Bedrock Incision Rates Research Articles

Canyon incision and knickpoint propagation recorded by apatite 4He/3He thermochronometry

In many regions of the world, deeply incised canyons demonstrate the net effects of physical processes active at Earth's surface in response to surface uplift. Low-temperature thermochronometry can constrain the timing and rates of bedrock incision, which is necessary for relating canyon incision to surface uplift over geological timescales. We analyzed four samples from the Cotahuasi–Ocoña canyon system in southwest Peru using 4He/3He thermochronometry and we present a new inversion model to identify continuous low-temperature cooling histories that are consistent with the observed data. Derived cooling histories limit the onset of fluvial incision to ∼13 to 8Ma. This is in agreement with previously reported interpretations based on a three-dimensional thermal model interpolation to a much larger set of conventional apatite (U–Th)/He ages. However, because 4He/3He thermochronometry constrains an independent cooling history for each sample, the results also permit testing of landscape-evolution models with greater spatial variability in exhumation compared to those models that can be tested by a geographically scattered set of conventional (U–Th)/He ages. The different cooling histories of the four samples require asynchronous incision along at least part of the canyon system that is best explained by headward propagation of fluvial incision by knickpoint migration.

Rapid incision of the Colorado River in Glen Canyon – insights from channel profiles, local incision rates, and modeling of lithologic controls

AbstractThe Colorado River system in southern Utah and northern Arizona is continuing to adjust to the baselevel fall responsible for the carving of the Grand Canyon. Estimates of bedrock incision rates in this area vary widely, hinting at the transient state of the Colorado and its tributaries. In conjunction with these data, we use longitudinal profiles of the Colorado and tributaries between Marble Canyon and Cataract Canyon to investigate the incision history of the Colorado in this region. We find that almost all of the tributaries in this region steepen as they enter the Colorado River. The consistent presence of oversteepened reaches with similar elevation drops in the lower section of these channels, and their coincidence within a corridor of high local relief along the Colorado, suggest that the tributaries are steepening in response to an episode of increased incision rate on the mainstem. This analysis makes testable predictions about spatial variations in incision rates; these predictions are consistent with existing rate estimates and can be used to guide further studies. We also present cosmogenic nuclide data from the Henry Mountains of southern Utah. We measured in situ 10Be concentrations on four gravel‐covered strath surfaces elevated from 1 m to 110 m above Trachyte Creek. The surfaces yield exposure ages that range from approximately 2·5 ka to 267 ka and suggest incision rates that vary between 350 and 600 m/my. These incision rates are similar to other rates determined within the high‐relief corridor. Available data thus support the interpretation that tributaries of the Colorado River upstream of the Grand Canyon are responding to a recent pulse of rapid incision on the Colorado. Numerical modeling of detachment‐limited bedrock incision suggests that this incision pulse is likely related to the upstream‐dipping lithologic boundary at the northern edge of the Kaibab upwarp. Copyright © 2009 John Wiley & Sons, Ltd.

