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Water in rivers is delivered via the critical zone (CZ)—the living skin of the Earth, extending from the top of the vegetation canopy through the soil and down to fresh bedrock and the bottom of significantly active groundwater. Consequently, the success of stream‐rearing salmonids depends on the structure and resulting water storage and release processes of this zone. Physical processes below the land surface (the subsurface component of the CZ) ultimately determine how landscapes "filter" climate to manifest ecologically significant streamflow and temperature regimes. Subsurface water storage capacity of the CZ has emerged as a key hydrologic variable that integrates many of these subsurface processes, helping to explain flow regimes and terrestrial plant community composition. Here, we investigate how subsurface storage controls flow, temperature, and energetic regimes that matter for salmonids. We illustrate the explanatory power of broadly applicable, storage‐based frameworks across a lithological gradient that spans the Eel River watershed of California. Study sites are climatically similar but differ in their geologies and consequent subsurface CZ structure that dictates water storage dynamics, leading to dramatically different hydrographs, temperature, and riparian regimes—with consequences for every aspect of salmonid life history. Lithological controls on the development of key subsurface CZ properties like storage capacity suggest a heretofore unexplored link between salmonids and geology, adding to a rich literature that highlights various fluvial and geomorphic influences on salmonid diversity and distribution. Rapidly advancing methods for estimating and observing subsurface water storage dynamics at large scales present new opportunities for more clearly identifying landscape features that constrain the distributions and abundances of organisms, including salmonids, at watershed scales. [ABSTRACT FROM AUTHOR]

The Glen Torridon (GT) region within Gale crater, Mars, occurs in contact with the southern side of Vera Rubin ridge (VRR), a well‐defined geomorphic feature that is comparatively resistant to erosion. Prior to detailed ground‐based investigation of GT, its geologic relationship with VRR was unknown. Distinct lithologic subunits within the Jura member (Murray formation), which forms the upper part of VRR, made it possible to be also identified within GT. This indicates that the strata pass across the geomorphic divide between regions. Furthermore, the cross‐bedded lower part of the overlying Knockfarril Hill member (Carolyn Shoemaker formation) also occurs within both VRR and GT. Correlation of both units demonstrates that the strata form a continuous stratigraphic succession regardless of large‐scale geomorphic expression. The lithologic change from mudstone (Jura member) to cross‐bedded sandstone (Knockfarril Hill member) heralds a significant shift in paleoenvironment from lacustrine to fluvial. The upper part of the Knockfarril Hill member consists of interbedded mudstone and sandstone that transitions to the overlying finely laminated mudstone of the Glasgow member, and a return to lacustrine deposition. In GT, the Stimson formation unconformably overlies the Glasgow member, where it demarks the southern boundary of GT. Contacts for each stratigraphic unit were defined and transferred to a high‐resolution image base to make a geologic map and cross sections perpendicular to the NE strike. Stratal dips cannot exceed 2° NW to retain the positions of stratigraphic units in the locations they are exposed throughout GT. Plain Language Summary: The Mars Science Laboratory Curiosity rover explored a region called Glen Torridon (GT) that is located on the northwest side of a large sedimentary central mound (Mount Sharp) within Gale crater. This study analyzed the rocks within GT to (a) identify the sedimentary features, (b) determine if or how the sedimentary layers correlate between the Vera Rubin ridge (VRR) and GT regions, (c) interpret ancient environments, (d) generate a geologic map, and (e) discuss the relationship between topography and the exposure of sedimentary layers. Even though the VRR and GT regions are very different in present‐day geomorphology, sedimentary layers correlate across the two regions indicating that the layers belong to a continuous sedimentary succession. Cross‐bedded sandstones of the Knockfarril Hill member represent a change to fluvial from the underlying lake and lake‐margin deposits of the Jura member, which represents a major shift in depositional environment. Packages of layers have distinct characteristics, which allows their distribution to be represented on a map. A geologic cross section limits the tilt of the layers to less than two degrees. We find that caution must be taken when attempting to derive primary stratigraphy only using datasets acquired from orbital platforms. Key Points: Cross‐bedded Knockfarril Hill member strata overlie mudstone of the Jura member indicating an environmental change from lake to fluvialA geologic map of Glen Torridon (GT) made using lithologically defined stratigraphic units shows that strata dip two degrees or less to the NNWStratigraphic units pass from Vera Rubin ridge into GT indicating that strata continue through a major geomorphic boundary [ABSTRACT FROM AUTHOR]

