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Inorganic sediment supply is a critical component of a salt marsh’s ability to vertically aggrade in response to relative sea level rise, yet there remains significant uncertainty on the primary sources, timing, and rates of sediment delivery to marshes. This is particularly true for the Northeastern, U.S. Atlantic coastline where the magnitude and sourcing of sediment varies widely due to post-glaciated landscapes. Here we present results from a 3-year study between 2020 and 2023 designed to inform management and restoration decisions related to northeast marshes through the development of a scalable method for assessing the availability and distribution of inorganic sediment to and within marshes, including the identification of thresholds of inorganic sediment delivery required to maintain a stable marsh platform under various rates of sea level rise for the region. Field investigations involved instrumental observations, deployment and recovery of seasonal sediment traps, and the collection and analysis of marsh core samples. The study targets 12 marsh systems spanning environmental gradients for the region that allowed us to examine different sources and delivery mechanisms of sediment. Our compilation of existing data reveals spatial variability in marsh accretion rates, but also highlights regional trends and the general agreement among rates determined through a variety of different methodologies and time spans. Our instrumental observations and sediment trap deployments confirm differences in sediment delivery among marshes. Back barrier marshes with relatively small watersheds predominantly accumulated inorganic sediment during the fall in response to large storms and wave activity suspending coastal and offshore sediment deposits (marine sources) that are carried into marshes through tidal advection. In contrast, marshes proximal to large rivers (>10,000 km2 watersheds) have higher accumulation rates and receive the bulk of their inorganic sediment in response to fluvial delivery of terrestrial sediment during spring freshet events. Among our 12 study marshes, only one experienced its highest rate of sediment accumulation during summer months, which we attribute to substantially greater crab herbivory promoting internal recycling of sediment. Overall, we have measured sediment accumulation in over 450 individual traps across spring, summer and fall seasons in twelve marshes. The results from the analysis of these samples represents the largest dataset of its kind for the region and enable defining regionally appropriate input variables for modeling the spatial variations of sedimentation across marsh surfaces as a function of tidal inundation and distance to the nearest channel, as well as providing defined sedimentation limits needed to sustain healthy marsh growth under future sea level rise and various potential restoration pathways.

With climate change and increased coastal land alteration, salt marshes globally are becoming increasingly degraded. Salt marshes of the Northeastern United States (Maine to Virginia) are particularly vulnerable given the history of intensive alteration such as ditching and tidal restrictions since European colonization. Such alterations reduce the accretionary potential of salt marshes in this region, in turn reducing their ability to keep up with accelerating relative sea level rise. This ultimately leads to reductions in marsh area and loss of ecosystem function, including flood protection, carbon burial, habitat provision, and nutrient filtration. Through collaboration between multiple government, academic, and non-profit organizations, we investigate the following questions: (1) What are the spatial patterns of salt marsh vulnerability to relative sea level rise across the Northeast United States? (2) Additionally, how is this vulnerability linked to specific salt marsh modifications (e.g., ditching, and tidal restrictions)? To address these questions, we combine the Unvegetated to Vegetated Ratio (UVVR) salt marsh vulnerability metric, computed from 2014-2018 using Landsat imagery, with mapped tidal restrictions (e.g., culverts, bridges, tide gates, dikes) and ditches for the Northeastern coast of the United States. We hypothesize that estimated salt marsh lifespans, a mass balance between relative sea level rise and sediment budget (estimated using UVVR), will be shortened where salt marsh modifications are most intense. Results will be used to drive science-based decision making through prioritization of salt marsh restoration.

