BEYOND SEDIMENT: DYNAMICS OF PARTICULATE TRANSPORT AND STORAGE IN URBAN STREAMS (Invited Presentation)

Far from rendering urban streams immutable and boring, humans alter relationships between precipitation, discharge, and sediment supply, they disrupt connectivity within catchments, and they introduce new materials into fluvial systems - together creating novel realizations ripe for study by intrepid geomorphologists. Here we draw on our studies of streams in several US cities to show that urban streams erode, transport, and store diverse particulate materials, within the constraints of climate, geology, and human engineering.

Urban development increases efficient overland drainage along flowpaths with few opportunities to entrain sediment until they reach the stream. Conversely, those flowpaths bring water in contact with particulate organic matter and a plethora of anthropogenic materials. The addition of green stormwater infrastructure creates distributed sinks within the catchment that further alter the relationships between precipitation, discharge, and material transport. Stormflows arriving in urban streams as “hungry water” may encounter free-formed, hardened, or simplified channels with variable floodplain access. As a consequence, turbidity dynamics are strongly influenced by the spatial availability of erodible material within the channel, with that availability mediated by both geology and infrastructure. Bed material transport is frequent. Organic carbon loads may increase overall, but large wood is often supply limited, and both large wood and particulate organic matter are highly mobile. Where present, wood, exposed roots, and stream bank vegetation are the dominant storage zone for anthropogenic debris, especially light, flexible trash (e.g., plastic bags). Long-term trash mobility and weathering rates are poorly constrained. Channel morphology is a significant predictor of wood and anthropogenic debris storage, but more work is needed to determine how interactions between wood, sediment, and anthropogenic debris influence the transport and storage of each.