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Between June and September each year, the Indian monsoon typically delivers about 80% of the Nepalese Himalaya's annual precipitation. Topography on the windward (southern) flank of the range modulates persistent spatial variations in precipitation along the length of the range. Where topography is stepped with an initial abrupt rise as the Himalaya abut the foreland and a second rise as topography ascends toward the high peaks, two bands of high precipitation prevail, each where relief passes a threshold value. In contrast, more uniform, northward rising topography localizes a single high-rainfall band near the front of the range. A dense meteorological network that was operated in the Marsyandi catchment from 1999 to 2004 in central Nepal gives more insight on spatial variability in precipitation. Annual precipitation decreases ten-fold between the rainfall peak (of ∼4. m/yr) on the southern flank of the Himalaya and the semi-arid rain shadow on its northern flank (40-50. km farther north). Modest contrasts in rainfall between ridges versus valleys during the monsoon are replaced by strong altitude-dependent precipitation contrasts in the winter. Strikingly, above ∼4. km altitude, ∼40% of the total precipitation arrives as winter snowfall. Four years of daily discharge and suspended sediment measurements on the main-stem and on several tributaries of the Marsyandi during the monsoon document a strong north-south gradient in average erosion rates. Based on a suspended-to-bedload ratio of 2:1 (as estimated from grain-sizes in a landslide-dammed paleo-lake), erosion rates range from ∼0.1. mm/yr in the northern rain shadow to ∼2. mm/yr in the monsoon-drenched south. This strong modern spatial gradient in erosion rates mimics the precipitation gradient across the same area and broadly scales with specific discharge. In the wetter regions, nearly a meter of rain is required before significant sediment fluxes occur. After this initial meter of rain, the daily rainfall required to trigger sediment pulses (attributable to landsliding) gradually decreases during the remaining monsoon season from ∼40. mm to 10. mm. In the higher altitude rain shadow to the north, water discharge is more closely linked to temperature than to precipitation: a linkage suggesting that melting of snow and ice, rather than rainfall, modulates the runoff. The sediment flux in the rain shadow during the monsoon season displays a marked temporal hysteresis: fluxes broadly scale with discharge during the first two months of the monsoon, but decouple from discharge later in the monsoon. This behavior suggests that the sediment flux is supply limited. We interpret that much of the sediment is subglacially derived and that its transport into the river network is restricted either by limited bedrock erosion or subglacial hydrology. © 2012 Académie des sciences.

We investigated the surficial processes that deliver sediment from hillslopes into channels in a mediterranean landscape and determined how these processes are controlled by climate and landscape characteristics (e.g. topography, vegetation). Watersheds in semi-arid regions of the United States are subject to a variety of disturbances, including grazing, fires, and vegetation cover changes and, while the consequences of sediment loading are well documented, presently our ability to predict the spatial and temporal pattern of delivery is poor. The processes that we have investigated are: shallow landslides, overland flow, dry ravel, and bioturbation. Through field experiments and monitoring, we have developed physically-based transport equations for each individual process. These transport equations are used as the governing equations for a computer model, driven by random sequences of rainstorms and fires, that predicts the spatial and temporal patterns of sediment delivery. As downstream problems become more closely linked to watershed conditions and perturbations, this type of fundamental research is relevant to issues such as the health of riverine ecosystems and the siltation of reservoirs. We anticipate that our fieldwork and modeling results will help land managers estimate the influx of sediment from surface processes according to different land-use practices.

Uplift of the Himalayas has been proposed to have locally accelerated chemical weathering, thus leading to enhanced C[O.sub.2] sequestration and global cooling. This hypothesis assumes that rapid erosion exposes fresh, highly reactive minerals at Earth's surface. Empirical studies quantifying the relationship between erosion and weathering have produced apparently conflicting results, where the nature of the relationship is dependent on the weathering regime of the sampled landscapes. We derive a quantitative model that defines this relationship across the range of weathering regimes, from supply-limited to kinetically limited conditions. The model matches trends in field data collected by others and reconciles apparently conflicting results. The model also demonstrates that, as erosion rates increase, potential increases in weathering rate from the exposure of fresher materials are offset by the decrease in the total volume of minerals exposed due to thinner regolith. We conclude that the relationship between weathering and erosion is one of diminishing returns, in which increases in erosion rate lead to progressively smaller increases in weathering rate; indeed, at the highest erosion rates, weathering rates may decline. The ability, therefore, of accelerated uplift and erosion to stimulate greater C[O.sub.2] sequestration may be significant in landscapes eroding at rates of [10.sup.0]-[10.sup.2] t [km.sup.- 2] [yr.sup.-l]. However, where erosion rates are greater than [10.sup.2] t [km.sup.-2] [yr.sup.-l,] increases in denudation may not be matched by increases in chemical weathering. Finally, our results suggest that watersheds with regolith thicknesses of ~0.5 m will yield the greatest solute fluxes.