Bedrock vadose zone water storage (i.e., rock moisture) dynamics are rarely observed but potentially key to understanding drought responses. Exploiting a borehole network at a Mediterranean blue oak savanna site—Rancho Venada—we document how water storage capacity in deeply weathered bedrock profiles regulates woody plant water availability and groundwater recharge. The site is in the Northern California Coast Range within steeply dipping turbidites. In a wet year (water year 2019; 647 mm of precipitation), rock moisture was quickly replenished to a characteristic storage capacity, recharging groundwater that emerged at springs to generate streamflow. In the subsequent rainless summer growing season, rock moisture was depleted by about 93 mm. In two drought years that followed (212 and 121 mm of precipitation) the total amount of rock moisture gained each winter was about 54 and 20 mm, respectively, and declines were documented exceeding these amounts, resulting in progressively lower rock moisture content. Oaks, which are rooted into bedrock, demonstrated signs of water stress in drought, including reduced transpiration rates and extremely low water potentials. In the 2020–2021 drought, precipitation did not exceed storage capacity, resulting in variable belowground water storage, increased plant water stress, and no recharge or runoff. Rock moisture deficits (rather than soil moisture deficits) explain these responses. Plain Language Summary: When rainfall is lower than normal, water stored belowground can sustain forests and drain to streams. Does the presence of deep, dynamic water storage in bedrock below soil provide enhanced drought resilience? We used a network of deep boreholes drilled into hillslopes to study how the weathered bedrock unsaturated zone acts as a water source for woody vegetation and mediates groundwater recharge. At our winter‐wet, summer‐dry oak savanna field site in the Northern California Coast Range, dry season reductions in rock moisture were driven by woody plant water use. However, the water storage capacity of the subsurface exceeded net precipitation inputs in dry winters. Thus, deep water storage was not replenished during an extreme drought to the same degree as in wetter years, resulting in decreased groundwater recharge and streamflow, and lower water availability for trees—even those deeply rooted in the bedrock. Trees using bedrock‐water exhibited dieback in the second year of the hot, dry 2020–2021 drought. These findings motivate expanded study of how moisture storage properties of deep bedrock influence the susceptibility of vegetation to drought over multiple years. Key Points: Multi‐year field study reveals role of deep bedrock moisture within hillslopesLarge deep storage capacity relative to precipitation in dry years drives variable interannual storageDrought reduced rock moisture replenishment, groundwater recharge, streamflow, and blue oak water potential, transpiration and leaf area [ABSTRACT FROM AUTHOR]

Aeolis Mons (informally, Mount Sharp) exhibits a number of canyons, including Gediz and Sakarya Valles. Poorly sorted debris deposits are evident on both canyon floors and connect with debris extending down the walls for canyon segments that cut through sulphate‐bearing strata. On the floor of Gediz Vallis, debris overfills a central channel and merges with a massive debris ridge located at the canyon terminus. One wall‐based debris ridge is evident. In comparison, the floor of Sakarya Vallis exhibits a complex array of debris deposits. Debris deposits on wall segments within Sakarya Vallis are mainly contained within chutes that extend downhill from scarps. Lateral debris ridges are also evident on chute margins. We interpret the debris deposits in the two canyons to be a consequence of one or more late‐stage hydrogeomorphic events that increased the probability of landslides, assembled and channelized debris on the canyon floors, and moved materials down‐canyon. The highly soluble nature of the sulphate‐bearing rocks likely contributed to enhanced debris generation by concurrent aqueous weathering to produce blocky regolith for transport downslope by fluvial activity and landslides, including some landslides that became debris flows. Subsequent wind erosion in Gediz Vallis removed most of the debris deposits within that canyon and partially eroded the deposits within Sakarya Vallis. The enhanced wind erosion within Gediz Vallis was a consequence of the canyon's alignment with prevailing slope winds. Plain Language Summary: Debris deposits on the walls and floors of canyons cut into Mount Sharp are a consequence of late stage precipitation, with ground