This paper presents a review of recent progress on the theory of orographic precipitation and a discussion of the role of preexisting atmospheric disturbances, especially their strong water vapor fluxes. I also introduce the basic elements of stable moist airflow dynamics and cloud physics, and a new linear theory of orographic precipitation. The theory is tested against two types of data: a single event of Alpine precipitation and the annual climatology of the Oregon coastal ranges. Different methods are used to determine the free “cloud-delay” parameters in the theory, including a statistical analysis of data from conventional rain gauges and isotope analysis of stream samples. The surprising threshold behavior of nonlinear accretion-dominated cloud physics is displayed. Finally, I consider the impact of scale-dependent precipitation patterns on erosion and terrain evolution.

The extent to which orography may be a product of climate-erosion interactions is largely unknown. One grand challenge is to quantify the precipitation regimes of mountainous regions at the spatial and temporal scales relevant for investigating the interplay of erosion and tectonics in active orogens. In this paper, our objective is to synthesize recent research integrating numerical model simulations, satellite data, and surface observations in the Himalaya to elucidate the role of weather and climate in mountain evolution. We focus on the seasonal and interannual space-time variability of precipitation in the Great Himalayas by studying two preeminent storm regimes in detail—monsoon onsets and depressions in general, and wintertime Western Disturbances (cold season events). High-resolution simulations of heavy precipitation storms for two monsoon onset conditions (1999 and 2001) and one wintertime storm (2000) are used to illustrate the complex patterns of interaction between the mountains and the atmosphere, and to show how these affect the spatial distribution of precipitation. Along with observations from an existing ground-based network, these simulations provide unique insights into the space-time features of seasonal and inter-annual variability of precipitation. Our analysis indicates that the trajectory of monsoon storms during onset events exerts a strong control on the precipitation amounts and rainfall penetration into the rain shadow. Spatial variability of subsequent storm tracks in any given year helps explain the interannual variability of monsoon precipitation. Both observational data and our simulations define striking spatial variability in precipitation on upwind and downwind fianks of ridges that project into obliquely impinging storms. Specifically, as southeasterly monsoon winds encounter north-south oriented ridges, forced lifting of moist air enhances precipitation on the upwind fianks, whereas less precipitation occurs on downwind fianks. This variability is observed at spatial scales as short as ∼10 km—a distance equivalent to the spacing of major ridge crests. Because infrequent, singular storm events appear to control the mass input to glaciers, and may determine the frequency and spatial distribution of landslides, these findings provide physically based insight into decoupling high-frequency (seasonal to interannual time scales) from low-frequency (multidecadal to centennial and longer time scales) signals in the interpretation of climate and erosion records in the Himalayas. Furthermore, this research suggests that integrative studies aimed at unraveling the role of climate in landscape evolution must include consideration of storm frequency and intensity along with spatial variability at scales consistent with regional climate forcing.

Spatial variability in precipitation has received little attention in the study of connections between climate, erosion, and tectonics. However, long-term precipitation patterns show large variations over spatial scales of ∼10 km and are strongly controlled by topography. We use precipitation rate estimates from Tropical Rainfall Measuring Mission (TRMM) satellite radar data to approximate annual precipitation over the Himalaya at a spatial resolution of 10 km. The resulting precipitation pattern shows gradients across the range, from east to west along the range, and fivefold differences between major valleys and their adjacent ridges. Basin-wide average precipitation estimates correlate well with available measured mean runoff for Himalayan rivers. Estimated errors of 15%–50% in TRMM-derived annual precipitation are much smaller than the spatial variability in predicted totals across the study area. A simple model of orographic precipitation predicts a positive relationship between precipitation and two topographically derived factors: the saturation vapor pressure at the surface and this pressure times the slope. This model captures significant features of the pattern of precipitation, including the gradient across the range and the ridge-valley difference, but fails to predict the east-west gradient and the highest totals. Model results indicate that the spatial pattern of precipitation is strongly related to topography and therefore must co-evolve with the topography, and suggest that our model may be useful for investigation of the relationships among the coupled climate-erosion-tectonic system.

