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

Abstract

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.