Quaternary history of the piedmont reach of Río Diamante, Argentina

The Río Diamante is located in a segment of the southern Central Andes that is transitional in terms of morphology and foreland tectonics (33–37°S). The piedmont reach of the river flows eastwards between the main mountain front and the San Rafael Block (the southernmost foreland uplift to the east of the southern Central Andes). Unlike adjacent rivers, the Río Diamante has incised into pre-Quaternary bedrock to form a deep canyon across the piedmont. Five fill and strath terraces were mapped, correlated, and surveyed along the ∼35 km piedmont reach to determine their paleo-long profiles. Chronological data for the terraces were provided by geochemical correlation of interbedded tephras to a previously dated ignimbrite, as well as by eight cosmogenic 10Be ages, and suggest a synchronous relationship between fill terrace deposition and glaciation upstream. Terraces are tentatively correlated with oxygen isotope stages 16 (Qt1), 12 (Qt2), 4 (Qt3) and 2 (Qt4 and Qt5). Minor spatial variation in incision along the reach is apparent from the long profile of the Qt2 strath (∼450 ka), which shows low-amplitude folding. The long-term reach average rate of bedrock incision is estimated to be ∼0.1–0.5 m/kyr and the recent shorter term (post-OIS 2) rate is estimated at ∼2 m/kyr. Mid and late Quaternary uplift of the piedmont area is a likely cause for the incision; however, other possible explanations include a delayed response to pre-mid Quaternary uplift, or a response to changes in climate and sediment supply on a tectonically stable portion of the piedmont flanked by subsiding basins. Base-level fall due to subsidence on the eastern side of the San Rafael Block creating a westward migrating knickzone may have contributed to the incision, but the depth of incision is greater than the observed subsidence and no major knickzone is present in the modern river profile of the study reach. Studies such as this on piedmont geomorphic processes are important for understanding the topographic and tectonic transitions that characterise the southern Central Andes.

Fashion and phases of late Pleistocene aggradation and incision in the Alaknanda River Valley, western Himalaya, India

We study the aggradation and incision of the Alaknanda River Valley during the late Pleistocene and Holocene. The morphostratigraphy in the river valley at Deoprayag shows the active riverbed, a cut terrace, and a fill terrace. The sedimentary fabric of the fill terrace comprises four lithofacies representing 1) riverbed accretion, 2) locally derived debris fan, 3) the deposits of waning floods and 4) palaeoflood records. The sedimentation style, coupled with geochemical analysis and Optically Stimulated Luminescence (OSL) dating, indicate that this terrace formed in a drier climate and the river valley aggraded in two phases during 21–18 ka and 13–9 ka. During these periods, sediment supply was relatively higher. Incision began after 10 ka in response to a strengthened monsoon and aided by increase of the tectonic gradient. The cut terrace formed at ~ 5 ka during a phase of stable climate and tectonic quiescence. The palaeoflood records suggest wetter climate 200–300 yr ago when the floods originated in the upper catchment of the Higher Himalaya and in the relatively drier climate ~ 1.2 ka when locally derived sediments from the Lesser Himalaya dominated flood deposits. Maximum and minimum limits of bedrock incision rate at Deoprayag are 2.3 mm/a and 1.4 mm/a.

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.

New constraints on sediment-flux–dependent river incision: Implications for extracting tectonic signals from river profiles

We present new field data from rivers draining across active normal faults that incise across the same lithology at the fault, have been subjected to similar climatic regimes and tectonic settings, and were perturbed by a well-documented increase in fault slip rate ca. 1 Ma. In spite of these similarities, the rivers exhibit markedly different long profiles and patterns of catchment incision. We use channel slope and hydraulic geometry data for each river to calculate bed shear stresses (τb), and show that there is no simple relationship between peak τb and the relative uplift rates across the faults, U , which differ by a factor of four. The long-term average sediment supply to each channel ( Q s), estimated from time-averaged catchment erosion rates, can explain the τb versus U data if bedload modulates bedrock incision rate, E , in a strongly nonlinear way. Together these field data allow us, for the first time, to evaluate theoretical predictions of the role of sediment on river profile evolution and to quantify the magnitude of the effect in natural systems.

40Ar/39Ar and field studies of Quaternary basalts in Grand Canyon and model for carving Grand Canyon: Quantifying the interaction of river incision and normal faulting across the western edge of the Colorado Plateau