water infiltration that triggered landslides and debris flows of blocky regolith. This is supported by their morphologies, and by calculations of slope stability in the presence of groundwater. The relatively soluble nature of the sulphate‐bearing strata facilitated chemical weathering to produce material conditioned to fail as landslides. Subsequent wind‐driven erosion degraded many of these features, including differential stripping of wall and floor rocks to generate debris ridges. Key Points: Debris deposits on the walls and floors of canyons cut into Mount Sharp were a consequence of late stage, water‐related eventsWind‐driven erosion degraded many of these features, including differential stripping of wall and floor deposits to generate debris ridges [ABSTRACT FROM AUTHOR]

This paper provides an overview of the Curiosity rover's exploration at Vera Rubin ridge (VRR) and summarizes the science results. VRR is a distinct geomorphic feature on lower Aeolis Mons (informally known as Mount Sharp) that was identified in orbital data based on its distinct texture, topographic expression, and association with a hematite spectral signature. Curiosity conducted extensive remote sensing observations, acquired data on dozens of contact science targets, and drilled three outcrop samples from the ridge, as well as one outcrop sample immediately below the ridge. Our observations indicate that strata composing VRR were deposited in a predominantly lacustrine setting and are part of the Murray formation. The rocks within the ridge are chemically in family with underlying Murray formation strata. Red hematite is dispersed throughout much of the VRR bedrock, and this is the source of the orbital spectral detection. Gray hematite is also present in isolated, gray‐colored patches concentrated toward the upper elevations of VRR, and these gray patches also contain small, dark Fe‐rich nodules. We propose that VRR formed when diagenetic event(s) preferentially hardened rocks, which were subsequently eroded into a ridge by wind. Diagenesis also led to enhanced crystallization and/or cementation that deepened the ferric‐related spectral absorptions on the ridge, which helped make them readily distinguishable from orbit. Results add to existing evidence of protracted aqueous environments at Gale crater and give new insight into how diagenesis shaped Mars' rock record. Plain Language Summary: Vera Rubin ridge is a feature at the base of Mount Sharp with a distinct texture and topography. Orbiter observations showed hematite, a mineral that sometimes forms by chemical reactions in water environments, was present atop the ridge. The presence of both water and chemical activity suggested the area preserved a past habitable environment. In this paper, we detail how the Curiosity science team tested this and other orbital‐based hypotheses. Curiosity data suggested that most ridge rocks were lain down in an ancient lake and had similar compositions to other Mount Sharp rocks. Curiosity confirmed that hematite was present in the ridge but no more abundantly than elsewhere. Larger grain size or higher crystallinity probably account for the ridge's hematite being more visible from orbit. We conclude Vera Rubin ridge formed because groundwater recrystallized and hardened the rocks that now make up the ridge. Wind subsequently sculpted and eroded Mount Sharp, leaving the harder ridge rocks standing because they resisted erosion compared with surrounding rocks. The implication of these results is that liquid water was present at Mount Sharp for a very long time, not only when the crater held a lake but also much later, likely as groundwater. Key Points: We summarize Curiosity's campaign at Vera Rubin ridge (Sols 1726–2302) and the high‐level results from articles in this special issueVera Rubin ridge formed when diagenesis hardened rocks along the base of Aeolis Mons; wind subsequently etched the feature into a ridgeResults add evidence for protracted aqueous environments at Gale crater and give new insight into how diagenesis shaped Mars' rock record [ABSTRACT FROM AUTHOR]

The spatiotemporal dynamics of plant water sources are hidden and poorly understood. We document water source use of Quercus garryana growing in Northern California on a profile of approximately 50 cm of soil underlain by 2–4 m of weathered bedrock (sheared shale mélange) that completely saturates in winter, when the oaks lack leaves, and progressively dries over the summer. We determined oak water sources by combining observations of water stable isotope composition, vadose zone moisture and groundwater dynamics, and metrics of tree water status (potential) and use (sapflow). During the spring, oak xylem water is isotopically similar to the seasonal groundwater and shallow, evaporatively enriched soil moisture pools. However, as soils dry and the water table recedes to the permanently saturated, anoxic, low‐conductivity fresh bedrock boundary, Q. garryana shifts to using a water source with a depleted isotopic composition