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.

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.

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.

In order to investigate if high elevations existed in the Rocky Mountains before the late Miocene, we examined oxygen isotope ratios of 63 Tertiary smectite samples as a proxy for the isotopic composition of precipitation. Of these samples, 51 were also analyzed for hydrogen isotope ratios. These smectites were formed as a result of the weathering of volcanic air-fall deposits that blanketed much of western North America during the Tertiary. Smectite-bearing ashfall samples were collected from Eocene, Oligocene, and Miocene deposits along a transect that extends from the western Great Plains to Yellowstone National Park at modern elevations from ∼900 to ∼2800 m. In general, oxygen and hydrogen δ values of smectite lie along a line parallel to the meteoric water line, which suggests that the isotopic composition of these ash-derived smectites records the meteoric water composition during its formation. There is little evidence for postdepositional exchange with basinal brine fluids, evaporative effects, or diagenesis of these smectites. The δ 18 O values of Oligocene and Miocene samples increase ∼6‰ linearly from sample sites located at the crest of the Rocky Mountains to sites in western Nebraska and South Dakota. These results mimic the distribution and values of calculated oxygen isotope ratios of theoretical modern smectite over this same geographic traverse of decreasing elevation. This result suggests modern atmospheric circulation patterns and that the resulting distribution of δ 18 O precipitation has persisted since the Oligocene. The δ 18 O values of Eocene samples increase ∼8‰ between the Yellowstone region and central Wyoming, a result that does not correlate with modern δ 18 O precipitation trends. Our Eocene results may be explained by climate conditions extant at that time, but tectonic modification in the region between 50 Ma and 37 Ma cannot be excluded as the cause of our results. Because the modern climate system requires interaction with and modification by high-elevation areas, our results suggest that the Rocky Mountains have been at high elevation since at least 50 Ma.

The character and distribution of cooling ages in modern river sediment provide useful constraints on rates and patterns of uplift and erosion within actively deforming mountain ranges. Such sediment effectively samples all locations within the catchment area, irrespective of remoteness. We evaluate how successfully detrital cooling ages may be used to constrain hinterland erosion rates by examining the modern catchment of the Marsyandi River in central Nepal. Over the 100–200-km-length scale of the catchment, laser fusion 40 Ar/ 39 Ar data for detrital muscovite collected from 12 separate sites illustrate the downstream development of a detrital cooling-age signal that is both systematic and representative of the contributing area. Comparisons of paired samples indicate that, at short spatial (tens of meters) and temporal (hundreds of years) scales, the detrital cooling-age signal is consistent. The distribution of bedrock cooling ages in a subcatchment and the resulting detrital signal at the catchment mouth can be modeled as a function of the erosion rate, relief, hypsometry, catchment area, and muscovite distribution. Given that independent constraints are available for most of these variables, the detrital age signal should be a robust indication of the spatially averaged erosion rate. In the Marsyandi, our model predicts erosion rate differences of approximately twofold, with higher rates (>2 mm/yr) along the southern topographic front of the Himalaya.

The geochemistry of river water, river sediments, and suspended matter in three mountainous watersheds in New Zealand and Taiwan is used to determine chemical erosion yields in regions of rapid tectonic uplift. Suspended matter from all three rivers is depleted in soluble alkali metals and alkaline earths compared to upper continental crust material and marine clays, reflecting the bedrocks' origin as marine sediments that had undergone previous weathering cycles prior to uplift and subjection to the current chemical weathering regime. The New Zealand rivers are depleted in Mg 2+ and enriched in Ca 2+ and Na + + K + compared to global average river water, but the Taiwan river is enriched only in Mg 2+ compared to global average. The Haast compared River, draining the Southern Alps of New Zealand, is depleted in Cl + SO 4 to the global average, but has higher alkalinity and slightly higher H 4 SiO 4 . The chemical weathering yields determined here compose only a small portion (1%–5%) of the total weathering in these systems, but are still among the highest chemical yields ever reported. Our new data, in comparison to previously determined physical erosion yields in these watersheds, show that physical erosion strongly enhances chemical erosion. This work demonstrates the importance of chemical erosion as a process denuding the landscape, especially in high-standing, tectonically active mountainous landscapes.