There is widespread concern that rapidly rising sea levels may drown salt marshes by exceeding the rate at which these important ecosystems can build elevation. A significant fraction of marshes reside within backbarrier estuaries, yet little attention has been paid to how changes in inlet geometry influences estuarine tides and marshes. In 1898, a coastal storm eroded a new inlet through the barrier beach that fronts the North-South Rivers Estuary in Massachusetts, USA. The new inlet shortened the North River by 5.6 km and lengthened the South River channel by the same amount. Modern measurements of tidal attenuation suggest that channel shortening abruptly increased mean high tide along the North River by at least 30 cm. Foraminifera communities within North River marsh sediments indicated an environmental change from infrequent to frequent inundation at the time of the 1898 switch in inlet location, which supports this hypothesis. Increased mineral sediment deposition after the inlet switch played a dominant role in allowing marshes along the North River channel to adjust to greater inundation. Following the inlet switch, sediment accreted in North River marshes at 2–5 times the rate of sea level rise (SLR). The North River channel widened by an average of 18% relative to pre-1898 conditions to accommodate the increased tidal prism. The role of mineral sediment accretion in making this marsh resilient to an abrupt increase in inundation depth highlights the importance of maintaining adequate sediment supplies in coastal regions as SLR accelerates.

Tidal marsh restoration and creation has been proposed as a tool to build coastal resilience in the face of rising sea level and increasing intensity of coastal storms. However, it is unclear what conditions within constructed settings will lead to the successful establishment of tidal marsh. We used sediment cores and historical geospatial data in the tidal freshwater Hudson River to identify the rapid creation and development of marshes that are sheltered by human-made structures including railroad berms, jetties, and dredge spoil islands. These backwater areas rapidly accumulated clastic material following anthropogenic modification that allowed for transition from tidal mudflat to emergent marsh. In one case, historical aerial photos document this transition occurring in less than 18 years, offering a timeframe for marsh development. Accretion rates for anthropogenic tidal marshes and mudflats average 0.8-1.1 cm yr-1 and 0.6-0.7 cm yr-1 respectively, equivalent to 2-3 times the rate of relative sea level rise as well as the observed accretion rate at a 6000+ year old reference marsh in the study area. Paired historical and geospatial analysis revealed that more than half of all the tidal wetlands on the Hudson are anthropogenic and developed since the industrial era, including two thirds of the emergent cattail marsh. These inadvertently constructed tidal wetlands currently trap roughly 6% of the Hudson River’s sediment load. Results indicate that when sediment is readily available freshwater tidal wetlands can develop relatively rapidly in sheltered estuarine settings, and serve as useful examples to help guide future tidal marsh creation and restoration efforts.

Observations and modeling are used to assess potential impacts of sediment releases due to dam removals on the Hudson River estuary. Watershed sediment loads are calculated based on sediment-discharge rating curves for gauges covering 80% of the watershed area. The annual average sediment load to the estuary is 1.2 Mt, of which about 0.6 Mt comes from side tributaries. Sediment yield varies inversely with watershed area, with regional trends that are consistent with substrate erodibility. Geophysical and sedimentological surveys in seven subwatersheds of the Lower Hudson were conducted to estimate the mass and composition of sediment trapped behind dams. Impoundments were classified as (1) active sediment traps, (2) run-of-river sites not actively trapping sediment, and (3) dammed natural lakes and spring-fed ponds. Based on this categorization and impoundment attributes from a dam inventory database, the total mass of impounded sediment in the Lower Hudson watershed is estimated as 4.9 ± 1.9 Mt. This represents about 4 years of annual watershed supply, which is small compared with some individual dam removals and is not practically available given current dam removal rates. More than half of dams impound drainage areas less than 1 km2, and play little role in downstream sediment supply. In modeling of a simulated dam removal, suspended sediment in the estuary increases modestly near the source during discharge events, but otherwise effects on suspended sediment are minimal. Fine-grained sediment deposits broadly along the estuary and coarser sediment deposits near the source, with transport distance inversely related to settling velocity.

Policy makers are interested in managing forests to store carbon. Optimizing this strategy requires understanding how carbon storage varies across environmental gradients. We explored variation in tree growth rate, tree longevity, and surface soil organic matter across 135 Connecticut River riparian forest plots. Tree growth rate did not vary significantly with climate but rather increased with sediment accretion rate, soil pH and decreased with plot elevation, where elevation was measured relative to the stage of the 2-year flood. By contrast, surface soil organic matter was negatively related to pH and tree growth rate. Tree species longevities were greater at higher elevations with coarser soils. The faster growth rates at lower elevations allow for restoring forest structure rapidly, whereas flood intolerant but longer-lived tree species allow more durable carbon sequestration at higher elevations. The close associations of growth rate, sediment accretion, and pH suggest that riverine nutrient inputs are important to maintaining the exceptionally high productivity of floodplains. Environmental assessments of river dams should consider impacts of intercepting sediments and reducing flooding on downstream floodplain fertility and productivity. Restoration of riparian locations with high deposition of sediments and associated nutrients may be an opportunity to maximize both nutrient and carbon sequestration.