Theories that propose feedbacks among climate, tectonics, and surface processes commonly assume that erosion is enhanced by glacial activity. Indeed, studies have shown that glaciers appear to limit the elevation of mountain ranges; however, comparisons between rates of glacial and nonglacial erosion are difficult to make. Ideally, such comparisons must hold precipitation and lithology constant, while only varying the erosional regime. Located in a climatic transition zone during the Pleistocene, the east-west-trending valleys of the Bitterroot Range present an opportunity for a quantitative analysis of glacial and nonglacial erosion because the north-facing sides of the valleys were glaciated, whereas the south-facing slopes were not. The different erosional regimes operating on either side of the valleys created strongly asymmetric ridges. Ridgelines separating the east-west-trending valleys have been pushed southward by glacial headwall retreat such that ridge-to-valley distances are ~50% greater on the north-facing slopes than on the south-facing slopes. In addition, mean hillslope angles are 6[degrees] lower on the glaciated slopes than on the unglaciated slopes, and calculations of geophysical relief suggest that, on average, glaciers have removed nearly twice as much rock as nonglacial processes. Finally, we conclude that, although rates of vertical incision by glacial processes in the Bitterroot Range were more rapid than nonglacial processes, the dominant geomorphological impact of glaciers was lateral erosion by headwall retreat. Keywords: glacial erosion, cirque headwall retreat, divide migration, relief, Bitterroot Range.

Sediment delivery to stream channels in mountainous basins is strongly episodic, with large pulses of sediment typically delivered by infrequent landslides and debris flows. Identifying the role of large but rare sediment delivery events in the evolution of channel morphology and fluvial sediment transport is crucial to an understanding of the development of mountain basins. In July 2001, intense rainfall triggered numerous debris flows in a severely burnt watershed in the Sapphire Mountains of Montana. Ten large debris flow fans were deposited on the valley floor, and investigations focused on the channel response to these sediment pulses. The channel has aggraded immediately upstream of each fan, and braided in reaches immediately downstream. Channel incision through the fans has created sets of coarse-grained terraces. The deposition upstream of the pulses consists almost exclusively of fine material, resulting in a median bed material size ([D.sub.50]) 1-2 orders of magnitude lower than the ambient channel material. The volume of sand being transported is so great that these aggrading reaches can extend hundreds of meters upstream of the fans, with 1-2 m of sand deposited across the entire valley floor. Along a 10 km study reach, cross section surveys, longitudinal profiles, and pebble counts chronicle channel response to a punctuated increase in sediment supply and provide insight on the processes of sediment wave dispersal. Keywords: fluvial geomorphology, sediment, debris flows, bed load, fire.

Chemical weathering of bedrock is critical to maintaining terrestrial life, and climate, typically as manifested by precipitation, is often identified as having a first-order control on rates of chemical weathering. The ability of precipitation to dissolve rock, however, is modulated by the properties of the overlying soil that influence the contact time between water and minerals. Flume experiments were conducted to investigate the hydrological controls on rates of chemical weathering. Solute concentrations of runoff flowing across synthetic bedrock overlain by nonreactive pseudosoils of differing hydraulic conductivities were measured to examine the role of seepage velocity in influencing weathering rates on steep slopes. The results suggest that, where weathering is not limited by the supply of fresh minerals, weathering rates should increase with decreasing hydraulic conductivity. In addition, a mathematical relationship between hydraulic conductivity and chemical weathering on hillslopes is introduced to explore the hydrological controls on feldspar and calcite dissolution rates. The mathematical model supports the results from the experiments. Keywords: chemical weathering, soil production, dissolution, hydraulic conductivity, advective-dispersion equation, hillslope hydrology.

Comparison of rainfall data and mean hillslope angles in the Himalayas of central Nepal shows that mean hillslope angles decrease with increasing mean annual rainfall. Higher pore pressures and higher rates of chemical weathering in the wetter regions may decrease the threshold angle of hillslopes prone to landsliding. When valley spacing is held constant, the sensitivity of mean hillslope angle to climate implies that relief, in the absence of limits due to rock strength, is also dependent on climate. These results suggest that wet-to-dry climatic changes increase relief in regions with incising bedrock channels and that dry-to-wet climatic changes reduce relief. Keywords: threshold hillslopes, climatic geomorphology, relief, Himalayas, Nepal, landslides.

