Gravel-Bed Rivers VI: From Process Understanding to River Restoration

Abstract

Sediment transport rates (dissolved, suspended, and bedload) measured over the course of several years are reported for two streams in the Gros Ventre Mountain range in western Wyoming, USA: Little Granite and Cache Creeks. Both streams drain watersheds that are in relatively pristine environments. The sites are about 20 km apart, have runoff dominated by snowmelt and are underlain by a similar geological setting, suggesting that sediment supply and rates of transport in the two watersheds may be comparable. Yet, estimated sediment yields for the two sites appear substantially different. On average, sediment load per unit watershed area was about 40% greater at Little Granite Creek than Cache Creek, with larger differences during wetter years. Moreover, while there were differences for all components of the sediment load, suspended, and bedload fractions showed the most noteworthy contrast where nearly three times more material was exported from Little Granite Creek on an annual basis. Speculatively, this is attributed to contributions of sediment from several chronic sources in the Little Granite Creek watershed.Similar to other studies of sediment transport in gravel-bed streams, the range of measured bedload and suspended sediment in this study were quite variable. An assessment of annual differences in the 13 years of bedload record for Little Granite Creek indicated that variability could not be ascribed to between-year differences. Instead, the data appear to represent the same population of highly variable transport rates. However, the inability to distinguish between years could be due to the infrequency with which data were collected each year. Seasonal variability was evident in the suspended sediment record of Little Granite Creek where there were higher rates on the rising limb of the snowmelt hydrograph, indicative of a flushing of sediment with the onset of snowmelt.Baseline data on rates of sediment transport provide useful information on the inherent variability of stream processes and may be used to assess departure due to natural or anthropogenic disturbances. In August 2000, wildfire burned portions of E-mail addresses: sryanburkett@fs.fed.us (S.E. Ryan) ISSN: 0928-2025 DOI: 10.1016/S0928-202511134-2 the Little Granite Creek watershed, presenting an opportunity to quantify increases in sediment loads associated with large-scale disturbance. The results of 3 years of postfire monitoring showed substantial increases in suspended sediment transport on the rising limb of the snowmelt hydrograph and during several summer thunderstorms. Suspended sediment yields calculated for the post-fire years were higher for the first year and have decreased over time, indicating a return to baseline levels within a few years following the wildfire. In contrast, there were no detectable increases in the rate of bedload transport over the pre-burn values suggesting differences in the rapidity with which the two sediment components respond to disturbance.