Bedrock River Incision Research Articles

Interpretation and downstream correlation of bedrock river terrace treads created from propagating knickpoints

Derivation of an analytical model for the slope of a strath terrace created following an upstream propagating wave of incision reveals that for detachment‐limited river incision the exponent on channel slope, n, governs terrace tread slope. Terrace elevations can increase upstream (n > 1), downstream (n < 1), or can be horizontal (n = 1). Numerical modeling confirms these results for temporally evolving knickpoint geometries and for incision due to sudden base level fall and an increase in rock uplift rate. Except in the case of bedrock river incision with a slope threshold only exceeded during knickpoint propagation, a terrace created from transient headward incision contains no information about the paleo‐channel gradient. Gradients in rock uplift rate along channels potentially complicate the interpretation of terraces by altering the primary tilt on strath terraces. In particular, monotonic gradients in rock uplift can produce terrace treads that are apparently folded. Because gradients in rock uplift rate are common, where terraces are not longitudinally traceable, care is warranted in terrace correlation. In simple tectonic settings where terraces are longitudinally traceable, the slope of a strath terrace created from headward incision may provide a means of estimating the dependency of river incision rate on channel slope. Terraces in four locations argued to have formed from headward incision are parallel or close to parallel with the active channel, implying a slope exponent on river incision rate that is much greater than one or a threshold slope for incision that is only exceeded during knickpoint propagation.

The influence of erosion thresholds and runoff variability on the relationships among topography, climate, and erosion rate

[1] Bedrock river incision occurs only during floods large enough to mobilize sediment and overcome substrate detachment thresholds. New data relating channel steepness and erosion rate provide the opportunity to evaluate the role of thresholds and discharge variability in landscape evolution. We augment an extensive erosion rate data set in the San Gabriel Mountains, California, with analysis of streamflow records and observations of channel width and sediment cover to evaluate the importance of climate and erosion thresholds on incision rates. We find the relationship between channel steepness and erosion rate in the San Gabriel Mountains can be explained using a simple stochastic-threshold incision model where the distribution of large floods follows an inverse power law, suggesting that details of incision mechanics, sediment effects, width adjustment, and debris flows do not significantly influence the steady state relationship between steepness and erosion rate. Using parameters tuned to this case, we vary climate parameters to explore a range of behavior for the steepness-erosion relationship. Erosion is enhanced by both increases in mean runoff and discharge variability. We explore the implications of an empirical relationship between mean runoff and variability to test whether dry, variable climates can erode more efficiently than wet, stable climates. For channels with high thresholds or low steepness, modeled erosion rate peaks at a mean runoff of 200–400 mm/yr. For much of the parameter space tested, erosion rates are predicted to be insensitive to increases in runoff above ∼500 mm/yr, with important implications for the hypothesized influence of climate on tectonics.

Development and application of 2-D soft bedrock river incision model

Development and application of 2-D soft bedrock river incision model

How rivers react to large earthquakes: Evidence from central Taiwan

Earthquakes and bedrock river incision are fundamental processes in the evolution of tectonically active landscapes, yet little work has focused on understanding how a bedrock river responds to a single large earthquake. Data from the 1999 M w = 7.6 Chi-Chi earthquake in central Taiwan show dramatic differences in river response that depend on the proximity to the rupture zone. Near the fault scarp, vertical ground deformation intensifi es river incision on a time scale of years to decades, while distal to the fault, landslides induced by the earthquake mantle bedrock on the river bed with sediment, impeding incision for decades to centuries as the material is evacuated. This surprising spatial and temporal variability in bedrock incision caused by earthquakes has implications for the paleoseismic interpretation of bedrock terraces and for the evolution of tectonically active landscapes.

