetry data for the central and southern rockies subregions (po0.03), while no trends were noted for the Northern Rockies subregion. Published by Elsevier Ltd.

Ground water occurrence, movement, and its contribution to streamflow were investigated in Loch Vale, an alpine catchment in the Front Range of the Colorado Rocky Mountains. Hydrogeomorphologic mapping, seismic refraction measurements, and porosity and permeability estimates indicate that talus slopes are the primary ground water reservoir, with a maximum storage capacity that is equal to, or greater than, total annual discharge from the basin (5.4 ± 0.8 � 10 6 m 3 ). Although snowmelt and glacial melt provide the majority of annual water flux to the basin, tracer tests and gauging along a stream transect indicate that ground water flowing from talus can account for ≥75% of streamflow during storms and the winter base flow period. The discharge response of talus springs to storms and snowmelt reflects rapid transmittal of water through coarse debris at the talus surface and slower release of water from finer-grained sediments at depth. Ice stored in permafrost (including rock glaciers) is the second largest ground water reservoir in Loch Vale; it represents a significant, but seldom recognized, ground water reservoir in alpine terrain. Mean annual air temperatures are sufficiently cold to support permafrost above 3460 m; however, air temperatures have increased 1.1° to 1.4°C since the early 1990s, consistent with long-term (1976‐2000) increases in air temperature measured at other high-elevation sites in the Front Range, European Alps, and Peruvian Andes. If other climatic factors remain constant, the increase in air temperatures at Loch Vale is sufficient to increase the lower elevational limit of permafrost by 150 to 190 m. Although this could cause a short-term increase in streamflow, it may ultimately result in decreased flow in the future.

Variability in atmospheric deposition across the Rocky Mountains is influenced by elevation, slope, aspect, and precipitation amount and by regional and local sources of air pollution. To improve estimates of deposition in mountainous regions, maps of average annual atmospheric deposition loadings of nitrate, sulfate, and acidity were developed for the Rocky Mountains by using spatial statistics. A parameter-elevation regressions on independent slopes model (PRISM) was incorporated to account for variations in precipitation amount over mountainous regions. Chemical data were obtained from the National Atmospheric Deposition Program/National Trends Network and from annual snowpack surveys conducted by the US Geological Survey and National Park Service, in cooperation with other Federal, State and local agencies. Surface concentration maps were created by ordinary kriging in a geographic information system, using a local trend and mathematical model to estimate the spatial variance. Atmospheric-deposition maps were constructed at 1-km resolution by multiplying surface concentrations from the kriged grid and estimates of precipitation amount from the PRISM model. Maps indicate an increasing spatial trend in concentration and deposition of the modeled constituents, particularly nitrate and sulfate, from north to south throughout the Rocky Mountains and identify hot-spots of atmospheric deposition that result from combined local and regional sources of air pollution. Highest nitrate (2.5–3.0 kg/ha N) and sulfate (10.0–12.0 kg/ha SO4) deposition is found in northern Colorado.

Long-term measurements (1983‐2001) of nutrients and seston in Emerald Lake (Sierra Nevada, California) have revealed ecologically significant patterns. Nitrate, both during spring runoff and during growing seasons, declined from 1983 through 1995. Declining snowmelt nitrate was caused primarily by changes in snow regime induced by the 1987‐1992 drought: years with shallow, early melting snowpacks had lower snowmelt nitrate concentrations owing to less labile N production in catchment soils and longer plant growing seasons. However, nitrate declines during growing seasons carried through the wetter years of 1993‐2000 and are likely the result of increased P loading to the lake and the release of phytoplankton from P limitation. Contemporaneous with these changes was an increase in algal biomass and a shift from P limitation toward more frequent N limitation of phytoplankton abundance. Particulate carbon concentrations in the late 1990s were two- to threefold greater than in the early 1980s. These trends were reflected in a larger set of Sierra Nevada lakes sampled as part of synoptic surveys ( n 5 28). Between 1985 and 1999, nitrate decreased and total P increased in .70% of the lakes sampled. Our data suggest that lakes throughout the Sierra Nevada are experiencing measurable eutrophication in response to the atmospheric deposition of nutrients.