Non-linear turbidity-discharge relationships are explored in the context of sediment sourcing and event-driven hysteresis using long-term (>12 year) turbidity observations from the tidal freshwater...

This study presents a new method to examine long-term dynamics in sediment yield using time-varying sediment-discharge rating curves. Dynamic linear models (DLMs) are introduced as a time series filter that can assess how the relationship between streamflow and sediment concentration or load changes over time in response to a wide variety of natural and anthropogenic watershed disturbances or long-term changes. The filter operates by updating parameter values using a recursive Bayesian design that responds to one-day-ahead forecast errors while also accounting for observational noise. The estimated time series of rating curve parameters can then be used to diagnose multi-scale (daily-decadal) variability in sediment yield after accounting for fluctuations in streamflow. The technique is applied in a case study examining changes in turbidity load, a proxy for sediment load, in the Esopus Creek watershed, part of the New York City drinking water supply system. The results show that turbidity load exhibits a complex array of variability across time scales. The DLM highlights flood event-driven positive hysteresis, where turbidity load remained elevated for months after large flood events, as a major component of dynamic behavior in the rating curve relationship. The DLM also produces more accurate one-day-ahead loading forecasts compared to other static and time-varying rating curve methods. The results suggest that DLMs provide a useful tool for diagnosing changes in sediment-discharge relationships over time and may help identify variability in sediment concentrations and loads that can be used to inform dynamic water quality management.

Off-river coves and embayments provide accommodation space for sediment accumulation, particularly for sandy estuaries where high energy in the main channel prevents significant long-term storage of fine-grained material. Seasonal sediment inputs to Hamburg Cove in the Connecticut River estuary (USA) were monitored to understand the timing and mechanisms for sediment storage there. Unlike in freshwater tidal coves, sediment was primarily trapped here during periods of low discharge, when the salinity intrusion extended upriver to the cove entrance. During periods of low discharge and high sediment accumulation, deposited sediment displayed geochemical signatures consistent with a marine source. Numerical simulations reveal that low discharge conditions provide several important characteristics that maximize sediment trapping. First, these conditions allow the estuarine turbidity maximum (ETM) to be located in the vicinity of the cove entrance, which increases sediment concentrations during flood tide. Second, the saltier water in the main channel can enter the cove as a density current, enhancing near-bed velocities and resuspending sediment, providing an efficient delivery mechanism. Finally, higher salinity water accumulates in the deep basin of the cove, creating a stratified region that becomes decoupled from ebb currents, promoting retention of sediment in the cove. This process of estuarine-enhanced sediment accumulation in off-river coves will likely extend upriver during future sea level rise.

Lacustrine sediment archives indicate that flooding during Tropical Storm Irene (2011) in the north-eastern United States caused the most severe erosion of any flood in the historic record, surpassing that of events with greater precipitation and peak discharges. Compared to deposition from historic floods, Irene's event layer was more massive and more enriched in unweathered upland sediments, indicating an anomalously high incidence of mass wasting and sediment entrainment. Precipitation records indicate that neither precipitation intensity nor total accumulation distinguished Irene from less erosive historic floods. However, cumulative precipitation prior to Irene exceeded the 95th percentile of all days in the record. When allowing for non-stationarity in the twentieth century background precipitation, we find a four-fold increase in the probability of Irene-like conditions, where impacts of extreme rainfall are enhanced by high antecedent precipitation. We conclude that irrespective of increases in extreme precipitation, the risk of highly erosive flooding in the region is increasing due to the influence of wetter baseline conditions associated with a changing climate. Copyright © 2015 John Wiley & Sons, Ltd.

