Presenter/s: Nick Legg
Symposium Session: 2020 - 02 Considering sediment dynamics in river restoration design
Topics covered: adaptive management and monitoring, fish passage, fish-salmon, floodplain, hydraulics, lessons learned, riparian, sediment transport, and stream
Restoration of incised alluvial streams is a common yet still challenging problem faced throughout the Pacific Northwest, where the negative effects of stream incision range from disconnected floodplains, armored streambeds, lowered water tables, and degraded riparian communities. A fundamental question in these efforts is whether passive restoration is a viable approach to meeting management objectives on required timescales (e.g. salmonid recovery prior to extinction or extirpation). In other words, can we reasonably wait for streams to build their beds back up to a point of reestablished connectivity and floodplain function? The timescales of potential recovery are directly informed by the metric D* [years], which we define as the volume deficit (D) of sediment lost through historical stream incision, divided by the annual rate of bedload sediment supply (S) to a reach. Through examination of regional patterns in D*, we find that bedload supply rates are often quite low relative to the sediment volume lost via historic stream incision. This suggests that potential timescales of recovery are on the order of multiple decades to centuries. This presentation will highlight application of this simple metric to inform restoration strategies, especially those that recognize gravel and bedload as a scarce resource.