In many young orogens of the Mediterranean region, Quaternary tectonics and regional uplift are traditionally considered strictly correlated, but few data are actually available for precise calculations of uplift rates. Such quantitative studies are needed to define the relationships between local faulting and large-scale uplift, and to formulate correct hypotheses on the geodynamic scenario in which the regional raising occurred. In addition, data about burial depths of sediments or tectonic loadings suffered by sedimentary and low-grade metamorphic rocks are essential for comparisons with the uplift rates obtained in the same areas. Such comparisons between quite different data sources improve the comprehension and choice of the most reliable mechanism responsible for the regional uplift and exhumation. Uplift rates have been calculated for a large sector of the Lucanian Apennine (axial zone of the southern Italian Apennines), and also reviewed for the Calabrian arc (southern termination of the Italian peninsula), using both geomorphological observations (elevation values, ages, and arrangement of depositional and erosional land surfaces and other morphotectonic indicators) and stratigraphical and structural data (sea-level related facies, base levels, fault kinematics, and offset estimations). Such data have been compared with those derived from clay mineralogy of Mesozoic pelagic deposits (Lagonegro units), outcropping in the same sector of the chain, which give information on burial depths. The values of the Quaternary uplift rates of the southern Apennines axial zone vary from a minimum of 0.2 mm/yr near the town of Potenza to a maximum of ∼1.2–1.3 mm/yr in Agri high valley, a severely deformed Quaternary intermontane basin, still tectonically active, in the Pollino Mountains, a carbonate ridge with elevation up to 2200 m above sea level (asl); intermediate values (0.5–0.7 mm/yr) have been calculated for the other studied areas. The erosion rate from a key area of the Lucanian Apennine, obtained from both quantitative geomorphic analysis and missing volumes calculations on a catchment basin as wide as 150 km 2 , has been estimated at 0.2 mm/yr for the middle Pleistocene to Holocene time span. Since in the upper part of that chronological interval erosion and uplift rates match well, the axial-zone landscape could have reached a flux steady-state during the late Pleistocene. Using geomorphological features and late Pliocene to Pleistocene deposits involved in the genesis of erosional and depositional land surfaces, similar rates (≈0.6 mm/yr) have been obtained for a quite large time span (∼2 m.y.) in the Melandro basin and adjacent Maddalena Mountains. Therefore, during the last 2 m.y., the total uplift amount of the axial zone of the Lucanian Apennine is ∼1.2–1.3 km, with local peaks of 1.5 km. On the other hand, the Mesozoic pelagic units experienced tectonic loading of 4–5 km, as estimated by means of illite crystallinity (in the range 0.6–1.1 ▵°2[thetas]), percentage of illitic layers in illite/smectite mixed layers (60%–90%) and white mica polytypes (in the range of 15%–35%), and confirmed by other independent data. The Quaternary uplift and the related erosion rates of the southern Apennines are unquestionably due to extensional tectonics coupled with thermal/isostatic regional raising and, in minor extent, to strike-slip faulting acting in the earlier deformational stage of the south Apennines chain. The gap of several kilometers deriving from the comparison between uplift rates and tectonic loading values may be explained only with different exhumation modalities starting from late Miocene times. This age can be obtained assuming a fixed rate of 0.6 mm/yr, which represents the best long-term estimate for the axial zone of the chain. Going back in time using such a conservative rate to get a denudation value of ∼4–5 km, Tortonian age is reached. At that time, contractional tectonics were still active in this sector of the southern Apennines. Tectonic denudation may be a reliable explanation for the discrepancy between the different sources of data. Such phenomena led to the exhumation of the Mesozoic core of the chain (Lagonegro units), causing low-angle extension on its Tyrrhenian side by reactivation and inversion of older thrusts, and stacking on its eastern margin of Cretaceous to Miocene pelagic and flysch units (i.e., frontal imbricate fan units) by gravity megasliding.

The conventional assumption that erosion by ice sheets is pervasive and effective in landscape evolution is tested in northern Sweden using geomorphic mapping and cosmogenic nuclide analyses of formerly glaciated surfaces. The following evidence indicates that recent glaciations in this region have produced only slow and patchy landscape evolution: (1) Geomorphic mapping shows that at least 20% of the repeatedly glaciated study region in the northern Swedish mountains has landforms that are relict, i.e., clearly nonglacial in origin. (2) The contrast between cosmogenic apparent exposure ages from relict landforms in the northern Swedish mountains and from overlying glacial erratics and juxtaposed glacially eroded bedrock surfaces, which are consistent with last deglaciation, implies that the relict landforms have been preserved through multiple glacial cycles. (3) Apparent 10 Be and 26 Al exposure ages for tor summit bedrock surfaces in the northern Swedish lowlands reveal that these relict landforms have survived at least eleven exposure and ten burial events with little or no erosion over the past ∼1 m.y. (4) The northern Swedish lowland and mountains are primarily covered by glacial landforms. However geomorphic mapping suggests that even these landforms may have undergone limited erosion during the last glacial cycle. Cosmogenic 10 Be and 36 Cl data from what appear to be heavily scoured areas in one glacial corridor indicate erosion of only ∼2 ± 0.4 m of bedrock during the last glaciation. These results suggest that in some areas the overall modification produced by ice sheets may be more restricted than previously thought, or it has occurred preferentially during earlier Quaternary glacial periods.