Tibetan plateau river incision inhibited by glacial stabilization of the Tsangpo gorge

A considerable amount of research has focused on how and when the Tibetan plateau formed in the wake of tectonic convergence between India and Asia. Although far less enquiry has addressed the controls on river incision into the plateau itself, widely accepted theory predicts that steep fluvial knick points (river reaches with very steep gradients) in the eastern Himalayan syntaxis at the southeastern plateau margin should erode rapidly, driving a wave of incision back into the plateau. Preservation of the plateau edge thus presents something of a conundrum that may be resolved by invoking either differential rock uplift matching erosional decay, or other mechanisms for retarding bedrock river incision in this region where high stream power excludes the potential for aridity as a simple limit to dissection of the plateau. Here we report morphologic evidence showing that Quaternary depression of the regional equilibrium line altitude, where long-term glacier mass gain equals mass loss, was sufficient to repeatedly form moraine dams on major rivers: such damming substantially impeded river incision into the southeastern edge of the Tibetan plateau through the coupled effects of upstream impoundment and interglacial aggradation. Such glacial stabilization of the resulting highly focused river incision centred on the Tsangpo gorge could further contribute to initiating and accentuating a locus of rapid exhumation, known as tectonic anaeurysm.

Epigenetic gorges in fluvial landscapes

AbstractEpigenetic gorges form when channels that have been laterally displaced during episodes of river blockage or aggradation incise down into bedrock spurs or side‐walls of the former valley rather than excavating unconsolidated fills and reinhabiting the buried paleovalley. Valley‐filling events that promote epigenetic gorges can be localized, such as a landslide dam or an alluvial/debris flow fan deposit at a tributary junction, or widespread, such as fluvial aggradation in response to climate change or fluctuating base‐level. The formation of epigenetic gorges depends upon the competition between the resistance to transport, strength and roughness of valley‐filling sediments and a river's ability to sculpt and incise bedrock. The former affects the location and lateral mobility of a channel incising into valley‐filling deposits; the latter determines rates of bedrock incision should the path of the incising channel intersect with bedrock that is not the paleovalley bottom. Epigenetic gorge incision, by definition, post‐dates the incision that originally cut the valley. Strath terraces and sculpted bedrock walls that form in relation to epigenetic gorges should not be used to directly infer river incision induced by tectonic activity or climate variability. Rather, they are indicative of the variability of short‐term bedrock river incision and autogenic dynamics of actively incising fluvial landscapes. The rate of bedrock incision associated with an epigenetic gorge can be very high (>1 cm/yr), typically orders of magnitude higher than both short‐ and long‐term landscape denudation rates. In the context of bedrock river incision and landscape evolution, epigenetic gorges force rivers to incise more bedrock, slowing long‐term incision and delaying the adjustment of rivers to regional tectonic and climatic forcing. Copyright © 2008 John Wiley & Sons, Ltd.

The influence of large landslides on river incision in a transient landscape: Eastern margin of the Tibetan Plateau (Sichuan, China)

Deep landscape dissection by the Dadu and Yalong rivers on the eastern margin of the Tibetan plateau has produced high-relief, narrow river gorges and threshold hillslopes that frequently experience large landslides. Large landslides inundate river valleys and overwhelm channels with large volumes (>10 5 m 3 ) of coarse material, commonly forming stable landslide dams that trigger extensive and prolonged aggradation upstream. These observations suggest that strong feedbacks among hillslope processes, channel morphology, and incision rate are prevalent throughout this landscape and are likely characteristic of transient landscapes experiencing large increases in local relief, in general. Landslide effects are a by-product of rapid incision initiated by regional uplift. However, over timescales relevant to landscape evolution (>10 4 yr), large landslides can also act as a primary control on channel morphology and longitudinal river profiles, inhibiting incision and further preventing the complete adjustment of rivers to regional tectonic, climatic, and lithologic forcing. We explore a probabilistic, numerical model to provide a quantitative framework for evaluating how landslides influence bedrock river incision and landscape evolution. The time-average number of landslide dams along a river course, and thus the magnitude of the landslide influence, is set by two fundamental timescales—the time it takes to erode landslide deposits and erase individual dams and the recurrence interval of large landslides that lead to stable dams. Stable, gradually eroding landslide dams create mixed bedrock-alluvial channels with spatial and temporal variations in incision, ultimately slowing long-term rates of river incision, thereby reducing the total amount of incision occurring over a given length of river. A stronger landslide effect implies that a higher percentage of channel length is buried by landslide-related sediment, leading to reduced river incision efficiency. The longer it takes a river channel to incise into a landslide dam and remove all landslide-related sediment, the more control these events have on the evolution of the river profile and landscape evolution. This can be the result of slow erosion of stable dams, or a higher frequency of large events.