40Ar/39Ar dates on basalts of Grand Canyon provide one of the best records in the world of the interplay among volcanism, differential canyon incision, and neotectonic faulting. Earlier 40K/40Ar dates indicated that Grand Canyon had been carved to essentially its present depth before 1.2 Ma. But new 40Ar/39Ar data cut this time frame approximately in half; new ages are all <723 ka, with age probability peaks at 606, 534, 348, 192, and 102 ka. Strategic sampling of basalts provides a semicontinuous record for deciphering late Quaternary incision and fault-slip rates and indicates that basalts flowed into and preserved a record of a progressively deepening bedrock canyon. The Eastern Grand Canyon block (east of Toroweap fault) has bedrock incision rates of 150–175 m/Ma over approximately the last 500 ka; western Grand Canyon block (west of Hurricane fault) has bedrock incision rates of 50–75 m/Ma over approximately the last 720 ka. Fault displacement rates are 97–106 m/Ma on the Toroweap fault (last 500–600 ka) and 70–100 m/Ma on the Hurricane fault (last 200–300 ka). As the river crosses each fault, the apparent incision rate is lowest in the immediate hanging wall, and this rate, plus the displacement rate, is sub-equal to the incision rate in the footwall. At the reach scale, variation in apparent incision rates delineates ∼100 m/Ma of cumulative relative vertical lowering of the western Grand Canyon block relative to the eastern block and 70–100 m of slip accommodated by formation of a hanging-wall anticline. Data from the Lake Mead region indicate that our refined fault-dampened incision model has operated over the last 6 Ma. Bedrock incision rate has been 20–30 m/Ma in the lower Colorado River block in the last 5.5 Ma, and displacement on the Wheeler fault has resulted in both lowering of the Lower Colorado River block and formation of a hanging-wall anticline of the 6-Ma Hualapai Limestone. In modeling long-term incision history, extrapolation of Quaternary fault displacement and incision rates linearly back 6 Ma only accounts for approximately two-thirds of eastern and approximately one-third of western Grand Canyon incision. This “incision discrepancy” for carving Grand Canyon is best explained by higher rates during early (5- to 6-Ma) incision in eastern Grand Canyon and the existence of Miocene paleocanyons in western Grand Canyon. Differential incision data provide evidence for relative vertical displacement across Neogene faults of the Colorado Plateau-Basin and Range transition, a key data set for evaluating uplift and incision models. Our data indicate that the Lower Colorado River block has lowered 25–50 m/Ma (150–300 m) relative to the western Grand Canyon block and 125–150 m/Ma (750–900 m) relative to the eastern Grand Canyon block in 6 Ma. The best model explaining the constrained reconstruction of the 5- to 6-Ma Colorado River paleoprofile, and other geologic data, is that most of the 750–900 m of relative vertical block motion that accompanied canyon incision was due to Neogene surface uplift of the Colorado Plateau.

Rates of fluvial bedrock incision within an actively uplifting orogen: Central Karakoram Mountains, northern Pakistan

Terrestrial cosmogenic nuclide (TCN) 10Be surface exposure ages for strath terraces along the Braldu River in the Central Karakoram Mountains range from 0.8 to 11 ka. This indicates that strath terrace formation began to occur rapidly upon deglaciation of the Braldu valley at ∼ 11 ka. Fluvial incision rates for the Braldu River based on the TCN ages for strath terraces range from 2 to 29 mm/a. The fluvial incision rates for the central gorged section of the Braldu River are an order of magnitude greater than those for the upper and lower reaches. This difference is reflected in the modern stream gradient and valley morphology. The higher incision rates in the gorged central reach of the Braldu River likely reflect differential uplift above the Main Karakoram Thrust that has resulted in the presence of a knickpoint and more rapid fluvial incision. The postglacial fluvial incision rate (2–3 mm/a) for the upper and lower reaches are of the same order of magnitude as the exhumation rates estimated from previously published thermochronological data for the Baltoro granite in the upper catchment region and for the adjacent Himalayan regions.