Empirical observations from fluvial systems across the globe reveal a consistent power-law scaling between channel slope and contributing drainage area. Theoretical arguments for both detachment- and transport-limited erosion regimes suggest that rock uplift rate should exert first-order control on this scaling. Here we describe in detail a method for exploiting this relationship, in which topographic indices of longitudinal profile shape and character are derived from digital topographic data. The stream profile data can then be used to delineate breaks in scaling that may be associated with tectonic boundaries. The description of the method is followed by three case studies from varied tectonic settings. The case studies illustrate the power of stream profile analysis in delineating spatial patterns of, and in some cases, temporal changes in, rock uplift rate. Owing to an incomplete understanding of river response to rock uplift, the method remains primarily a qualitative tool for neotectonic investigations; we conclude with a discussion of research needs that must be met before we can extract quantitative information about tectonics directly from topography.

The Western Escarpment of the Andes at 18.30°S (Arica area, northern Chile) is a classical example for a transient state in landscape evolution. This part of the Andes is characterized by the presence of >10,000 km 2 plains that formed between the Miocene and the present, and >1500 m deeply incised valleys. Although processes in these valleys scale the rates of landscape evolution, determinations of ages of incision, and more importantly, interpretations of possible controls on valley formation have been controversial. This paper uses morphometric data and observations, stratigraphic information, and estimates of sediment yields for the time interval between ca. 7.5 Ma and present to illustrate that the formation of these valleys was driven by two probably unrelated components. The first component is a phase of base-level lowering with magnitudes of∼300–500 m in the Coastal Cordillera. This period of base-level change in the Arica area, that started at ca. 7.5 Ma according to stratigraphic data, caused the trunk streams to dissect headward into the plains. The headward erosion interpretation is based on the presence of well-defined knickzones in stream profiles and the decrease in valley widths from the coast toward these knickzones. The second component is a change in paleoclimate. This interpretation is based on (1) the increase in the size of the largest alluvial boulders (from dm to m scale) with distal sources during the last 7.5 m.y., and (2) the calculated increase in minimum fluvial incision rates of ∼0.2 mm/yr between ca. 7.5 Ma and 3 Ma to ∼0.3 mm/yr subsequently. These trends suggest an increase in effective water discharge for systems sourced in the Western Cordillera (distal source). During the same time, however, valleys with headwaters in the coastal region (local source) lack any evidence of fluvial incision. This implies that the Coastal Cordillera became hyperarid sometime after 7.5 Ma. Furthermore, between 7.5 Ma and present, the sediment yields have been consistently higher in the catchments with distal sources (∼15 m/m.y.) than in the headwaters of rivers with local sources (<7 m/m.y.). The positive correlation between sediment yields and the altitude of the headwaters (distal versus local sources) seems to reflect the effect of orographic precipitation on surface erosion. It appears that base-level change in the coastal region, in combination with an increase in the orographic effect of precipitation, has controlled the topographic evolution of the northern Chilean Andes.

The Coastal Cordillera of northern Chile is the only subaerial part of the South American continental crust in direct contact with the subducted Nazca plate. Deformation of the cordillera can be directly related to plate coupling at the subduction zone. Knowledge of uplift rate variation along the coast is however limited, mainly due to difficulties in dating and correlating the discontinuous remnants of marine terraces. This paper uses geomorphology and the stream gradient index (SGI) to examine 80 drainages along 200 km of coastline between 21°S and 23°S. Selected catchments are located on similar geology (basaltic andesite and granodiorite) and within the same coastal climate zone. The southernmost part of the transect overlaps with an area of known uplift rates based on dated marine terraces. Characteristics of the SGI index, combined with the geomorphology, enable the coast to be divided into two sectors. Extrapolation of the SGI values from an area of known (sector 1) to unknown uplift rates (sector 2) enables an estimate of relative uplift rates for sector 2. Sector 1 has known uplift rates of 240–384 mm k.y. −1 . The SGI results suggest sector 2 has overall uplift rates which may be 2–3 times greater than sector 1. The large-scale (70–100 km) differences in apparent differential uplift reflected by the SGI along the Coastal Cordillera are tentatively attributed to aseismic ridge subduction. Smaller-scale (30 km) variations in the SGI associated with the major regional drainages in sector 2 suggest that the latter drainages may be focused in structural lows (with low SGI values) controlled by NE-SW– to E-W–trending reverse faults, although further work is required to clarify this.

