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.
Key Points
Erosion thresholds and discharge variability are critical for incision laws
The distribution of floods shapes the relief‐erosion rate relationship
Dry, but variable climates can be more erosive than wet, stable climates
Steep channel networks commonly show a transition from constant‐gradient colluvial channels associated with debris flow activity to concave‐up fluvial channels downstream. The trade‐off between ...debris flow and fluvial erosion in steep channels remains unclear, which obscures connections among topography, tectonics, and climate in steep landscapes. Here, we analyze steep debris‐flow‐prone channels across the western United States and observe: (1) similar maximum channel gradients across a range of catchment erosion rates and geologic settings; and (2) lengthening colluvial channels with coarsening sediment cover. Following this compilation, we hypothesize that steep channel gradients are controlled by two competing thresholds of motion for bed‐sediment cover: bed failure by mass‐wasting and fluvial entrainment. We use downstream patterns in discharge, channel geometry, and sediment size to calculate discharges needed to mobilize sediment cover by both mechanisms across channels in the San Gabriel Mountains (SGM) and northern San Jacinto Mountains (NSJM) in southern California. Across steep colluvial channels in both landscapes, decadal discharges are below fluvial entrainment thresholds but above mass‐wasting entrainment thresholds for
D50 (median) sediment sizes, consistent with recent debris flows captured by repeat imagery. Colluvial channel gradient is similar despite > 3× contrasts in surface sediment grain size. In concave‐up fluvial channels downstream, decadal discharges exceed fluvial entrainment thresholds, and mass‐wasting is not predicted on lower gradients. In both landscapes, fluvial channels steepen downstream compared to gradients needed to mobilize sediment cover, which we interpret to reflect downstream increases in sediment flux. Coarser sediment supply in the NSJM than the SGM increases fluvial entrainment thresholds, which increases total channel relief in the NSJM by (1) lengthening colluvial channels shaped by debris flows and (2) increasing fluvial channel gradients. Our compilation and downstream analysis show how differing sensitivity of fluvial and debris flow processes to sediment grain size impacts the relative relief of colluvial and fluvial regimes in headwater channel networks.
Steep mountain channels show pronounced changes in slope‐area scaling often attributed to the downstream efficiency of debris flow and fluvial erosion. We show that bed‐sediment grain sizes across steep channels are consistent with a transition in dominant sediment motion process, from in‐channel mass‐wasting to fluvial entrainment. Also, the corresponding transition in channel morphology moves downstream in steep catchments with coarser sediment supply. Lengthening debris‐flow channels affects how total topographic relief is divided between areas above and below fluvial channel heads.
It has been long hypothesized that topography, as well as climate and rock strength, exert first order controls on erosion rates. Here we use detrital cosmogenic
10Be from 50 basins, ranging in size ...from 1 to 150
km
2, to measure millennial erosion rates across the San Gabriel Mountains in southern California, where a strong E–W gradient in relief compared to weak variation in precipitation and lithology allow us to isolate the relationship between topographic form and erosion rate. Our erosion rates range from 35 to 1100
m/Ma, and generally agree with both decadal sediment fluxes and long term exhumation rates inferred from low temperature thermochronometry. Catchment-mean hillslope angle increases with erosion rate until ∼
300
m/Ma, at which point slopes become invariant with erosion rate. Although this sort of relation has been offered as support for non-linear models of soil transport, we use 1-D analytical hillslope profiles derived from existing soil transport laws to show that a model with soil flux linear in slope, but including a slope stability threshold, is indistinguishable from a non-linear law within the scatter of our data. Catchment-mean normalized channel steepness index increases monotonically, though non-linearly, with erosion rate throughout the San Gabriel Mountains, even where catchment-mean hillslope angles have reached a threshold. This non-linearity can be mostly accounted for by a stochastic threshold incision model, though additional factors likely contribute to the observed relationship between channel steepness and erosion rate. These findings substantiate the claim that the normalized channel steepness index is an important topographic metric in active ranges.
The connection between topography and erosion rate is central to understanding landscape evolution and sediment hazards. However, investigation of this relationship in steep landscapes has been ...limited due to expectations of: (a) decoupling between erosion rate and “threshold” hillslope morphology; and (b) bias in detrital cosmogenic nuclide erosion rates due to deep‐seated landslides. Here we compile 120 new and published 10Be erosion rates from catchments in the San Gabriel Mountains, California, and show that hillslope morphology and erosion rate are coupled for slopes approaching 50° due to progressive exposure of bare bedrock with increasing erosion rate. We find no evidence for drainage area dependence in 10Be erosion rates in catchments as small as 0.09 km2, and we show that landslide deposits influence erosion rate estimates mainly by adding scatter. Our results highlight the potential and importance of sampling small catchments to better understand steep hillslope processes.