that matches residual moisture in the deep soil and underlying weathered bedrock vadose zone. Sapflow rates remain high as late‐summer predawn water potentials drop below −2.5 MPa. Neutron probe surveys reveal late‐summer rock moisture declines under the oaks in contrast to constant rock moisture levels under grass‐dominated areas. We therefore conclude that the oaks temporarily use seasonal groundwater when it occupies the weathered profile but otherwise use deep unsaturated zone moisture after seasonal groundwater recedes. The ample moisture, connected porosity, and oxygenated conditions of the weathered bedrock vadose zone make it a key tree water resource during the long summer dry season of the local Mediterranean climate. Plain Language Summary: What are the water sources that allow oaks to transpire through extended dry periods? Oaks in California are thought to tap into groundwater to meet dry season water demand. Here, we show that oaks growing in a savanna woodland instead use tightly held moisture within deep soil and weathered bedrock above the saturated zone. We determined this by matching stable isotopes in the trees' water to these moisture pools. Deep soil and rock moisture were isotopically lighter than shallow soil moisture, which was affected by evaporative enrichment, and the groundwater, which looked more like average wet season rainfall. Monitored moisture declines within the weathered bedrock corroborate the isotopic interpretation that rock moisture sustained oak transpiration. Although the saturated zone was only a few meters below the ground surface in late summer, the trees did not use it due to its low oxygen content and residence in low permeability fresh bedrock. Our findings matter to forest management because of the need to determine when and where trees use groundwater, which regulates ecologically critical baseflow. Moisture extraction from the weathered bedrock unsaturated zone is likely important globally, as forests are widespread on hillslopes with thin soils overlying weathered bedrock. Key Points: Isotopic, physiologic, and hydrologic observations were combined to identify oak water sourcesRock and deep soil moisture rather than groundwater sustain transpiration in summer dry seasonDeep unsaturated zone moisture sources and oak water were isotopically depleted [ABSTRACT FROM AUTHOR]

Plant water stress in response to rainfall variability is mediated by subsurface water storage, yet the controls on stored plant‐available water remain poorly understood. Here we develop a probabilistic water balance model for Mediterranean climates that relates the amount of water stored over the wet season to annual rainfall statistics and subsurface storage capacity in soil and weathered bedrock. This model predicts that low storage capacity—relative to winter rainfall—results in similar year‐to‐year summer water availability, as both relatively wet and dry winters replenish storage. Observed water balances in seven catchments in the Northern California Coast Ranges exhibited this dynamic. We hypothesized that plants would be decoupled from precipitation variability at these storage‐capacity‐limited sites and observed that summer productivity and water use (inferred from the enhanced vegetation index) were independent of winter rainfall totals. These areas emerged largely unscathed from recent extreme drought, despite widespread plant mortality elsewhere. Plain Language Summary: When does a shortage of precipitation become a shortage of water supply to plants? In rain‐dominated seasonally dry climates, the answer depends on how water is stored belowground. Here we propose—perhaps counterintuitively—that low water storage capacity in Earth's critical zone (which includes soil and weathered bedrock) relative to average rainfall can decouple plant community productivity and water use from rainfall variability, and conversely that relatively large storage capacity increases plant sensitivity to annual swings in rainfall totals. A simple model and analysis of watersheds in winter wet, summer dry climates in California reveal that where it consistently rains much more than the subsurface can store, a similar amount of water is stored belowground in both relatively wet and dry years, with excess rainfall leaving as runoff. We hypothesized that this would result in similar year‐to‐year summer plant water availability, in spite of highly variable winter rainfall. We found, via satellite observations, that summer plant greenness was insensitive to swings in precipitation at these "storage‐capacity‐limited" sites. Contrary to predictions based primarily on tree density and rainfall deficits, these sites did not experience widespread mortality in the 2011‐2016 extreme drought. Key Points: Where annual rainfall reliably exceeds subsurface storage capacity, plant productivity should be insensitive to rainfall variabilityWater balances reveal sites where storage is replenished in both wet and dry winters, resulting in consistent summer plant water supplySuch storage‐capacity limited sites are inherently resilient to meteorological drought [ABSTRACT FROM AUTHOR]