Several process-based models of river incision have been proposed in recent years that attempt to describe fluvial landform development. Although some field tests have been performed, more data are required to test the ability of these models to predict the observed evolution of fluvial landforms. We have investigated several tens of rivers located in the French western Alps that flow across folded sedimentary rocks with strongly contrasting rock strengths. These rivers record significant variations in some of the parameters controlling river incision, notably bedrock lithology, stream power, incision rate, and sediment flux, potentially allowing discrimination between existing models. Variations in incision rates are driven by variations in the amount of disequilibrium introduced in the river profiles during the last glaciation. We use diagnostic indices to investigate transport- and detachment-limited conditions, which include the channel morphology, the occurrence of lithogenic knickpoints, the continuity of alluvial and bedrock reaches, and the slope-area scaling of the river long profile. We observe transitions from detachment-limited to transport-limited conditions with increasing discharge/drainage area and decreasing incision rate. Bedrock strength influences the location of the transition predictably. The formation of transport-limited rivers coincides with the development of a valley flat wider than the active channel, which accommodates variations in bedrock strength, stream power, and incision rate along the transport-limited reaches. We propose and calibrate a model for the development of valley flats along transport-limited rivers and explore some properties of landscape development in mountain ranges controlled by transport-limited rivers.

We use a numerical model to investigate disequilibrium conditions in detachment-limited river networks. Erosion rates are modeled using two different equations that include sediment flux as a variable for determining incision rates into bedrock. A number of numerical simulations are performed to explore erosion patterns, channel profile shape, and network concavity after an increase in uplift rate across the network. In the case where an increase in sediment flux (relative to carrying capacity) is considered only to decrease incision rates, the main channel has a two-part response to a faster uplift rate; initially a knickpoint steepens channel slopes locally, but at later times channel slopes rise throughout the network. However, in the case where an increase in sediment flux can both enhance and suppress incision rates, the transient network response can be much more dynamic; channel slopes (and also elevations) can both rise and fall, all in response to a single increase in uplift rate. The response varies depending on the magnitude of change in uplift rate and the initial ratio of sediment flux to sediment carrying capacity. In all examples, the lower parts of the network respond quickly to an increase in uplift rates by increasing channel slopes, while the response of erosion rates in the upper parts of the network occurs later. As a result, the change in sediment flux delivered to higher order channels lags the initial changes in the slope of these channels and causes a complex response in erosion rates. These findings highlight that erosion rates at any point in the network respond to changes both downstream and upstream, and therefore variables such as sediment flux that integrate the upstream response can play an important role is shaping the transient morphology of river networks.