Giant landslides, topography, and erosion

Distributions of slope angles in tectonically active mountain belts point to the development of threshold conditions, where hillslopes attain a critical inclination or height at which they fail readily because of limitations in material strength. It has been proposed that hillslopes adjust to rapid uplift and bedrock incision through an increase in the rate of relief-limiting landsliding rather than gradual slope steepening. Here we test this concept by investigating the relationship between mean local relief H¯, which we take to be a proxy of long-term erosion rates E, and the occurrence of over 300 of the largest ( V > 10 8 m 3) terrestrial landslides on Earth. We find that nearly two-thirds of these giant landslides have occurred in the steepest 5% of the Earth's land surface, where relief is close to its proposed upper strength limit. They are primarily located in deeply incised valleys, along fault-bounded fringes of active mountain belts, and in volcanic arcs. This distribution coincides with areas of high long-term erosion rates (∼ 4 mm yr − 1 ), confirming that giant landslides contribute to rapid denudation of mountains. Most of the eroded volume is concentrated in the smallest, but steepest parts of mountain belts and volcanic arcs. First-order estimates of minimum erosion rates accomplished by the largest landslides are ≥ 0.01 mm yr − 1 ; these rates are between 1% and 10% of the Late Pleistocene to Holocene mean erosion rates in a given area. Importantly, the landslide erosion rates show a nonlinear increase with mean local relief, suggesting that the contribution of giant landslides in total and per event increases significantly with increasing overall erosion rates. However, giant landslides also occur in areas of lower-than-average relief ( H¯ ∼ 300–700 m), irrespective of whether threshold hillslopes have developed or not. Factors contributing to these failures include soft rocks, extensive low-angle discontinuities, high rates of fluvial bedrock incision, and tectonically driven deformation and slope loading.

Predictions of steady state and transient landscape morphology using sediment‐flux‐dependent river incision models

Recent experimental and theoretical studies support the notion that bed load in mountain rivers can both enhance incision rates through wear and inhibit incision rates by covering the bed. These effects may play an important role in landscape evolution and, in particular, the response of river channels to tectonic or climatic perturbation. We use the channel‐hillslope integrated landscape development (CHILD) numerical model with two different bedrock incision models that include the dual role of the sediment flux to explore the transient behavior of fluvial landscapes. Both models predict that steady state channel slopes increase in landscapes with higher rock uplift rates. However, the incision models predict different transient responses to an increase in uplift rate, and the behavior of each incision model depends on both the magnitude of change in uplift rate and the local drainage area. In some cases, the transient channel behavior is indistinguishable from that predicted for transport‐limited alluvial rivers. In other cases, knickpoints form in some or all of the drainage network, as predicted by the detachment‐limited stream power model. In all cases the response in the lower parts of the network is highly dependent on the response in the upper parts of the network as well as the hillslopes. As the upper parts of the network send more sediment downstream, channel incision rates may rise or fall, and slopes in the lower parts of the channel may, in fact, decrease at times during the transient adjustment to an increase in rock uplift rate. In some cases, channel incision in the upper parts of the network ceases during the transient while the hillslopes adjust to the new uplift rate; drainage density may also change as a function of uplift rate. Our results suggest that if the sediment flux strongly controls bedrock incision rates, then (1) the transient fluvial response will take longer than predicted by the detachment‐limited stream power model, (2) changes in channel slope may be much more complex than predicted by the detachment‐limited stream power model, and (3) changes in the fluvial system will be closely tied to sediment delivery from the hillslopes. Importantly, our results outline quantitative differences in system behavior produced by competing models and provide a framework for identifying locations in natural systems where differences in channel morphology can be used to discern between competing fluvial erosion models.

Bedload-to-suspended load ratio and rapid bedrock incision from Himalayan landslide-dam lake record

About 5400 cal yr BP, a large landslide formed a > 400-m-tall dam in the upper Marsyandi River, central Nepal. The resulting lacustrine and deltaic deposits stretched > 7 km upstream, reaching a thickness of 120 m. 14C dating of 7 wood fragments reveals that the aggradation and subsequent incision occurred remarkably quickly (∼ 500 yr). Reconstructed volumes of lacustrine (∼ 0.16 km 3) and deltaic (∼ 0.09 km 3) deposits indicate a bedload-to-suspended load ratio of 1:2, considerably higher than the ≤ 1:10 that is commonly assumed. At the downstream end of the landslide dam, the river incised a new channel through ≥ 70 m of Greater Himalayan gneiss, requiring a minimum bedrock incision rate of 13 mm/yr over last 5400 yr. The majority of incision presumably occurred over a fraction of this time, suggesting much higher rates. The high bedload ratio from such an energetic mountain river is a particularly significant addition to our knowledge of sediment flux in orogenic environments.