Erosion of steepland valleys by debris flows

Episodic debris flows scour the rock beds of many steepland valleys. Along recent debris-flow runout paths in the western United States, we have observed evidence for bedrock lowering, primarily by the impact of large particles entrained in debris flows. This evidence may persist to the point at which debris-flow deposition occurs, commonly at slopes of less than ∼0.03–0.10. We find that debris-flow–scoured valleys have a topographic signature that is fundamentally different from that predicted by bedrock river-incision models. Much of this difference results from the fact that local valley slope shows a tendency to decrease abruptly downstream of tributaries that contribute throughgoing debris flows. The degree of weathering of valley floor bedrock may also decrease abruptly downstream of such junctions. On the basis of these observations, we hypothesize that valley slope is adjusted to the long-term frequency of debris flows, and that valleys scoured by debris flows should not be modeled using conventional bedrock river-incision laws. We use field observations to justify one possible debris-flow incision model, whose lowering rate is proportional to the integral of solid inertial normal stresses from particle impacts along the flow and the number of upvalley debris-flow sources. The model predicts that increases in incision rate caused by increases in flow event frequency and length (as flows gain material) downvalley are balanced by rate reductions from reduced inertial normal stress at lower slopes, and stronger, less weathered bedrock. These adjustments lead to a spatially uniform lowering rate. Although the proposed expression leads to equilibrium long-profiles with the correct topographic signature, the crudeness with which the debris-flow dynamics are parameterized reveals that we are far from a validated debris-flow incision law. However, the vast extent of steepland valley networks above slopes of ∼0.03–0.10 illustrates the need to understand debris-flow incision if we hope to understand the evolution of steep topography around the world.

Flood magnitude–frequency and lithologic control on bedrock river incision in post-orogenic terrain

Mixed bedrock–alluvial rivers–bedrock channels lined with a discontinuous alluvial cover–are key agents in the shaping of mountain belt topography by bedrock fluvial incision. Whereas much research focuses upon the erosional dynamics of such rivers in the context of rapidly uplifting orogenic landscapes, the present study investigates river incision processes in a post-orogenic (cratonic) landscape undergoing extremely low rates of incision (< 5 m/Ma). River incision processes are examined as a function of substrate lithology and the magnitude and frequency of formative flows along Sandy Creek gorge, a mixed bedrock–alluvial stream in arid SE-central Australia. Incision is focused along a bedrock channel with a partial alluvial cover arranged into riffle-pool macrobedforms that reflect interactions between rock structure and large-flood hydraulics. Variations in channel width and gradient determine longitudinal trends in mean shear stress ( τ b) and therefore also patterns of sediment transport and deposition. A steep and narrow, non-propagating knickzone (with 5% alluvial cover) coincides with a resistant quartzite unit that subdivides the gorge into three reaches according to different rock erodibility and channel morphology. The three reaches also separate distinct erosional styles: bedrock plucking (i.e. detachment-limited erosion) prevails along the knickzone, whereas along the upper and lower gorge rock incision is dependent upon large formative floods exceeding critical erosion thresholds ( τ c) for coarse boulder deposits that line 70% of the channel thalweg (i.e. transport-limited erosion). The mobility of coarse bed materials (up to 2 m diameter) during late Holocene palaeofloods of known magnitude and age is evaluated using step-backwater flow modelling in conjunction with two selective entrainment equations. A new approach for quantifying the formative flood magnitude in mixed bedrock–alluvial rivers is described here based on the mobility of a key coarse fraction of the bed materials; in this case the d 84 size fraction. A 350 m 3/s formative flood fully mobilises the coarse alluvial cover with τ b∼200–300 N/m 2 across the upper and lower gorge riffles, peaking over 500 N/m 2 in the knickzone. Such floods have an annual exceedance probability much less than 10 − 2 and possibly as low as 10 − 3. The role of coarse alluvial cover in the gorge is discussed at two scales: (1) modulation of bedrock exposure at the reach-scale, coupled with adjustment to channel width and gradient, accommodates uniform incision across rocks of different erodibility in steady-state fashion; and (2) at the sub-reach scale where coarse boulder deposits (corresponding to τ b minima) cap topographic convexities in the rock floor, thereby restricting bedrock incision to rare large floods. While recent studies postulate that decreasing uplift rates during post-orogenic topographic decay might drive a shift to transport-limited conditions in river networks, observations here and elsewhere in post-orogenic settings suggest, to the contrary, that extremely low erosion rates are maintained with substantial bedrock channel exposure. Although bed material mobility is known to be rate-limiting for bedrock river incision under low sediment flux conditions, exactly how a partial alluvial cover might be spatially distributed to either optimise or impede the rate of bedrock incision is open to speculation. Observations here suggest that the small volume of very stable bed materials lining Sandy Creek gorge is distributed so as to minimise the rate of bedrock fluvial incision over time.