Understanding and quantifying fluvial transport and bedrock abrasion processes have become major concerns in modeling landform response to tectonic and climatic forcing. Recent theoretical and experimental investigations have in particular stressed the importance of sediment supply and size in controlling bedrock incision rate. Many studies on the downstream evolution of pebble size have focused on unraveling the respective roles of selective sorting and abrasion, without paying much attention to sediment sources. In order to track sediment supply and characteristics from source to sink in an active tectonic setting, where long-term selective deposition can be excluded, we systematically measured sediment size and lithology on gravel bars along the Marsyandi River and its tributaries (Himalayas of central Nepal), and also in sediment source material from hillslopes (landslides, moraines, terrace deposits). The downstream evolution in lithological distribution is found to be in close agreement with common views on pebble abrasion and present views on denudation in the range: (1) pebbles from the more rapidly uplifted and eroded Higher Himalayan gneissic units are over-represented, due to their major contribution to sediment influx, (2) easily erodible lithologies such as schists, sandstones, and limestone are under-represented relative to resistant rock types like quartzite. More surprisingly, we observe a general downstream coarsening of gravel bar material along the middle and lower Marsyandi River, whereas downstream sediment fining is typical of most river systems. A simple integrative model that tracks pebbles from hillslope to the main stem of the river and includes abrasion coefficients for the different Himalayan lithologies and size distribution of hillslopes sediment supplies accounts for both changing lithologic proportion along the Marsyandi and for the downstream coarsening of gravel bar material. This coarsening mainly results from differences in sediment sources along the Marsyandi Valley, in particular from differences in size distributions of landslide and moraine material. However, the median pebble size of subsurface material in gravel bars is coarser than median size of the blocky material in the source. The choice of the measurement methods and their potential bias are discussed but cannot explain this surprising feature displayed by our measurements. We suspect that due to sediment transport modalities in active tectonic settings, the subpavement grain-size distribution on gravel bars is not representative of the average bed-load size distribution. Consequently, pebble abrasion is more easily demonstrated by description of pebble lithology than by the downstream evolution of pebble size. Our study also shows, in contrast with previous studies, that experimentally derived abrasion coefficients can account for the downstream evolution of pebbles without calling for additional fining processes. We conclude that the eroded lithology and hillslope sediment source exert a major influence on the downstream evolution of sediment characteristics, on bedload ratio, and probably on bedrock erosion efficiency. These conclusions have important implications in terms of river profile evolution, landscape denudation, internal erosion coupling, and the response of the fluvial network to glacial-interglacial fluctuations.

Passive margin escarpments are extensively studied around the world, and understanding their evolution continues to present one of the more compelling interdisciplinary challenges tackled by earth scientists today. Escarpments reflect the morphotectonic development of passive margins and can separate regions with different climatic histories, but the inferred rapid rates of escarpment retreat have been at odds with actual measurements of land surface denudation. In this paper we present results from extensive cosmogenic 10 Be and 26 Al analyses across the escarpment of southeastern Australia to quantify the erosional processes evolving the highland, lowland, and scarp face landscapes. We document new relationships between soil production rates and soil thicknesses for the highland and lowland landscapes and compare these soil production functions with those published in our earlier studies from the highlands and at the base of the escarpment. Both new functions define exponential declines of soil production rates with increasing soil depths, with inferred intercepts of 65 and 42 m/m.y. for the highland and lowland sites, respectively, and slopes of –0.02. Exposed bedrock at both of the new sites erodes more slowly than the maximum soil production rates, at 22 ± 3 and 9 ± 2 m/m.y., respectively, thus suggesting a “humped” soil production function. We suggest that instead of a humped function, lithologic variations set the emergence of bedrock, which evolves into the tors that are found extensively across the highlands and at the crest of the escarpment by eroding more slowly than the surrounding soil-mantled landscape. Compared to soil production rates from previous work using 10 Be and 26 Al measurements from two different sites, these results show remarkable agreement and specifically quantify a soil production function for the region where soil production rates decline exponentially with increasing soil thickness, with an intercept of 53 m/m.y. and a slope of –0.02. Erosion rates determined from 10 Be concentrations from outcropping tors, bedrock, and saprolite from a main spur ridge perpendicular to the escarpment, and sediments from first- and zero-order catchments draining the main ridges, show a clear linear decline with elevation, from ∼35 m/m.y. near the escarpment base to ∼3 m/m.y. at the escarpment crest. This order of magnitude difference in erosion rates may be due to increases in stream incision with distance downslope on the escarpment, or to decreases in precipitation with elevation, neither of which we quantify here. The rates do agree, in general, with our soil production functions, suggesting that the biogenic processes actively eroding soil-mantled landscapes are shaping the evolution of the escarpment despite our observations of block fall and debris-flow processes across the steep regions near the scarp crest. Our results support recent results from studies using low-temperature thermochronology, which suggest that the escarpment is relatively stable after having retreated rapidly immediately following rifting. Differences between our rates of surface erosion caused by processes active today and the scarp retreat rates needed to place the escarpment in its present position need to be explained by future work to untangle the mysteries of escarpment evolution.