Plain Language Summary
In general, erosion rates increase as landscapes steepen. But where landslides are common, this relationship is thought to break down as hillslopes approach their angle of repose. The main tracer for measuring erosion rates, 10Be in sediment, can also be affected by landslides, and models predict it is unreliable for small watersheds in steep landscapes. Here, we compile an extensive data set of 10Be erosion rates from the San Gabriel Mountains of California. We show that slope and erosion rate are coupled well above the soil angle of repose due to systematic exposure of bedrock cliffs, supporting a new conceptual model for steep landscapes. The presence of landslides adds scatter but does not bias 10Be erosion rates, which yield robust results even in small, steep watersheds that have previously been avoided.
Key Points
Progressive exposure of bare rock on steeper slopes leads to correlation of 10Be erosion rate and mean hillslope angle up to 47°
Deep seated landslide deposits add scatter, but do not systematically bias 10Be erosion rate estimates in the San Gabriel Mountains
No evidence for drainage area dependence of 10Be erosion rates in upland catchments
A fundamental long‐standing question regarding Mars history is whether the flat and low‐lying northern plains ever hosted an ocean. The best opportunity to solve this problem is provided by ...stratigraphic observations of sedimentary deposits onlapping the crustal dichotomy. Here, we use high‐resolution imagery and topography to analyze a branching network of inverted channel and channel lobe deposits in the Aeolis Dorsa region, just north of the dichotomy boundary. Observations of stacked, cross‐cutting channel bodies and stratal geometries indicate that these landforms represent exhumed distributary channel deposits. Observations of depositional trunk feeder channel bodies, a lack of evidence for past topographic confinement, channel avulsions at similar elevations, and the presence of a strong break in dip slope between topset and foreset beds suggest that this distributary system was most likely a delta, rather than an alluvial fan or submarine fan. Sediment transport calculations using both measured and derived channel geometries indicate a minimum delta deposition time on the order of 400 years. The location of this delta within a thick and widespread clastic wedge abutting the crustal dichotomy boundary, unconfined by any observable craters, suggests a standing body of water potentially 105 km2 in extent or greater and is spatially consistent with hypotheses for a northern ocean.
Key Points
Stratigraphic analysis reveals paleoflow direction of branching channel networksBackwater scaling relationships enable flow reconstructions of deltaic depositsDeltaic deposits at Aeolis Dorsa support the presence of a past unconfined sea
Interpreting catchment-mean erosion rates from in situ
produced cosmogenic 10Be concentrations in stream sediments requires
calculating the catchment-mean 10Be surface production rate and
effective ...mass attenuation length, both of which can vary locally due to
topographic shielding and slope effects. The most common method for
calculating topographic shielding accounts only for the reduction of nuclide
production rates due to shielding at the surface, leading to catchment-mean
corrections of up to 20 % in steep landscapes, and makes the simplifying
assumption that the effective mass attenuation length for a given nuclide
production mechanism is spatially uniform. Here I evaluate the validity of
this assumption using a simplified catchment geometry with mean slopes
ranging from 0 to 80∘ to calculate the spatial
variation in surface skyline shielding, effective mass attenuation length,
and the total effective shielding factor, defined as the ratio of the
shielded surface nuclide concentration to that of an unshielded horizontal
surface. For flat catchments (i.e., uniform elevation of bounding
ridgelines), the effect of increasing vertical attenuation length as a
function of hillslope angle and skyline shielding exactly offsets the effect
of decreasing surface production rate, indicating that no topographic
shielding correction is needed when calculating catchment-mean vertical
erosion rates. For dipping catchments (as characterized by a plane fit to
the bounding ridgelines), the catchment-mean surface nuclide concentrations
are also equal to that of an unshielded horizontal surface, except for cases
of extremely steep range-front catchments, where the surface nuclide
concentrations are counterintuitively higher than the unshielded case due to
added production from oblique cosmic ray paths at depth. These results
indicate that in most cases topographic shielding corrections are
inappropriate for calculating catchment-mean erosion rates, and are only needed
for steep catchments with nonuniform distributions of quartz and/or erosion
rate. By only accounting for shielding of surface production, existing
shielding approaches introduce a slope-dependent systematic error that could
lead to spurious interpretations of relationships between topography and
erosion rate.
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