Leonard Sklar | Search USGS

USGS2009

Transport slopes, sediment cover, and bedrock channel incision in the Henry Mountains, Utah

[1] Field data from channels in the Henry Mountains of Utah demonstrate that abundant coarse sediment can inhibit fluvial incision into bedrock by armoring channel beds (the cover effect). We compare several small channels that share tributary junctions and have incised into the same sedimentary bedrock unit (Navajo Sandstone) but contain differing amounts of coarse diorite clasts owing to the spatial distribution of localized sediment sources. Bedrock channels that contain abundant clasts (diorite-rich) have steeper longitudinal slopes than tributaries of these channels with smaller drainage areas and less sediment (diorite-poor). The diorite-poor tributaries have incised more deeply to lower average slopes and have more reach-scale slope variability, which may reflect bedrock properties, longitudinal sediment sorting, and incision at lower sediment supply. Diorite-rich channels have less bedrock exposed and smoother longitudinal profiles than diorite-poor channels. We find that (1) coarse sediment can mantle bedrock channel beds and reduce the efficiency of incision, validating the hypothesized cover effect in fluvial incision models; (2) the channel slope needed to transport the sediment load can be larger than that needed to erode bedrock, suggesting that the slope of incising bedrock channels can become adjusted to the sediment load; (3) when abundant sediment is available, transport capacity rather than thresholds of motion can be dominant in setting bedrock channel slope; and (4) cover effects can be important even when moderate amounts of bedrock are exposed in channel beds.

Utah↗

USGS2005

Field measurements of incision rates following bedrock exposure: Implications for process controls on the long profiles of valleys cut by rivers and debris flows

Until recently, published rates of incision of bedrock valleys came from indirect dating of incised surfaces. A small but growing literature based on direct measurement reports short-term bedrock lowering at geologically unsustainable rates. We report observations of bedrock lowering from erosion pins monitored over 1–7 yr in 10 valleys that cut indurated volcanic and sedimentary rocks in Washington, Oregon, California, and Taiwan. Most of these channels have historically been stripped of sediment. Their bedrock is exposed to bed-load abrasion, plucking, and seasonal wetting and drying that comminutes hard, intact rock into plates or equant fragments that are removed by higher flows. Consequent incision rates are proportional to the square of rock tensile strength, in agreement with experimental results of others. Measured rates up to centimeters per year far exceed regional long-term erosion-rate estimates, even for apparently minor sediment-transport rates. Cultural artifacts on adjoining strath terraces in Washington and Taiwan indicate at least several decades of lowering at these extreme rates. Lacking sediment cover, lithologies at these sites lower at rates that far exceed long-term rock-uplift rates. This rate disparity makes it unlikely that the long profiles of these rivers are directly adjusted to either bedrock hardness or rock-uplift rate in the manner predicted by the stream power law, despite the observation that their profiles are well fit by power-law plots of drainage area vs. slope. We hypothesize that the threshold of motion of a thin sediment mantle, rather than bedrock hardness or rock-uplift rate, controls channel slope in weak bedrock lithologies with tensile strengths below &sim;3–5 MPa. To illustrate this hypothesis and to provide an alternative interpretation for power-law plots of area vs. slope, we combine Shields' threshold transport concept with measured hydraulic relationships and downstream fining rates. In contrast to fluvial reaches, none of the hundreds of erosion pins we installed in steep valleys recently scoured to bedrock by debris flows indicate any postevent fluvial lowering. These results are consistent with episodic debris flows as the primary agent of bedrock lowering in the steepest parts of the channel network above &sim;0.03–0.10 slope.

California, Washington, Oregon↗