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.

Using fill terraces to understand incision rates and evolution of the Colorado River in eastern Grand Canyon, Arizona

The incision and aggradation of the Colorado River in eastern Grand Canyon through middle to late Quaternary time can be traced in detail using well‐exposed fill terraces dated by a combination of optically stimulated luminescence, uranium series, and cosmogenic nuclide dating. This fluvial history provides the best bedrock incision rate for this important landscape and highlights the complications and advantages of fill terrace records for understanding river long‐profile evolution and incision. The use of fill terraces, as distinct from strath terraces, for calculating incision rates is complicated by the cyclic alluviation and incision they record. In the example of the Grand Canyon this has led to various rates being reported by different workers and rates that tend to be overestimates in shorter records. We illustrate that a meaningful long‐term bedrock incision rate of ∼140 m/m.y. can be extracted from the Grand Canyon record by linking episodes when the Colorado River is floored on bedrock. Variable incision rates reported in the greater region may be, to some degree, due to inconsistent calculations. Our data also highlight that the Colorado River has been a mixed alluvial‐bedrock river through both time and space and has been a bedrock river for less than half of its Pleistocene history. This strong temporal variation, combined with the varying bedrock the river encounters on its path, heightens the challenge of understanding the tectonic, climatic, and drainage integration controls on the form and evolution of the Colorado River's long profile.

Quaternary fans and terraces in the Khumbu Himal south of Mount Everest: their characteristics, age and formation

Large fans and terraces are frequent in the Khumbu Himal within the high Himalayan valleys south of Mt. Everest. These features are composed of massive matrix- and clast-supported diamicts that were formed from both hyperconcentrated flows and coarse-grained debris flows. Cosmogenic radionuclide (CRN) exposure ages for boulders on fans and terraces indicate that periods of fan and terrace formation occurred at c . 16, c . 12, c . 8, c . 4 and c . 1.5 ka, and are broadly coincident with the timing of glaciation in the region. The dating precision is insufficient to resolve whether the surfaces formed before, during or after the correlated glacial advance. However, the sedimentology, and morphostratigraphic and geomorphological relationships suggest that fan and terrace sedimentation in this part of the Himalaya primarily occurs during glacier retreat and is thus paraglacial in origin. Furthermore, modern glacial-lake outburst floods and their associated deposits are common in the Khumbu Himal as the result of glacial retreat during historical times. We therefore suggest that Late Quaternary and Holocene fan and terrace formation and sediment transfer are probably linked to temporal changes in discharge and sediment load caused by glacier oscillations responding to climate change. The timing of major sedimentation events in this region can be correlated with fans and terraces in other parts of the Himalaya, suggesting that major sedimentation throughout the Himalaya is synchronous and tied to regional climatic oscillations. Bedrock incision rates calculated from strath terrace ages average c . 3.9 mm a −1 , suggesting that the overall rate of incision is set by regional uplift.

An Episode of Rapid Bedrock Channel Incision During the Last Glacial Cycle, Measured with 10Be