Earth System Models (ESMs) are essential tools for understanding and predicting global change, but they cannot explicitly resolve hillslope‐scale terrain structures that fundamentally organize water, energy, and biogeochemical stores and fluxes at subgrid scales. Here we bring together hydrologists, Critical Zone scientists, and ESM developers, to explore how hillslope structures may modulate ESM grid‐level water, energy, and biogeochemical fluxes. In contrast to the one‐dimensional (1‐D), 2‐ to 3‐m deep, and free‐draining soil hydrology in most ESM land models, we hypothesize that 3‐D, lateral ridge‐to‐valley flow through shallow and deep paths and insolation contrasts between sunny and shady slopes are the top two globally quantifiable organizers of water and energy (and vegetation) within an ESM grid cell. We hypothesize that these two processes are likely to impact ESM predictions where (and when) water and/or energy are limiting. We further hypothesize that, if implemented in ESM land models, these processes will increase simulated continental water storage and residence time, buffering terrestrial ecosystems against seasonal and interannual droughts. We explore efficient ways to capture these mechanisms in ESMs and identify critical knowledge gaps preventing us from scaling up hillslope to global processes. One such gap is our extremely limited knowledge of the subsurface, where water is stored (supporting vegetation) and released to stream baseflow (supporting aquatic ecosystems). We conclude with a set of organizing hypotheses and a call for global syntheses activities and model experiments to assess the impact of hillslope hydrology on global change predictions. Plain Language Summary: Hillslopes are key landscape features that organize water availability on land. Valley bottoms are wetter than hilltops, and sun‐facing slopes are warmer and drier than shaded ones. This hydrologic organization leads to systematic differences in soil and vegetation between valleys and hilltops, and between sunny and shady slopes. Although these patterns are fundamental to understanding the structures and functions of water and terrestrial ecosystems, they are too fine grained to be represented in global‐scale Earth System Models. Here we bring together Critical Zone scientists who study the interplay of vegetation, the porous upper layer of the continental crust from vegetation to bedrock, and moisture dynamics deep into the weathered bedrock underlying hillslopes and Earth System Model scientists who develop global models, to ask: Do hillslope‐scale processes matter to predicting global change? The answers will help scientists understand where and why hillslopes matter, and to better predict how terrestrial ecosystems, including societies, may affect and be affected by our rapidly changing planet. Key Points: Lateral flow from ridges to valleys, and contrasts between sunny and shady slopes organize water, energy and vegetation across landscapesThese processes may affect Earth System Model predictions of terrestrial water storage and fluxes, as well as ecosystem resilience to stressThe greatest knowledge gap is the subsurface structure; Critical Zone science can offer new insights into terrestrial water storage/fluxes [ABSTRACT FROM AUTHOR]

The topography provided by altimetry, synthetic aperture radar-topography, and stereo radargrammetry has opened new doors for Titan research by allowing for quantitative analysis of morphologic form. Using altimetry measurements, we show that Titan's Maria are consistent with an equipotential surface but that several filled lakes are found to be hundreds of meters above this sea level, suggesting that they exist in isolated or perched basins. Within a given drainage basin, empty lake floors are typically higher than the liquid elevation of nearby lakes/seas, suggesting local subsurface connectivity. The majority of Titan's lakes reside in topographically closed, sharp-edged depressions whose planform curvature suggests lateral expansion through uniform scarp retreat. Many, but not all, empty lake basins exhibit flat floors and hectometer-scale raised rims that present a challenge to formation models. We conclude that dissolution erosion can best match the observed constraints but that challenges remain in the interpretation of formation processes and materials. [ABSTRACT FROM AUTHOR]

