The influence of valley plugs in channelized streams on floodplain sedimentation dynamics over the last century

Abstract

Channelization of alluvial systems alters the functions and processes of floodplain ecosystems. In the loess belt region of western Tennessee and northern Mississippi, channelization, the geology of the region, and past land-use practices have resulted in geomorphic readjustments of streams and the formation of valley plugs. Valley plugs completely block stream channels with sediment and debris, and can result in greater deposition rates on floodplain surfaces. We used dendrogeomorphic techniques to quantify floodplain sediment deposition rates from the last 100+ years. The data were used to determine the spatial and temporal patterns of sedimentation in floodplains affected by channelization and the subsequent formation of valley plugs and in floodplains along unchannelized streams. We measured sediment accumulation by coring trees for age determination and measuring depth to the root collar of 63 trees at unchannelized sites and 214 trees at channelized valley plug sites. Mean floodplain deposition rates were greater at valley plug sites (-x = 1.71 ± 0.08 cm/yr) compared to rates at unchannelized sites (-x = 0.31 ± 0.02 cm/yr). The formation of valley plugs in channelized streams also affected the spatial and temporal patterns of sedimentation. Increases in deposition rates at the valley plug sites occurred approximately 30 years ago and corresponds to the timing of regional activities including: channelization projects, increases in agricultural activity, and the termination of stream dredging projects.