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The depth of the Earth’s soil cover is controlled by the competing processes of soil production and erosion. Estimates of the rates of these processes over rugged topography suggest that soil-production rates will increase over surfaces that are subject to rapid erosion. The extent and persistence of the Earth’s soil cover depends on the long-term balance between soil production and erosion. Higher soil production rates under thinner soils provide a critical stabilizing feedback mechanism 1 , 2 , 3 , and climate- and lithology-controlled soil production is thought to set the upper limit for steady-state hillslope erosion 4 . In this framework, erosion rates exceeding the maximum soil production rate can be due only to bedrock mass wasting 5 . However, observation of pervasive, if patchy, soil cover in areas of rugged topography and rapid erosion indicates additional stabilizing mechanisms. Here we present 10 Be-derived estimates of soil-production and detrital erosion rates that show that soil production rates increase with increasing catchment-averaged erosion rates, a feedback that enhances soil-cover persistence. We show that a process transition to landslide-dominated erosion in steeper, more rapidly eroding catchments results in thinner, patchier soils and rockier topography, but find that there is no sudden transition to bedrock landscapes. Instead, using our global data compilation, we suggest that soil production may increase in frequency and magnitude to keep up with increasing erosion rates. We therefore conclude that existing models 6 , 7 , 8 greatly exaggerate changes in critical-zone processes in response to tectonic uplift.

The connection between topography and erosion rate is central to understanding landscape evolution and sediment hazards. However, investigation of this relationship in steep landscapes has been limited due to expectations of: (a) decoupling between erosion rate and “threshold” hillslope morphology; and (b) bias in detrital cosmogenic nuclide erosion rates due to deep‐seated landslides. Here we compile 120 new and published 10Be erosion rates from catchments in the San Gabriel Mountains, California, and show that hillslope morphology and erosion rate are coupled for slopes approaching 50° due to progressive exposure of bare bedrock with increasing erosion rate. We find no evidence for drainage area dependence in 10Be erosion rates in catchments as small as 0.09 km2, and we show that landslide deposits influence erosion rate estimates mainly by adding scatter. Our results highlight the potential and importance of sampling small catchments to better understand steep hillslope processes. Plain Language Summary In general, erosion rates increase as landscapes steepen. But where landslides are common, this relationship is thought to break down as hillslopes approach their angle of repose. The main tracer for measuring erosion rates, 10Be in sediment, can also be affected by landslides, and models predict it is unreliable for small watersheds in steep landscapes. Here, we compile an extensive data set of 10Be erosion rates from the San Gabriel Mountains of California. We show that slope and erosion rate are coupled well above the soil angle of repose due to systematic exposure of bedrock cliffs, supporting a new conceptual model for steep landscapes. The presence of landslides adds scatter but does not bias 10Be erosion rates, which yield robust results even in small, steep watersheds that have previously been avoided. Key Points Progressive exposure of bare rock on steeper slopes leads to correlation of 10Be erosion rate and mean hillslope angle up to 47° Deep seated landslide deposits add scatter, but do not systematically bias 10Be erosion rate estimates in the San Gabriel Mountains No evidence for drainage area dependence of 10Be erosion rates in upland catchments

In the future, Earth will be warmer, precipitation events will be more extreme, global mean sea level will rise, and many arid and semiarid regions will be drier. Human modifications of landscapes will also occur at an accelerated rate as developed areas increase in size and population density. We now have gridded global forecasts, being continually improved, of the climatic and land use changes (C&LUC) that are likely to occur in the coming decades. However, besides a few exceptions, consensus forecasts do not exist for how these C&LUC will likely impact Earth‐surface processes and hazards. In some cases, we have the tools to forecast the geomorphic responses to likely future C&LUC. Fully exploiting these models and utilizing these tools will require close collaboration among Earth‐surface scientists and Earth‐system modelers. This paper assesses the state‐of‐the‐art tools and data that are being used or could be used to forecast changes in the state of Earth's surface as a result of likely future C&LUC. We also propose strategies for filling key knowledge gaps, emphasizing where additional basic research and/or collaboration across disciplines are necessary. The main body of the paper addresses cross‐cutting issues, including the importance of nonlinear/threshold‐dominated interactions among topography, vegetation, and sediment transport, as well as the importance of alternate stable states and extreme, rare events for understanding and forecasting Earth‐surface response to C&LUC. Five supplements delve into different scales or process zones (global‐scale assessments and fluvial, aeolian, glacial/periglacial, and coastal process zones) in detail. Key Points We review models and data useful for forecasting Earth surface changes We identify key knowledge gaps required to forecast Earth surface changes We strategize how geomorphologists and Earth‐systems modelers can collaborate

It has been assumed for over 100 years that bedrock disintegration into erodable soil declines with increasing soil mantle thickness. Heimsath et al apply two independent field methods for determining soil production rates to hillslopes in northern California.

Steep mountain regions can weather faster and produce soil more quickly than previously thought. [Also see Report by Larsen et al. ] Rocky mountain ranges may appear static but are constantly in motion. Tectonic forces push the mountains up, while physical and chemical processes break rocks down to sediment that is transported to river plains and ultimately to the sea. This cycle is thought to regulate global climate over million-year time scales ( 1 ) while also responding to climate forcing itself ( 2 ). It remains unclear whether mountain uplift drives climate change, or whether climatic cooling drives uplift by causing faster erosion ( 3 ). On page 637 of this issue, Larsen et al. ( 4 ) provide data that help to quantify these controls on mountain building, reporting faster sediment production rates and higher chemical weathering rates than previously measured. Their results also provide key insights into soil sustainability over shorter time scales ( 5 ).