ABSTRACT Preferential flow in hillslope systems through subsurface networks developed from a range of botanical, faunal and geophysical processes have been observed and inferred for decades and may provide a large component of the bulk transport of water and solutes. However, our dominant paradigm for understanding and modelling hillslope hydrologic processes is still based on the Darcy-Richards matric flow framework, now with a set of additional methods to attempt to reproduce some of the aggregate function of the two-phase system of network and matrix flow. We call for a community effort to design and implement a set of well planned experiments in different natural and constructed hillslopes, coupled with the development of new theory and methods to explicitly incorporate and couple the co-evolution of subsurface flow networks as intrinsic components of hydrological, ecological and geomorphic systems. This is a major community challenge that can now benefit from new experimental infrastructure, renewal of older infrastructure and recent advances in sensor systems and computational capacity but will also require a sustained and organized interdisciplinary approach. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The past decade has seen an explosive increase in the number of peer reviewed papers reporting new scientific findings in geomorphology (including fans, channels, floodplains and landscape evolution), geologic mapping, tectonics and faulting, coastal processes, lava flows, hydrology (especially snow and runoff routing), glaciers and geo-archaeology. A common genesis of such findings is often newly available decimeter resolution 'bare Earth' geodetic images, derived from airborne laser swath mapping, a.k.a. airborne LiDAR, observations. In this paper we trace nearly a half century of advances in geodetic science made possible by space age technology, such as the invention of short-pulse-length high-pulse-rate lasers, solid state inertial measurement units, chip-based high speed electronics and the GPS satellite navigation system, that today make it possible to map hundreds of square kilometers of terrain in hours, even in areas covered with dense vegetation or shallow water. To illustrate the impact of the LiDAR observations we present examples of geodetic images that are not only stunning to the eye, but help researchers to develop quantitative models explaining how terrain evolved to its present form, and how it will likely change with time. Airborne LiDAR technology continues to develop quickly, promising ever more scientific discoveries in the years ahead. [ABSTRACT FROM AUTHOR]

Many models of river meander migration rely upon a simple formalism, whereby the eroding bank is cut back at a rate that is dictated by the flow, and the depositing bank then migrates passively in response, so as to maintain a constant bankfull channel width. Here a new model is presented, in which separate relations are developed for the migration of the eroding bank and the depositing bank. It is assumed that the eroding bank consists of a layer of fine-grained sediment that is cohesive and/or densely riddled with roots, underlain by a purely noncohesive layer of sand and/or gravel. Following erosion of the noncohesive layer, the cohesive layer fails in the form of slump blocks, which armor the noncohesive layer and thereby moderate the erosion rate. If the slump block material breaks down or is fluvially entrained, the protection it provides for the noncohesive layer diminishes and bank erosion is renewed. Renewed bank erosion, however, rejuvenates slump block armoring. At the depositing bank, it is assumed that all the sediment delivered to the edge of vegetation due to the transverse component of sediment transport is captured by encroaching vegetation, which is not removed by successive floods. Separate equations describing the migration of the eroding and depositing banks are tied to a standard morphodynamic formulation for the evolution of the flow and bed in the central region of the channel. In this model, the river evolves toward maintenance of roughly constant bankfull width as it migrates only to the extent that the eroding bank and depositing bank 'talk' to each other via the medium of the morphodynamics of the channel center region. The model allows for both (a) migration for which erosion widens the channel, forcing deposition at the opposite bank, and (b) migration for which deposition narrows the channel forcing erosion at the opposite bank. Copyright © 2010 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Additions of sand to gravel beds greatly increase the mobility and flux of gravel. However, it is not known how additions of finer gravel to coarser gravel beds will affect the mobility of bed material. Here we examine the effect of fine gravel pulses on gravel bed material transport and near-bed flow dynamics in a series of flume experiments. Bed material refers exclusively to sediment in the channel prior to the pulse introduction. The observations indicate that fine sediment pulses tend to migrate downstream in low-amplitude waves. As the waves pass over the gravel bed, the interstitial pockets in the bed material surface fill and coarse gravel particles are entrained. This increases bed material transport rates and causes a distinct shift from a selective mobility transport regime where particles coarser than the bed material median (8 mm) make up <30% of the load to an equal mobility transport regime where bed materials coarser than 8 mm and finer than 8 mm are transported