|Shields Jr, Fletcher|
Submitted to: Management of Landscapes Disturbed by Channel Incision Stabilization Rehabi
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/1997
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 two of the watersheds 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: Bed material samples were taken from thirteen streams in Northwestern Mississippi in 1986 and from two streams in 1996. Thalweg profiles for 1985 and 1991 were available for the two streams sampled in 1996. Median grain sizes ranged from 0.1 to 19.9 mm. Over the length of stream channel sampled, downstream fining was the exception rather than the rule, as lateral sources of coarse sediment delimited fining reaches. Sources of coarse material were either gravel-bearing tributaries or failing banks containing strata of weakly-cemented gravel. 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. Exponential downstream fining could be observed in reaches isolated from sources of coarse material. Thalweg profiles indicated that bed material became finer in aggrading reaches and coarser in degrading reaches.