Studies worldwide have shown that mercury (Hg) is a ubiquitouscontaminant, reaching even the most remote environments such ashigh-altitude lakes via atmospheric pathways. However, very fewstudies have been conducted to assess Hg contamination levels ofthese systems. We sampled 90 mid-latitude, high-altitude lakes from seven national parks in the western United States during afour-week period in September 1999. In addition to the synoptic survey, routine monitoring and experimental studies were conducted at one of the lakes (Mills Lake) to quantify MeHg fluxrates and important process rates such as photo-demethylation. Results show that overall, high-altitude lakes have low total mercury (HgT) and methylmercury (MeHg) levels (1.07 and 0.05 ng L-1, respectively), but a very good correlation of Hg to MeHg (r2= 0.82) suggests inorganic Hg(II) loading is a primary controlling factor of MeHg levels in dilute mountain lakes. Positive correlations were also observed for dissolved organic carbon (DOC) and both Hg and MeHg, although to a much lesser degree. Levels of MeHg were similar among the seven national parks, with the exception of Glacier National Park where lowerconcentrations were observed (0.02 ng L-1), and appear to berelated to naturally elevated pH values there. Measured rates ofMeHg photo-degradation at Mills Lake were quite fast, and thisprocess was of equal importance to sedimentation and stream flowfor removing MeHg. Enhanced rates of photo-demethylation are likely an important reason why high-altitude lakes, with typically high water clarity and sunlight exposure, are low in MeHg.

A chemical survey of 69 high-altitude lakes in seven national parks in the western United States was conducted during the fallof 1999; the lakes were previously sampled during the fall of 1985, as part of the Western Lake Survey. Lakes in parks in the Sierra/southern Cascades (Lassen Volcanic, Yosemite, Sequoia/Kings Canyon National Parks) and in the southern RockyMountains (Rocky Mountain National Park) were very dilute; medianspecific conductance ranged from 4.4 to 12.2 μS cm-1 andmedian alkalinity concentrations ranged from 32.2 to 72.9 μeqL-1. Specific conductances and alkalinity concentrations were substantially higher in lakes in the central and northernRocky Mountains parks (Grand Teton, Yellowstone, and GlacierNational Parks), probably due to the prevalence of more reactivebedrock types. Regional patterns in lake concentrations of NO3 and SO4 were similar to regional patterns in NO3 and SO4 concentrations in precipitation, suggestingthat the lakes are showing a response to atmospheric deposition.Concentrations of NO3 were particularly high in Rocky Mountain National Park, where some ecosystems appear to be undergoing nitrogen saturation.

During 1993‐97, samples of the full depth of the Rocky Mountain snowpack were collected at 52 sites from northern New Mexico to Montana and analyzed for major-ion concentrations. Concentrations of acidity, sulfate, nitrate, and calcium increased from north to south along the mountain range. In the northern part of the study area, acidity was most correlated (negatively) with calcium. Acidity was strongly correlated (positively) with nitrate and sulfate in the southern part and for the entire network. Acidity in the south exceeded the maximum acidity measured in snowpack of the Sierra Nevada and Cascade Mountains. Principal component analysis indicates three solute associations we characterize as: (1) acid (acidity, sulfate, and nitrate), (2) soil (calcium, magnesium, and potassium), and (3) salt (sodium, chloride, and ammonium). Concentrations of acid solutes in the snowpack are similar to concentrations in nearby wetfall collectors, whereas, concentrations of soil solutes are much higher in the snowpack than in wetfall. Thus, dryfall of acid solutes during the snow season is negligible, as is gypsum from soils. Snowpack sampling o!ers a coste!ective complement to sampling of wetfall in areas where wetfall is di"cult to sample and where the snowpack accumulates throughout the winter. Published by Elsevier Science Ltd.

Stable sulfur isotope ratios and major ions in bulk snowpack samples were monitored at a network of 52 highelevation sites along and near the Continental Divide from 1993 to 1999. This information was collected to better de"ne atmospheric deposition to remote areas of the Rocky Mountains and to help identify the major source regions of sulfate in winter deposition. Average annual ! "# S values at individual sites ranged from #4.0 to #8.2! and standard deviations ranged from 0.4 to 1.6! . The chemical composition of all samples was extremely dilute and slightly acidic; average sulfate concentrations ranged from 2.4 to 12.2! eql$% and pH ranged from 4.82 to 5.70. The range of ! "# S values measured in this study indicated that snowpack sulfur in the Rocky Mountains is primarily derived from anthropogenic sources. A nearly linear relation between ! "# S and latitude was observed for sites in New Mexico, Colorado, and southern Wyoming, which indicates that snowpack sulfate in the southern part of the network was derived from two isotopically distinct source regions. Because the major point sources of SO & in the region are coal-"red powerplants, this pattern may re#ect variations in the isotopic composition of coals burned by the plants. The geographic pattern in ! "# S for sites farther to the north in Wyoming and Montana was much less distinct, perhaps re#ecting the paucity of major point sources of SO & in the northern part of the network. Published by Elsevier Science Ltd.