in equal proportions. The only possible source for this coarser bed load material is the sediment bed, suggesting that portions of the coarse surface layer are being mobilized. Observations of near-bed velocity and turbulence suggest that fine gravel pulses cause fluid acceleration in the near-bed region associated with a reduction in the level of turbulence produced at the sediment boundary. This accelerated fluid at the bed increases drag exerted on coarse surface layer particles, promoting their mobilization. Our findings suggest that, in general, finer bed sediment (not just sand) can mobilize coarser sediment and that expressions for the influence of sand on bed mobility need to be generalized on the basis of grain ratios. [ABSTRACT FROM AUTHOR]

Steep, rough channels occupy a large fraction of the total channel length in mountainous regions. Most sediment mobilized on hillslopes must pass through these streams before reaching lower-gradient channels. Steep channels have wide grain size distributions that are composed of finer, more mobile sediment and large, rarely mobile grains. The large grains can bear a significant portion of the total shear stress and thereby reduce the stress available to move the finer sediment. Conventional bed load transport equations often overpredict the sediment flux in steep channels by several orders of magnitude. We hypothesize that sediment transport equations overpredict the sediment flux because they do not (1) account for the stress borne by rarely mobile grains, (2) differentiate between highly and rarely mobile sediment, and (3) account for the limited availability of mobile sediment. Here we modify a conventional bed load transport equation to include these three effects. We use measurements of the flow, bed properties, and sediment flux in a small, steep flume to test this equation. We supply gravel at a constant rate through fields of regularly spaced immobile spheres and measure the bed coverage by gravel and sphere protrusion (the percent of the sphere that protrudes above the gravel deposit). For a given sphere spacing, the proportion of the bed covered by gravel increases and the sphere protrusion decreases with greater sediment supply. Thus bed coverage and immobile grain protrusion may serve as proxies for sediment availability in steep, rough streams. Unlike most transport equations that we tested, our modified bed load equation predicts sediment fluxes to within an order of magnitude of the measured values. Our results demonstrate that accurately predicting bed load transport in steep, rough streams may require accounting for the effects of local sediment availability (coverage by mobile sediment) and drag due to rarely mobile particles. [ABSTRACT FROM AUTHOR]

The sculpture of valleys by flowing water is widely recognized, and simplified models of incision by this process (e.g., the stream power law) are the basis for most recent landscape evolution models. Under steady state conditions a stream power law predicts that channel slope varies as an inverse power law of drainage area. Using both contour maps and laser altimetry, we find that this inverse power law rarely extends to slopes greater than ∼0.03 to 0.10, values below which debris flows rarely travel. Instead, with decreasing drainage area the rate of increase in slope declines, leading to a curved relationship on a log-log plot of slope against drainage area. Fieldwork in the western United States and Taiwan indicates that debris flow incision of bedrock valley floors tends to terminate upstream of where strath terraces begin and where area-slope data follow fluvial power laws. These observations lead us to propose that the steeper portions of unglaciated valley networks of landscapes steep enough to produce mass failures are predominately cut by debris flows, whose topographic signature is an area-slope plot that curves in log-log space. This matters greatly as valleys with curved area-slope plots are both extensive by length (>80% of large steepland basins) and comprise large fractions of main stem valley relief (25-100%). As a consequence, valleys carved by debris flows, not rivers, bound most hillslopes in unglaciated steeplands. Debris flow scour of these valleys appears to limit the height of some mountains to substantially lower elevations than river incision laws would predict, an effect absent in current landscape evolution models. We anticipate that an understanding of debris flow incision, for which we currently lack even an empirical expression, would substantially change model results and inferences drawn about linkages between landscape morphology and tectonics, climate, and geology. [ABSTRACT FROM AUTHOR]

