Importance of Fine Sediment Dynamics in Ecological Alterations of Urban Headwater Streams

Abstract:

Headwater streams in urbanizing watersheds are some of the nation’s most ecologically impaired aquatic ecosystems. However, the relation between fine sediment dynamics and ecological impairment is not well understood. We investigated mechanisms of stream impairment in two neighboring urban watersheds: one with exclusively in-channel best-management practices (BMPs) (i.e., stream restoration structures and bank stabilization) and the other with exclusively watershed BMPs (e.g., stormwater retention ponds). The watershed with the in-channel BMPs was moderately more urbanized and its stream had a simpler (engineered) channel planform with higher flow velocities and more stable banks. Consequently the stream with the in-channel BMPs experienced higher peak flows and higher overall water and sediment yields compared with the stream having watershed BMPs. Further, the stream with the in-channel BMPs also had a coarser bed with less sand and fine particulates, which was consistent with hydraulic modeling and sediment source tracking data suggesting that in-channel BMPs impart greater hydraulic shear stress on the bed and cause greater mobilization of bed sediment. Stream metabolism also differed between the two streams, with higher levels of gross primary productivity and lower than expected ecosystem respiration which produced more frequent periods of autotrophy in the stream with the in-channel BMPs that contributed to greater downstream loading of labile organic carbon compared to the more characteristic heterotrophy of the stream with the watershed BMPs. We conclude that different styles of management in urban watersheds drastically influence channel complexity and fine sediment dynamics, which has cascading influences on stream ecology.