Carbon dioxide (CO2) and methane (CH4) exchange between the atmosphere and a subalpine wetland located in Rocky Mountain National Park, Colorado, at 3200 m elevation were measured during 1996–1998. Respiration, net CO2 flux, and CH4 flux were measured using the closed chamber method during snow-free periods and using gas diffusion calculations during snow-covered periods. The ranges of measured flux were 1.2-526 mmol CO2 m−2 d−1 (respiration), −1056−100 mmol CO2m−2 d−1 (net CO2exchange), and 0.1–36.8 mmol CH4m−2 d−1 (a positive value represents efflux to the atmosphere). Respiration and CH4 emission were significantly correlated with 5 cm soil temperature. Annual respiration and CH4 emission were modeled by applying the flux-temperature relationships to a continuous soil temperature record during 1996–1998. Gross photosynthesis was modeled using a hyperbolic equation relating gross photosynthesis, photon flux density, and soil temperature. Modeled annual flux estimates indicate that the wetland was a net source of carbon gas to the atmosphere each of the three years: 8.9 mol C m−2 yr−1 in 1996, 9.5 mol C m−2 yr−1 in 1997, and 9.6 mol C m−2 yr−1 in 1998. This contrasts with the long-term carbon accumulation of ∼0.7 mol m−2 yr−1 determined from 14C analyses of a peat core collected from the wetland.

Alpine/subalpine basins may exhibit substantial variability in solute fluxes despite many apparent similarities in basin characteristics. An evaluation of controls on spatial patterns in solute fluxes may allow development of predictive tools for assessing basin sensitivity to outside perturbations such as climate change or deposition of atmospheric pollutants. Relationships between basin physical characteristics, determined from geographical information system (GIS) tools, and solute fluxes and mineral weathering rates were explored for nine alpine/subalpine basins in Rocky Mountain National Park, Colorado, using correlation analyses for 1993 and 1994 data. Stream-water nitrate fluxes were correlated positively with basin characteristics associated with the talus environment; i.e., the fractional amounts of steep slopes (≥ 30°), unvegetated terrain and young debris (primarily Holocene till) in the basins, and were correlated negatively with fractional amounts of subalpine meadow terrain. Correlations with nitrate indicate the importance of the talus environment in promoting nitrate flux and the mitigating effect of areas with established vegetation, such as subalpine meadows. Total mineral weathering rates for the basins ranged from about 300 to 600 mol ha−1 year−1. Oligoclase weathering accounted for 30 to 73% of the total mineral weathering flux, and was positively correlated with the amount of old debris (primarily Pleistocene glacial till) in the basins. Although calcite is found in trace amounts in bedrock, calcite weathering accounted for up to 44% of the total mineral weathering flux. Calcite was strongly correlated with steep slope, unvegetated terrain, and young debris—probably because physical weathering in steep-gradient areas exposes fresh mineral surfaces that contain calcite for chemical weathering. Oligoclase and calcite weathering are the dominant sources of alkalinity in the basins. However, atmospherically deposited acids consume much of the alkalinity generated by weathering of calcite and other minerals in the talus environment. Published in 2001 by John Wiley & Sons, Ltd.

Relations between stream water chemistry and topographic, vegetative, and geologic characteristics of basins were evaluated for nine alpine/subalpine basins in Rocky Mountain National Park, Colorado, to identify controlling parameters and to better understand processes governing patterns in stream water chemistry. Fractional amounts of steep slopes ($308), unvegetated terrain, and young surficial debris within each basin were positively correlated to each other. These terrain features, which commonly occur on steep valley side slopes underlain by talus, were negatively correlated with concentrations of base cations, silica, and alkalinity and were positively correlated with nitrate, acidity, and runoff. These relations might result from the short residence times of water and limited soil development in the talus environment, which limit chemical weathering and nitrogen uptake. Steep, unvegetated terrains also tend to promote high Ca/Na ratios in stream water, probably because physical weathering rates in those areas are high. Physical weathering exposes fresh bedrock that contains interstitial calcite, which weathers relatively quickly. The fractional amounts of subalpine meadow and, to a lesser extent, old surficial debris in the basins were positively correlated to concentrations of weathering products and were negatively correlated to nitrate and acidity. These relations may reflect more opportunities for silicate weathering and nitrogen uptake in the lower-energy environments of the valley floor, where soils are finer-grained, older, and better developed and slopes are relatively flat. These results indicate that in alpine/subalpine basins, slope, vegetation (or lack thereof), and distribution and age of surficial materials are interrelated and can have major effects on stream water chemistry.