We use 10Be to infer when, how fast, and why the Susquehanna River incised through bedrock along the U.S. Atlantic seaboard, one of the world’s most prominent and ancient passive margins. Although the rate at which large rivers incise rock is a fundamental control on the development of landscapes, relatively few studies have directly measured how quickly such incision occurs either in tectonically active environments or along passive margins. Exposure ages of fluvially carved, bedrock strath terraces, preserved along the lower Susquehanna River, demonstrate that even along a passive margin, large rivers are capable of incising through rock for short periods of time at rates approaching those recorded in tectonically active regions, such as the Himalayas. Over eighty samples, collected along and between three prominent levels of strath terraces within Holtwood Gorge, indicate that the Susquehanna River incised more than 10 meters into the Appalachian Piedmont during the last glacial cycle. Beginning ∼36 ka, incision rates increased dramatically, and remained elevated until ∼14 ka. The northern half of the Susquehanna basin was glaciated during the late Wisconsinan; however, similar rates and timing of incision occurred in the unglaciated Potomac River basin immediately to the south. The concurrence of incision periods on both rivers suggests that glaciation and associated meltwater were not the primary drivers of incision. Instead, it appears that changing climatic conditions during the late Pleistocene promoted an increase in the frequency and magnitude of flood events capable of exceeding thresholds for rock detachment and bedrock erosion, thus enabling a short-lived episode of rapid incision into rock. Although this study has constrained the timing and rate of bedrock incision along the largest river draining the Atlantic passive margin, the dates alone cannot explain fully why, or by what processes, this incision occurred. However, cosmogenic dating offers compelling evidence that episodes of rapid incision into bedrock are tied to glacial cycles and changes in global climate. These results, and the methods we employ, provide valuable insights into the nature of bedrock channel incision, not only along the Susquehanna River and passive margins, but also across a wide range of settings around the globe. Because river incision into bedrock transmits the effects of changing climate and tectonics through fluvial networks to hillslopes, comprehending when, where, and why rivers incise has important implications for the evolution of landscapes.

Relative rates of fluvial bedrock incision on Titan and Earth

Observations of likely fluvial channels on the surface of Titan, along with Titan's geologically youthful surface, motivate this study of comparative fluvial erosion rates on Titan and the Earth. The roles of bedload abrasion, suspended load abrasion, plucking, and cavitation are considered. Despite orders of magnitude differences in some of the physical parameters that control fluvial incision on Titan and the Earth, incision rates on Titan are likely to be similar to terrestrial rates, given similar conditions of slope, discharge, and sediment supply.

Discharge, discharge variability, and the bedrock channel profile

Long‐term bedrock incision is driven by daily discharge events of variable magnitude and frequency, with ineffective events below an incision threshold. We explore theoretically how this short‐term stochastic behavior controls long‐term steady state incision rates and bedrock channel profiles, combining a realistic frequency‐magnitude distribution of discharge with a deterministic, detachment‐limited incision model in which incision rate is a power function of basal shear stress above a critical shear stress. Our model predicts a power law relationship between steady state slope and drainage area consistent with observations. The exponent of this power law is independent of discharge mean and variability, while the amplitude factor, which controls mountain belt relief, is a power law function of mean runoff (with an exponent of −0.5) and a complex function of runoff variability. In accordance with evidence that incision occurs between 6 and 20% of time in rapidly incising rivers (&gt;1 mm/yr) our model predicts that channel steepness is virtually insensitive to runoff variability. Runoff variability can only decrease channel steepness for very slow incision rates and/or weak lithologies. The relationship between channel steepness and incision rate is always a power law whose exponent depends on the channel cross‐sectional geometry and runoff variability. This contradicts models neglecting discharge stochasticity in which the steepness‐incision scaling is set by the incision law exponent. Our results suggest that changes in climate variability cannot explain an increase in bedrock incision rates during the Late Cenozoic within the context of a detachment limited model.

Episodic incision of the Colorado River in Glen Canyon, Utah

AbstractIncision rates of the Colorado River are integral to understanding the development of the Colorado Plateau. Here we calculate episodic incision rates of the Colorado River based on absolute ages of two levels of Quaternary deposits adjacent to Glen Canyon, Utah, along the north flank of Navajo Mountain. Minimum surface ages are determined by a combination of cosmogenic radionuclide surface exposure ages, uranium series and soil‐development formation times. Bedrock incision rates of the Colorado River between c. 500 ka and c. 250 ka, and c. 250 ka to present are c. 0·4 m ka−1 and c. 0·7 m ka−1, respectively. These rates are more than double the rates reported in the Grand Canyon, suggesting that the Colorado River above Lees Ferry is out of equilibrium with the lower section of the river. We also determine incision rates of two tributaries to the Colorado River. Oak Creek and Bridge Creek flow off Navajo Mountain into Glen Canyon from the southeast. Oak Creek and Bridge Creek both have incision rates of c. 0·6 m ka−1 over the past c. 100 ka at points about 9 km away from the main stem of the Colorado River. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Controls on the channel width of rivers: Implications for modeling fluvial incision of bedrock