Thirty to forty m.y. of post-Laramide degradation of the southern Rocky Mountains likely produced relatively low-relief topography within the crystalline cores of the ranges, and capped the adjacent sedimentary basins with easily eroded sediments. We focus on the modern, more dissected topography of these ranges, reflecting late Cenozoic evolution driven by fluvial and glacial exhumation, each of which affects different portions of the landscape in characteristic ways. Ongoing exhumation of the adjacent basins, in places by more than 1 km, is effectively lowering base level of streams draining the crystalline range cores. The streams have incised deep bedrock canyons that now cut the flanks of the range. Over the same time scales, glaciation of the headwaters of the major streams has modified the range crests. We utilize the topography of the northern Front Range of Colorado to explore the response of a Laramide range both to the exhumation of the adjacent basin and to glaciation in the high elevations. We break the problem of whole landscape evolution into three related, one-dimensional problems: evolution of the high smooth summit surfaces; evolution of the longitudinal profiles of adjacent glacial troughs; and evolution of the fluvial profiles downstream of the glacial limit. We review work on the high summit surfaces, showing quantitatively that they are steady-state features lowering at rates on the order of 5 μm/yr, and are entirely decoupled from the adjacent glacial troughs. Glaciers not only truncate these high surfaces, but greatly alter the longitudinal profiles of the major streams: major steps occur at tributary junctions, and profiles above the glacial limit are significantly flattened from their original fluvial slopes. We extend existing models of glacial valley evolution by including processes that allow head-wall retreat. This serves to enhance the headward retreat of east-facing valleys, and explains the asymmetric truncation of the high smooth surfaces that form the spine of the range. Fluvial profiles downstream of the glacial limit commonly display a prominent convexity inboard of the range edge. Stream-power–based numerical models of profile evolution of specific rivers demonstrate that this reflects a transient response of the streams to base-level lowering. This response varies significantly with drainage basin area. We explore the degree to which this differential response controls the location of major remnants of pediments on the edge of the Great Plains, such as the prominent Rocky Flats and adjacent surfaces.

Mountain ranges in the southern Rocky Mountains, first uplifted during the early Cenozoic Laramide orogeny, have followed separate landscape evolutionary pathways in the late Cenozoic. We present a model that reconstructs the post-Laramide tectonic and geomorphic history of Sierra Nacimiento and the Taos Range, two nearly adjacent rift-flank ranges in north-central New Mexico that serve to illustrate the various processes shaping landscapes across the southern Rocky Mountains. The Sierra Nacimiento landscape reflects the exhumation of hard Precambrian rocks from beneath a softer Phanerozoic sedimentary cover. The exhumation is continuous, but not steady, being driven by distal base-level fall. Downstream diverging river terraces in the Jemez River valley on the eastern flank of Sierra Nacimiento and late Pliocene to Holocene fluvial deposits on the western Sierra Nacimiento piedmont document the base-level fall. The timing and contemporary rates of incision from these river systems suggest that exhumation is being propagated from south to north as knickzones work their way headward from the Rio Grande. In contrast, the Taos Range landscape reflects alternating active stream incision and aggradation astride, and throttled by, an active range-front normal fault. The distinction between the exhumation-dominated and tectonic-dominated mountain front is best quantified by analyses of first-order stream gradients and a watershed metric we call the drainage basin volume to drainage basin area ratio ( R va ). Gradients of first-order streams in the exhumation-dominated Sierra Nacimiento have a mode of 6.8 degrees, significantly less than the 17.7 degrees obtained from a comparable data set of Taos Range first-order streams. The distinct stream gradient and R va populations hint at an important change in the processes shaping hillslopes and low-order channels, which is supported by the lack of slope-clearing landslides in the Sierra Nacimiento landscape and the presence of such landslides in the Taos Range. Analogue and numeric models find that steep, rugged, faceted topography associated with tectonically active mountain fronts like the Taos Range can only be produced and maintained by creep and landslides where the sediment flux scales as a power law with respect to average hillslope or low-order channel gradient. Here, the fingerprint of active tectonics is recorded by both high R va values and steep modal channel gradients. By comparison, the Sierra Nacimiento landscape is shaped primarily by creep where the sediment flux has a linear relationship to average hillslope and low-order channel gradient. In this situation, the signatures of distal base-level fall are low R va values and relatively gentle modal channel gradients.