Numerical models are used to help constrain empirical parameterizations of soil production and transport mechanisms on soil-mantled hillslopes. The neighbourhood algorithm is used to invert soil thickness versus surface curvature data to provide not only more rigorous estimates of model parameter values but determine which of the model parameters are constrained by cosmogenic exposure ages ( Heimsath et al., 2000 ). We show that linear and depth-dependent creep constants can be constrained by simple geomorphometric measurements, such as the distribution of soil thickness on the landform and its relationship to surface curvature. We also show that this unique data set cannot be used on its own to constrain the parameterization of overland flow, another transport mechanism that is thought to play an important role on soil-mantled hillslopes, or to determine if a soil distribution has reached local steady state. We also demonstrate that, to explain the data, soil production must be a function of soil thickness. These conclusions have important implications for our understanding of landscape evolution on medium to long time scales.

Scaled two-dimensional sandbox experiments are used to investigate the effect of (1) the location of erosion with respect to the convergence geometry and (2) two erosion modes, distributed or focused, which are thought to represent end members, on the distribution and propagation of deformation within bivergent orogens. This study applies to the brittle part of medium-sized, natural orogens. Particle image velocimetry (PIV) is used to analyze the experiments with respect to surface uplift, thrust displacement, and finite strain. The experiments suggest that deformation responds immediately to erosion. Retro-wedge erosion amplifies the displacement of the basally accreted material, whereas pro-wedge erosion accelerates and additionally redirects the particle flux of the frontally accreted material. Pro- and retro-wedge erosion retards the propagation of deformation within the pro-wedge. This effect is stronger for pro-wedge erosion. Retro-wedge erosion amplifies vertical growth and leads to strain accumulation along the retro shear-zone and the mid-level detachment. Thus, during retro-wedge erosion, cause (erosion) and response (deformation) are significantly offset in space. Since pro-wedge erosion evokes a complete decoupling of the retro-wedge from the pro-wedge, cause and response are spatially more closely related. With respect to the erosion mode, we found that a more focused erosion leads to a more focused strain accumulation. Similarly, focused erosion applied at the pro-wedge deformation front prohibits accumulation of out-of-sequence displacement. In contrast, distributed pro-wedge erosion amplifies out-of-sequence displacement. Thus, a forward breaking or “piggyback” sequence of thrusting might involve considerable out-of-sequence displacement, which is strongly controlled by the erosion mode.

The theories of critical orogenic wedges and fluvial erosion are combined to explore the interactions between tectonics, erosion, and climate. A model framework is developed which allows the derivation of an exact analytical scaling relationship for how orogen width, height, and rock uplift rate vary as a function of accretionary flux and precipitation rate. Compared to a model with prescribed uplift rate, incorporating the tectonic response introduces a powerful negative feedback on the orogen, which strongly damps the system's equilibrium response to changes in forcing. Furthermore, for the most commonly assumed forms of the fluvial erosion law, the orogen is more sensitive to changes in the accretionary flux than in the precipitation rate. And while increases in accretionary flux and precipitation rate both cause an increase in exhumation rate, they have opposite tendencies on the orogen relief. Further analysis shows that the pattern of rock uplift does not affect the scaling relationship and that it is only weakly dependent on the hillslope condition.