Research Article| March 01, 2005 Controls on the channel width of rivers: Implications for modeling fluvial incision of bedrock Noah J. Finnegan; Noah J. Finnegan 1Quaternary Research Center and Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Gerard Roe; Gerard Roe 1Quaternary Research Center and Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar David R. Montgomery; David R. Montgomery 1Quaternary Research Center and Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Bernard Hallet Bernard Hallet 1Quaternary Research Center and Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Geology (2005) 33 (3): 229–232. https://doi.org/10.1130/G21171.1 Article history received: 10 Sep 2004 rev-recd: 19 Nov 2004 accepted: 26 Nov 2004 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Noah J. Finnegan, Gerard Roe, David R. Montgomery, Bernard Hallet; Controls on the channel width of rivers: Implications for modeling fluvial incision of bedrock. Geology 2005;; 33 (3): 229–232. doi: https://doi.org/10.1130/G21171.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract On the basis of the Manning equation and basic mass conservation principles, we derive an expression for scaling the steady-state width (W) of river channels as a function of discharge (Q), channel slope (S), roughness (n), and width-to-depth ratio (α): W = [α(α + 2)2/3]3/8Q3/8S−3/16n3/8. We propose that channel width-to-depth ratio, in addition to roughness, is a function of the material in which the channel is developed, and that where a river is confined to a given material, width-to-depth ratio and roughness can be assumed constant. Given these simplifications, the expression emulates traditional width-discharge relationships for rivers incising bedrock with uniformly concave fluvial long profiles. More significantly, this relationship describes river width trends in terrain with spatially nonuniform rock uplift rates, where conventional discharge-based width scaling laws are inadequate. We suggest that much of observed channel width variability in river channels confined by bedrock is a simple consequence of the tendency for water to flow faster in steeper reaches and therefore occupy smaller channel cross sections. We demonstrate that using conventional scaling relationships for channel width can result in underestimation of stream-power variability in channels incising bedrock and that our model improves estimates of spatial patterns of bedrock incision rates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Processes of oscillatory basin filling and excavation in a tectonically active orogen: Quebrada del Toro Basin, NW Argentina

Intramontane basins may act as important sediment storage areas, serve as recorders of the history of deformation, record syntectonic deposition, and document the evolution of climatic conditions during deposition. We document the timing, cyclicity, and processes that led to the filling and reexcavation of the intramontane Quebrada del Toro basin in NW Argentina. Geomorphic and geologic observations indicate that the basin was filled with sediment that has been subsequently excavated at least two times in the last ∼8 m.y. The last filling and excavation cycle occurred within the last 0.98 m.y. and has led to the deposition and removal of ∼61.4 km 3 of material from the basin, leading to a basin-wide averaged minimum denudation rate of 0.16 mm/yr. Aggradation within the basin took place due to channel steepening of the downstream fluvial system that connects the intramontane basin to the foreland. This portion of the fluvial system is actively incising through an uplifting bedrock zone. We use observations within the Toro to test a quasi-physically based model of channel aggradation behind a rising base level that rises due to downstream channel steepening. Our work shows that the bedrock incision rate constant required to reproduce conditions observed within the Toro basin is consistent with values measured independently in similar rock types. Therefore, in intramontane basins that experience similar processes of filling and evacuation, this model may be used to assess the relative importance of tectonic rock uplift, bedrock resistance to fluvial incision, and climate in determining the geomorphic and sedimentologic history of these basins.