The effects of large organic debris on sediment processes and stream morphology in Vermont

Research conducted on a small third-order stream in the Green Mountains of Vermont suggests that large organic debris (LOD) has an important influence on areal sorting and storage of sediment, spacing of pool-riffle sequences, and channel geometry. Large organic debris forms local base-levels that trap over 26 cubic meters of sediment and cause over 14 percent of the vertical drop along a 412 meter study reach on Tiger Brook, Vermont. The local base-levels created by large organic debris accumulations are the most important control on sediment storage between large bedrock knickpoints along the study reach, focusing fine-grained sediment deposition in areas stable at low and moderate discharges. The micro-steps created by these LOD local base-levels dominate gradient and sediment storage at short time-scales between the slowly evolving large bedrock macrosteps. The relation that exists among the volumes of standing timber adjacent to the channel, LOD and sediment stored behind LOD seems to indicate that large organic debris causes a negative feedback-type mechanism where channel degradation leads to increased standing timber recruitment and large organic debris sediment-storage sites.