Decades of quantitative measurement indicate that roots can mechanically reinforce shallow soils in forested landscapes. Forests, however, have variations in vegetation species and age which can dominate the local stability of landslide-initiation sites. To assess the influence of this variability on root cohesion we examined scarps of landslides triggered during large storms in February and November of 1996 in the Oregon Coast Range and hand-dug soil pits on stable ground. At 41 sites we estimated the cohesive reinforcement to soil due to roots by determining the tensile strength, species, depth, orientation, relative health, and the density of roots 1 mm in diameter within a measured soil area. We found that median lateral root cohesion ranges from 6.8–23.2 kPa in industrial forests with significant understory and deciduous vegetation to 25.6–94.3 kPa in natural forests dominated by coniferous vegetation. Lateral root cohesion in clearcuts is uniformly 10 kPa. Some 100-year-old industrial forests have species compositions, lateral root cohesion, and root diameters that more closely resemble 10-year-old clearcuts than natural forests. As such, the influence of root cohesion variability on landslide susceptibility cannot be determined solely from broad age classifications or extrapolated from the presence of one species of vegetation. Furthermore, the anthropogenic disturbance legacy modifies root cohesion for at least a century and should be considered when comparing contemporary landslide rates from industrial forests with geologic background rates.Key words: root strength, cohesion, landslide, debris flow, land use, anthropogenic disturbance.Des décades de mesures quantitatives indiquent que les racines peuvent renforcer mécaniquement les sols de surface dans des paysages boisés. Cependant, les forêts ont des variations d'espèces et d'âge qui peuvent dominer la stabilité locale des sites d'initiation de glissements. Pour évaluer l'influence de cette variabilité sur la cohésion due aux racines, on a examiné les escarpements des glissements déclenchés durant de gros orages en février et novembre 1996 dans le Oregon Coast range et dans des fosses creusées à la main dans le terrain stable. Sur 41 sites, on a estimé le renforcement du sol dû à la cohésion fournie par les racines en déterminant la résistance en traction, les espèces, la profondeur, l'orientation, la santé relative, et la densité des racines 1 mm de diamètre à l'intérieur d'une surface mesurée de sol. On a trouvé que la cohésion médiane latérale variait de 6,8 – 23,2 kPa dans les forêts industrielles en revégétation caduque significative, à 25,6 – 94,3 kPa dans les forêts naturelles dominées par une végétation de conifères. La cohésion latérale des racines dans des coupes nettes est uniformément 10 kPa. Des forêts industrielles vieilles de 100 ans ont des compositions d'espèces, une cohésion latérale de racines, et des diamètres de racines qui ressemblent plus à des coupes à blanc de 10 années qu'à des forêts naturelles. Comme telle, l'influence de la variabilité de la cohésion due aux racines sur la susceptibilité au glissement ne peut pas être déterminée seulement sur la base des classifications générales d'âge ou extrapolée en partant de la présence d'une espèce de végétation. De plus, l'héritage des remaniements anthropogéniques modifie la cohésion due aux racines pour au moins un siècle et devrait être pris en considération lorsque l'on compare la fréquence des glissements contemporains dans les forêts industrielles aux fréquences des glissements d'âge géologique.Mots clés : résistance des racines, cohésion, glissement, coulée de débris, utilisation des terres, remaniement anthropogénique.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

The use of cosmogenic isotopes to determine surface exposure ages has grown rapidly in recent years. The extent to which cosmogenic nuclides can distinguish between mechanistic hypotheses of landscape evolution is an important issue in geomorphology. We present a case study to determine whether surface exposure dating techniques can elucidatethe role knickpoint propagation plays in longitudinal profile evolution. Cosmogenically produced{sup}10{end}Be, {sup}26{end}Al, {sup}36{end}Cl, {sup}3{end}He, and {sup}21{end}Ne were measured in olivines collected from 5.2 Ma basalt flows on Kauai, Hawaii. Several obstacles had to be overcome prior to the measurement of in situ-produced radionuclides, including removal of meteoric lobe from the olivine grains.Discrepancies between the radionuclide and noble gas data may suggest limits for exposure dating. Approximate surface exposure ages calculated from the nuclide concentrations indicate that large boulders may remain in the Hawaiian valley below the knickpoint for hundreds of thousands of years. The ages of samples collected above the knickpoint are consistent with estimates of erosion based on the preservation of palaeosurfaces. Although the exposure ages can neither confirm norreject the knickpoint hypothesis, boulder ages downstream of the knickpoint are consistent with a wave of incision passing upvalley. The long residence time off the coarse material in the valley bottom further suggests that knickpomt propagation beneath a boulder pile is necessary for incision of the bedrock underlying the boulders to occur. [ABSTRACT FROM AUTHOR]