Regional Controls on the Rate and Magnitude of Channel Incision

Stream channels in the southeastern and southwestern U.S. are both subject to channel incision, but controls on the rates and magnitudes of incision differ between regions. Distinct differences in hydroclimatology and land use appear to be the primary controls on variations in channel incision. Differences in the timing, frequency, magnitude, duration, and intensity of precipitation in the two regions, as well as in rainfall-runoff relations, result in different characteristic hydrographs. The substantially wetter climate of the southeast, where precipitation is in the form of low intensity, long duration events during the winter months, results in higher unit peak discharges of longer duration. Conversely, the high intensity, short duration rainfall events typical of the southwest monsoon season create flashy hydrographs with smaller unit peak discharges. Hydroclimatology also affects riparian vegetation patterns, including root density and depth, which influence bank strength. Hydrologic regime and bank resistance are responsible for regional differences in sediment concentrations and yields, which also influence the rate and magnitude of channel incision. Maximum suspended sediment concentrations occur in the southwest, but annual sediment yields in the southeast exceed those of southwest streams as a result of increased streamflow frequency. Finally, type and intensity of land use occurring within each region affect channel incision. Two watersheds, Chinle Wash in Arizona and James Creek in Mississippi, are chosen to represent the southwestern and southeastern regions, respectively. These channels are characterized by incised, unstable stream banks with headcutting tributary gullies. Primary land uses in the James Creek watershed are agriculture and stream channelization, whereas land uses in Canyon de Chelly are grazing and the invasion of exotic riparian plants. In this study, we examine the role of climate and land use on differences in the rate and magnitude of incision in these two watersheds. Specifically, we hypothesize that (i) bank resistance is greater in the southwestern streams as a result of greater rooting depths for riparian vegetation, (ii) hydraulic driving forces are greater in the southeastern streams as a result of longer flow duration, and (iii) streams in the southeast incise more rapidly than comparable streams in the southwest because of a larger ratio of hydraulic driving force to substrate resistance.