Predicting landscape-scale erosion rates using digital elevation models

Functional relationships between erosion rates and topography are central to understanding controls on global sediment flux and interactions among tectonics, climate, and erosion in shaping topography. Based on such relations digital elevation models (DEMs) allow predicting landscape-scale erosion rates to the degree that process models can be calibrated and to the extent that such processes reflect elevation, drainage area, and aspect, or their derivatives such as slope and curvature. Digital elevation models allow investigating the influence of erosional processes on landscape form and evolution through generalized quantitative expressions often referred to as ‘erosion laws’. The analytical forms of such expressions are derived from physical principles, but only limited data are available to guide calibration to particular landscapes. In addition, few studies have addressed how different transport laws interact to set landscape-scale erosion rates in different environments. Conventionally, landscape-scale sediment flux is considered to be linearly related to slope or relief, but recent analyses point toward non-linear relations for steep terrain in which changes in the frequency of landsliding accommodate increased rates of rock uplift. In such situations, landscape-scale erosion rates are more closely tied to erosion potential predicted by models of bedrock river incision. Consequently, I propose that using DEMs to predict absolute or relative erosion rates at the landscape-scale counter-intuitively involves the rate of fluvial processes as governing the sediment flux from steep landscapes, and rates of hillslope processes as governing sediment flux from low-gradient landscapes. To cite this article: D.R. Montgomery, C. R. Geoscience 335 (2003).

Landscape disequilibrium on 1000–10,000 year scales Marsyandi River, Nepal, central Himalaya

In an actively deforming orogen, maintenance of a topographic steady state requires that hillslope erosion, river incision, and rock uplift rates are balanced over timescales of 10 5–10 7 years. Over shorter times, <10 5 years, hillslope erosion and bedrock river incision rates fluctuate with changes in climate. On 10 4-year timescales, the Marsyandi River in the central Nepal Himalaya has oscillated between bedrock incision and valley alluviation in response to changes in monsoon intensity and sediment flux. Stratigraphy and 14C ages of fill terrace deposits reveal a major alluviation, coincident with a monsoonal maximum, ca. 50–35 ky BP. Cosmogenic 10Be and 26Al exposure ages define an alluviation and reincision event ca. 9–6 ky BP, also at a time of strong South Asian monsoons. The terrace deposits that line the Lesser Himalayan channel are largely composed of debris flows which originate in the Greater Himalayan rocks up to 40 km away. The terrace sequences contain many cubic kilometers of sediment, but probably represent only 2–8% of the sediments which flushed through the Marsyandi during the accumulation period. At ∼10 4-year timescales, maximum bedrock incision rates are ∼7 mm/year in the Greater Himalaya and ∼1.5 mm/year in the Lesser Himalayan Mahabarat Range. We propose a model in which river channel erosion is temporally out-of-phase with hillslope erosion. Increased monsoonal precipitation causes an increase in hillslope-derived sediment that overwhelms the transport capacity of the river. The resulting aggradation protects the bedrock channel from erosion, allowing the river gradient to steepen as rock uplift continues. When the alluvium is later removed and the bedrock channel re-exposed, bedrock incision rates probably accelerate beyond the long-term mean as the river gradient adjusts downward toward a more “equilibrium” profile. Efforts to document dynamic equilibrium in active orogens require quantification of rates over time intervals significantly exceeding the scale of these millennial fluctuations in rate.