Differences in the timing of glacial advances, which are commonly attributed to climatic changes, can be due to variations in valley topography. Cosmogenic 10Be dates from 24 glacial moraine boulders in 5 valleys define two age populations, late‐glacial and early Holocene. Moraine ages correlate with paleoglacier valley hypsometries. Moraines in valleys with lower maximum altitudes date to the late‐glacial, whereas those in valleys with higher maximum altitudes are early Holocene. Two valleys with similar equilibrium‐line altitudes (ELAs), but contrasting ages, are <5 km apart and share the same aspect, such that spatial differences in climate can be excluded. A glacial mass‐balance cellular automata model of these two neighboring valleys predicts that change from a cooler‐drier to warmer‐wetter climate (as at the Holocene onset) would lead to the glacier in the higher altitude catchment advancing, while the lower one retreats or disappears, even though the ELA only shifted by ∼120 m. Key Points Glaciers with higher max source areas advance in wider range of climate regimes Common practice of assuming synchroneity for similar ELA depressions is flawed Largest glacial chronology available for central or western Nepalese Himalaya

The connection between topography and erosion rate is central to understanding landscape evolution and sediment hazards. However, investigation of this relationship in steep landscapes has been limited due to expectations of: (a) decoupling between erosion rate and “threshold” hillslope morphology; and (b) bias in detrital cosmogenic nuclide erosion rates due to deep‐seated landslides. Here we compile 120 new and published 10 Be erosion rates from catchments in the San Gabriel Mountains, California, and show that hillslope morphology and erosion rate are coupled for slopes approaching 50° due to progressive exposure of bare bedrock with increasing erosion rate. We find no evidence for drainage area dependence in 10 Be erosion rates in catchments as small as 0.09 km 2 , and we show that landslide deposits influence erosion rate estimates mainly by adding scatter. Our results highlight the potential and importance of sampling small catchments to better understand steep hillslope processes. In general, erosion rates increase as landscapes steepen. But where landslides are common, this relationship is thought to break down as hillslopes approach their angle of repose. The main tracer for measuring erosion rates, 10 Be in sediment, can also be affected by landslides, and models predict it is unreliable for small watersheds in steep landscapes. Here, we compile an extensive data set of 10 Be erosion rates from the San Gabriel Mountains of California. We show that slope and erosion rate are coupled well above the soil angle of repose due to systematic exposure of bedrock cliffs, supporting a new conceptual model for steep landscapes. The presence of landslides adds scatter but does not bias 10 Be erosion rates, which yield robust results even in small, steep watersheds that have previously been avoided. Progressive exposure of bare rock on steeper slopes leads to correlation of 10 Be erosion rate and mean hillslope angle up to 47° Deep seated landslide deposits add scatter, but do not systematically bias 10 Be erosion rate estimates in the San Gabriel Mountains No evidence for drainage area dependence of 10 Be erosion rates in upland catchments

Quantifications of Earth surface topography are essential for modeling the connections between physical and chemical processes of erosion and the shape of the landscape. Enormous investments are made in developing and testing process-based landscape evolution models. These models may never be applied to real topography because of the difficulties in obtaining high-resolution (1-2 m) topographic data in the form of digital elevation models (DEMs). Here we present a simple methodology to extract the high-resolution three-dimensional topographic surface from photographs taken with a hand-held camera with no constraints imposed on the camera positions or field survey. This technique requires only the selection of corresponding points in three or more photographs. From these corresponding points the unknown camera positions and surface topography are simultaneously estimated. We compare results from surface reconstructions estimated from high-resolution survey data from field sites in the Oregon Coast Range and northern California to verify our technique. Our most rigorous test of the algorithms presented here is from the soil-mantled hillslopes of the Santa Cruz marine terrace sequence. Results from three unconstrained photographs yield an estimated surface, with errors on the order of 1 m, that compares well with high-resolution GPS survey data and can be used as an input DEM in process-based landscape evolution modeling.[PUBLICATION ABSTRACT]

For hillslopes undergoing “diffusive” soil transport, it is often assumed that the soil flux is proportional to the local land‐surface gradient, where the coefficient of proportionality is like a diffusion coefficient. Inasmuch as transport involves quasi‐random soil particle motions related to biomechanical mixing and similar dilational processes, a slope‐dependent relation arises from a balance between particle fluxes that tend to loft a soil and gravitational settling of particles into available pore space. A specialized form of the Fokker‐Planck equation adapted to such particle motions clarifies how the particle flux involves advective and diffusive parts. This in turn contributes to a kinematic description of the diffusion‐like coefficient. Ingredients of this coefficient include an active soil thickness, a characteristic particle size, the porosity in excess of a consolidated porosity, and the rate of particle activation as a function of depth. These last two ingredients, vertical porosity structure and activation rate, in effect characterize the magnitude and frequency of settling particle motions related to biological activity and thereby set the rate constant of the transport process. The significance of land‐surface slope is that it is a measure of the downslope component of slope‐normal lofting that is balanced by settling. Because the diffusion‐like coefficient contains the soil thickness, the analysis suggests that the soil flux is proportional to the “depth‐slope” product. The analysis is consistent with published profiles of soil creep displacement and with published estimates of soil flux obtained by downslope integration of soil production rates for hillslopes in California and Australia.