Both human activity and climate change can influence erosion rates and initiate rapid landscape change. Understanding the relative impact of these factors is critical to managing the risks of extreme erosion related to flooding and landslide occurrence. Here we present a 2100 year record of sediment mass accumulation and inferred erosion based on lacustrine sediment cores from Amherst Lake, Vermont, USA. Using deposition from August 2011 Tropical Storm Irene as a modern analogue, we identified distinct event deposits indicative of destructive erosion events. These deposits record a prolonged (multidecadal) interval of enhanced erosion following the initial storm-induced landscape disturbance. The direct impact of human land cover alteration is minimal in comparison to the more recent twentieth century increase in the occurrence of catastrophic erosion linked to overall wetter conditions that favor high erosion rates and more easily trigger landslides during periods of extreme precipitation.

In humid regions, where gaining river conditions generally prevail, daily hydroelectric dam releases alter downstream surface water–groundwater interactions by reversing the head gradient between river and adjacent groundwater. Previously, it has been noted that artificial stage changes due to dam releases enhance hyporheic exchange. Here we investigate the regulated Deerfield River in northwestern Massachusetts at multiple scales to evaluate how changing downstream geologic conditions along the river mediate this artificial hyporheic pumping. Water budget analysis indicates that roughly 10% of bank-stored water is permanently lost from the 19.5-km river reach, likely as a result of transpiration by bank vegetation. An adjacent reference stream with similar dimensions and geomorphology, but without hydropeaking, shows predictable gaining conditions. Field observations from streambed piezometers and thermistors show that water losses are not uniform throughout the study reach. Riparian aquifer transmissivity in river sub-reaches largely determines the magnitude of surface water–groundwater exchange as well as net water loss from the river. These newly documented losses from hydropeaking river systems should inform decisions by river managers and hydroelectric operators of additional tradeoffs of oscillatory dam-release river management. Copyright © 2015 John Wiley & Sons, Ltd.

Hurricane Irene passed directly over the Connecticut River valley in late August, 2011. Intense precipitation and high antecedent soil moisture resulted in record flooding, mass wasting and fluvial erosion, allowing for observations of how these rare but significant extreme events affect a landscape still responding to Pleistocene glaciation and associated sediment emplacement. Clays and silts from upland glacial deposits, once suspended in the stream network, were routed directly to the mouth of the Connecticut River, resulting in record-breaking sediment loads fifteen-times greater than predicted from the pre-existing rating curve. Denudation was particularly extensive in mountainous areas. We calculate that sediment yield during the event from the Deerfield River, a steep tributary comprising 5% of the entire Connecticut River watershed, exceeded at minimum 10–40 years of routine sediment discharge and accounted for approximately 40% of the total event sediment discharge from the Connecticut River.A seriesof surface sediment corestaken in floodplainponds adjacent to the tidalsec

Observations and modeling are used to assess potential impacts of sediment releases due to dam removals on the Hudson River estuary. Watershed sediment loads are calculated based on sediment-discharge regressions for gauges covering 80% of the watershed area. The annual average sediment load to the estuary is 1.2 Mt, of which about 0.6 Mt comes from tributaries entering below the head of tides. Sediment yield varies inversely with watershed area, with regional trends that are consistent with differences in substrate erodibility. Geophysical and sedimentological surveys in five subwatersheds of the Lower Hudson were conducted to characterize the mass and composition of sediment trapped behind dams. Impoundments were classified as 1) active sediment traps, 2) run-of-river sites not actively trapping, and 3) dammed natural lakes and spring-fed ponds. Based on this categorization and impoundment attributes from the dam inventory database, the total mass of impounded sediment in the Lower Hudson watershed is estimated as 3.1 Mt. Assuming that roughly half of the impounded sediment is typically released downstream with dam removal, then the potential inputs represent less than 2 years of annual watershed supply. Modeling of simulated dam removals shows that modest suspended sediment increases occur in the estuary within about a tidal excursion of the source tributary, primarily during discharge events. Transport in the estuary depends strongly on settling velocity, but fine particles, which are important for accretion in tidal wetlands, deposit broadly along the estuary rather than primarily near the source.