|Shields Jr, Fletcher|
Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2000
Publication Date: N/A
Interpretive Summary: Streams in many agricultural watersheds are unstable and experience accelerated erosion and sediment deposition. Among the worst types of erosion is channel incision, where channels initially deepen and then rapidly widen until they are three to ten times or more larger than before. Cost-effective strategies to restore stream channel stability are based on an understanding of the incision process and how the channels in a watershed change as incision progresses. Previous studies have generated conceptual models that describe the way channel width, depth, and slope respond to incision, but little has been written about the way that the size of channel bed sediments change as incision progresses. Sediment transport, channel stability, and stream ecology are all quite sensitive to stream bed sediment size. This paper documents a preliminary effort to describe channel bed sediments in incising watersheds, and how bed sediments changed in three streams over a ten-year period. Results show that bed sediment size may change relatively rapidly and somewhat unpredictably in watersheds with significant gravel deposits. These findings will guide future efforts to understand, predict, and manage incised channel development.
Technical Abstract: Models developed to describe spatial changes in stream bed material size have underlying assumptions that hinder application to natural channels. In particular, most models neglect the influence of local sediment sources (channel bed and banks) on sediment size distribution. In addition, temporal variation of bed size in response to disturbance has not been considered. These shortcomings are most evident when models are applied to incising channels, which are extremely dynamic and convey sediments primarily derived from bed and bank erosion. Existing conceptual models of the evolution of incised channels do not consider bed material size. In order to incorporate bed size into an existing incised channel evolution model, samples were taken at 300-m intervals along three streams in Northwestern Mississippi in 1986 and 1996. Bed material was sand and gravel, with all particles smaller than 45 mm. Over the length of stream channel sampled, downstream fining was the exception rather than the rule, as lateral sources of coarse sediment (gravel-bearing tributaries or failing banks) delimited fining reaches. Temporal changes varied for the two streams: one becoming coarser in time and the other finer. Sources of coarse material were not spatially consistent over time. The integration of bed material into the channel evolution model did not allow reliable postdiction of bed material trends due to the difficulty of predicting the location and magnitude of sediment sources.