Correction to “Importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem”

[1] In the paper “Importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem” by Noah P. Snyder, Kelin X. Whipple, Gregory E. Tucker, and Dorothy J. Merritts (Journal of Geophysical Research, 108(B2), 2117, doi:10.1029/2001JB001655, 2003), the authors wish to correct several minor errors in equations contained in the published version. [4] There are several errors in Appendix A. The entire corrected appendix is as follows. A full discussion of the stochastic threshold bedrock channel incision model is given by Tucker [2003]. [6] This research was funded by National Science Foundation grants EAR-9725723 to Whipple and EAR-9725348 to Merritts.

Importance of a stochastic distribution of floods and erosion thresholds in the bedrock river incision problem

Fluvial erosion of bedrock occurs during occasional flood events when boundary shear stress exceeds a critical threshold to initiate incision. Therefore efforts to model the evolution of topography over long timescales should include an erosion threshold and should be driven by a stochastic distribution of erosive events. However, most bedrock incision models ignore the threshold as a second‐order detail. In addition, climate is poorly represented in most landscape evolution models, so the quantitative relationship between erosion rate and measurable climatic variables has been elusive. Here we show that the presence of an erosion threshold, when combined with a well‐constrained, probabilistic model of storm and flood occurrence, has first‐order implications for the dynamics of river incision in tectonically active areas. First, we make a direct calculation of the critical shear stress required to pluck bedrock blocks for a field site in New York. Second, we apply a recently proposed stochastic, threshold, bedrock incision model to a series of streams in California, with known tectonic and climatic forcing. Previous work in the area has identified a weak relationship between channel gradient or relief and rock uplift rate that is not easily explained by simpler detachment‐limited models. The results with the stochastic threshold model show that even low erosion thresholds, which are exceeded in steep channels during high‐frequency flood events, fundamentally affect the predicted relationship between gradient and uplift rate in steady state rivers, in a manner consistent with the observed topography. This correspondence between theory and data is, however, nonunique; models in which a thin alluvial cover may act to inhibit channel incision in the low uplift rate zone also provide plausible explanations for the observed topography. Third, we explore the broader implications of the stochastic threshold model to the development of fluvial topography in active tectonic settings. We suggest that continued field applications of geomorphic models, including physically meaningful thresholds and stochastic climate distributions, are required to advance our knowledge of interactions among surficial, climatic, and crustal processes.

Spatial coincidence of rapid inferred erosion with young metamorphic massifs in the Himalayas

A spatially distributed rate-of-erosion index (EI) based on models of bedrock river incision documents a strong spatial correspondence between areas of high erosion potential and young metamorphic massifs as well as structural highs throughout the Himalayas. The EI is derived from slopes and drainage areas calculated from a hydrologically cor- rected digital elevation model (GTOPO30) combined with precipitation data (IIASA) to generate synthetic annual stream discharges. These variables drive three generalized pro- cess models to produce EI maps that, while differing in detail, provide an internally con- sistent, spatially continuous index of large-scale erosion rates. The large spatial variation in potential erosion rates in the Himalayas suggested by the EI patterns contrasts with the uniform convergence of the Indian subcontinent. If these EI gradients persist through time, they support the emerging view of a positive feedback between localized, rapid ero- sion and upward advection of lower crust.

Geologic constraints on bedrock river incision using the stream power law

Denudation rate in unextended terranes is limited by the rate of bedrock channel incision, often modeled as work rate on the channel bed by water and sediment, or stream power. The latter can be generalized as KAmSn, where K represents the channel bed's resistance to lowering (whose variation with lithology is unknown), A is drainage area (a surrogate for discharge), S is local slope, and m and n are exponents whose values are debated. We address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method. We vary m, n, and K to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles. We find at least two end‐member incision laws, KA0.3–0.5S1 for Australian rivers with stable base levels and KfA0.1–0.2Sn for rivers in Kauai subject to abrupt base level change. The long‐term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion, characterized by Kf. Incision patterns from Japan and California could follow either expression. If they follow the first expression with m = 0.4, K varies from 10−7–10−6 m0.2/yr for granite and metamorphic rocks to 10−5–10−4 m0.2/yr for volcaniclastic rocks and 10−4–10−2 m0.2/yr for mudstones. This potentially large variation in K with lithology could drive strong variability in the rate of long‐term landscape change, including denudation rate and sediment yield.