Sulphur 35, a cosmogenically produced radioisotope with a short half-life (87 days), was measured in snowpack during 1993–1997 and at four locations within the Loch Vale watershed during 1995–1997. The four sites include the two main drainages in the watershed, Andrews Creek and Icy Brook, a small south facing catchment flowing into Andrews Creek (Andrews Spring 1), and a similar north facing catchment flowing out of a scree field into Icy Brook (Spring 19). Concentrations ranged from a high of almost 50 mBq/L for a sample from Spring 19 in June 1996 to a concentration near the detection limit for a sample from Andrews Creek in April 1997. Sulphur 35 concentrations were normalized to sulphate (as mBq/mg SO4−2) and were decay-corrected to a Julian day of 90 (April 1) for each year. Snowpack had the highest 35S concentration with an average concentration of 53 mBq/mg SO4−2. Concentrations in the streams were much lower, even when corrected for decay relative to JD 90. The large 35S concentrations found in Spring 19 were the result of increases in concentration due to sublimation and/or evapotranspiration and were lower than snowpack when normalized to sulphate. Using 35S concentrations found in snowpack as of JD 90 as a beginning concentration, the fraction of sulphate in streamflow that was derived from atmospheric deposition within the prior water year was estimated. For Icy Brook and Andrews Creek the fraction of the sulphate in streamflow derived from that year's snowpack and precipitation was low prior to the beginning of the main spring melt, reached a maximum during the period of maximum flow, and decreased as the summer progressed. A calculation of the seasonal flux indicated that about 40% of the sulphate that flowed out of the watershed was derived from atmospheric sulphate deposited during the previous year. This suggests that more than half of the sulphate deposited in the watershed by atmospheric processes during the previous year was removed during the following summer. Thus sulphate retention in alpine watersheds like Loch Vale is very limited, and changes in sulphate deposition should be quickly reflected in stream chemistry.

Accessory calcite, present at concentrations between 300 and 3000 mg kg−1, occurs in fresh granitoid rocks sampled from the Merced watershed in Yosemite National Park, CA, USA; Loch Vale in Rocky Mountain National Park CO USA; the Panola watershed, GA USA; and the Rio Icacos, Puerto Rico. Calcite occurs as fillings in microfractures, as disseminated grains within the silicate matrix, and as replacement of calcic cores in plagioclase. Flow-through column experiments, using de-ionized water saturated with 0.05 atm. CO2, produced effluents from the fresh granitoid rocks that were dominated by Ca and bicarbonate and thermodynamically saturated with calcite. During reactions up to 1.7 yr, calcite dissolution progressively decreased and was superceded by steady state dissolution of silicates, principally biotite. Mass balance calculations indicate that most calcite had been removed during this time and accounted for 57–98% of the total Ca released from these rocks. Experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution. The lack of excess Ca and alkalinity indicated that calcite had been previously removed by natural weathering. The extent of Ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposed in granitoid rocks. High Ca/Na ratios relative to plagioclase stoichiometries indicate excess Ca in the Yosemite, Loch Vale, and other alpine watersheds in the Sierra Nevada and Rocky Mountains of the western United States. This Ca enrichment correlates with strong preferential weathering of calcite relative to plagioclase in exfoliated granitoids in glaciated terrains. In contrast, Ca/Na flux ratios are comparable to or less than the Ca/Na ratios for plagioclase in the subtropical Panola and tropical Rio Icacos watersheds, in which deeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase during transport-limited weathering. These results indicate that the weathering of accessory calcite may strongly influence Ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces.

Stream water data from five headwater basins in the northeastern United States covering water years 1968–1996 and precipitation data from eight nearby precipitation monitoring sites covering water years 1984-1996 were analyzed for temporal trends in chemistry using the nonparametric seasonal Kendall test. Concentrations of SO4declined at three of five streams during 1968–1996 (p < 0.1), and all of the streams exhibited downward trends in SO4 over the second half of the period (1984–1996). Concentrations of SO4 in precipitation declined at seven of eight sites from 1984 to 1996, and the magnitudes of the declines (−0.7 to −2.0 µeq L−1 yr−1) generally were similar to those of stream water SO4. These results indicate that changes in precipitation SO4 were of sufficient magnitude to account for changes in stream water SO4. Concentrations of Ca + Mg declined at three of five streams and five of eight precipitation sites from 1984 to 1996. Precipitation acidity decreased at five of eight sites during the same period, but alkalinity increased in only one stream. In most cases the decreases in stream water SO4 were similar in magnitude to declines in stream water Ca + Mg, which is consistent with the theory of leaching by mobile acid anions in soils. In precipitation the magnitudes of SO4 declines were similar to those of hydrogen, and declines in Ca +Mg were much smaller. This indicates that recent decreases in SO4 deposition are now being reflected in reduced precipitation acidity. The lack of widespread increases in stream water alkalinity, despite the prevalence of downward trends in stream water SO4, suggests that at most sites, increases in stream water pH and acid-neutralizing capacity may be delayed until higher soil base-saturation levels are achieved.