During the 1997-1998 El Nino, record rainfall triggered >150 shallow landslides within a 9.5 [km.sup.2] area near Santa Barbara, California. They were studied to analyze the sediment delivery to valley floors from landslides in coastal sage scrub and converted grasslands. The conversion of coastal sage to grasslands, primarily to provide pasturage for cattle, is common in the region, and the landscape's response may affect water quality, reservoir infilling, and debris flow hazards. We explore the relationship between lateral-root reinforcement and landslide volume by developing a slopestability analysis that incorporates root cohesion along the sides of the failure. The stability analysis correctly predicts an inverse relationship between landslide volume and hillslope angle in the sage. The volumes of failures in the grasslands do not vary systematically with slope and are generally smaller than those in the sage. From aerial-photograph analysis and field mapping, we find that there are 22.9 failures per square kilometer in the grasslands compared to 13.2 failures per square kilometer in the sage. Despite the lower failure density in the coastal sage, greater failure volumes and longer transport distances delivered more sediment to valley floors, with a specific volumetric flux of 2.8 x [10.sup.-2] [m.sup.3].[m.sup.-1] for this El Nino compared to 1.7 x [10.sup.-2] [m.sup.3].[m.sup.-2] in the grasslands. We conclude that the conversion from vegetation with stronger and deeper roots (coastal sage) to vegetation with weaker and shallower roots (grass) has caused a pulse of increased landsliding in the grasslands because the soils are currently too thick for the prevailing root reinforcement. We suggest that, over time, soils in the grassland hollows will become thinner as the evacuation by landslides is repeated until the landsliding rate declines to balance the soil supplied from local colluvium production and diffusive processes upslope. Keywords: debris flows, geomorphology, sediment transport, slope stability.

In this study, the strong lithological heterogeneity of the northern Sierra Nevada (California, USA) is exploited to elucidate the role of lithology on river profiles and patterns at the mountain-range scale. The analyses indicate that plutonic, metavolcanic, and quartzite bedrock generally host the steepest river reaches, whereas gentle reaches flow across non-quartzite metasedimentary rocks and fault zones. In addition, the largest immobile boulders are often in the steepest reaches, suggesting that wide joint spacing plays a role in creating steep channels, and a positive relationship between boulder size and hillslope angle highlights the coupling of the hillslope and fluvial systems. With respect to river network configurations, dendritic patterns dominate in the plutonic bedrock, with channels aligned down the slope of the range; in contrast, river reaches in the metamorphic belts are mainly longitudinal and parallel to the structural grain. River profiles and patterns in the northern Sierra Nevada, therefore, bear a strong lithological imprint related to differential erosion. These observations indicate that attempts to infer uplift and tilting of the range based on the gradients and orientations of paleochannel remnants should first account for the effect of bedrock erodibility. Indeed, the differences in gradients of Tertiary paleochannel remnants used to argue for late Cenozoic uplift of the range can be wholly explained by differences in lithology. [ABSTRACT FROM AUTHOR]

Insights into the process of bioturbation are provided and particular focus is given on the bioturbatiuon within a quantitative framework that links the form of hillslopes with the processes of sediment transport and soil production. The effects of bioturbation on soil processes and sediment transport are also discussed.

From the earliest days of California geology, the ramp-like profile of the northern half of the Sierra Nevada mountains and putative signs of recent incision have been interpreted as evidence that the range was formed by the tilting of a rigid block in the late Cenozoic. Over the years, various geomorphic analyses have been used to quantify the magnitude of uplift and to establish its timing, such as analyzing the gradients of ancient channels, examining the tilt of sedimentary beds, and reconstructing the incisional history of rivers. Most studies that have used these methods have supported substantial (>500 m) recent uplift of the Sierra. In contrast, investigations based on other sources of paleotopographic information, such as isotope records, thermochronology, and detrital zircon geochronology, have found that the Sierra have been at high elevations for much of the Cenozoic. This set of contradictory results motivates a re-examination of the geomorphic evidence for late Cenozoic uplift. A critical assessment of these geomorphic studies, based on new topographic analyses and field investigations, reveals that their conclusions are not well supported. For example, several studies based their results on reconstructions of ancient channels that would have flowed up and over bedrock ridges as high as 190 m, a physical impossibility. Other weaknesses include unjustified assumptions regarding the original tilt of fluvial deposits, misinterpretations of stratigraphic relationships, and inadequate recognition of the effect of lithology on channel profiles. The studies supporting recent tilting in the northern Sierra Nevada are inconclusive and rely on observations not unique to tectonic forcing. Indeed, much of the evidence based on the paleogradients of the Tertiary channels is consistent with an early trellis drainage network formed across alternating bands of resistant and weak lithologies. In addition, analyses are presented to demonstrate that deep northern Sierran canyons thought to have been recently incised were, instead, cut as early as the Eocene-Oligocene. Two geomorphic studies from the southern Sierra are consistent with late Cenozoic tilting and uplift although ongoing tectonic activity may be insignificant. Finally, I present a conceptual model of the evolution of the Sierran landscape, applicable primarily to the northern half of the range, illustrating the development of three different drainage networks since the late Jurassic. [ABSTRACT FROM AUTHOR]