WOOD IN URBAN STREAMS IS A FUNCTION OF WATERSHED IMPERVIOUS AREA AND RIPARIAN BUFFERS

Large wood can play an important role in storing sediment and controlling pool spacing in headwater streams, as well as providing ecological benefits. But when large wood is mobile in urban streams, it can be a hazard to infrastructure, so many cities remove in-stream wood. We hypothesized that abundance and mobility of wood in urban streams is a function of connectivity between streams and wood sources (i.e., forested riparian zones) as well as connectivity of hydrologic flowpaths (as measured by impervious surface cover) that could generate wood-mobilizing flows. Repeat wood surveys were conducted at 11 stream reaches in the Cleveland, Ohio area, where wood removal is not practiced and forest riparian zones exist. Study reaches span watersheds with 0.5 to 40.1% impervious surface cover and 0.5 to 82.8% forested riparian zones. Channel morphology and sediment size were also measured.

Wood abundance was strongly inversely correlated with impervious surface cover, and less strongly correlated with the percent of the drainage network with forested riparian zones. Conversely, the percent of wood mobilized in each reach was inversely correlated with the percent of the drainage network with forested riparian zones, and less strongly correlated with impervious surfaces. Surprisingly, hydrology does not change when imperviousness is 12-40% in this region, so the correlations with impervious surfaces may instead stand in for a broader suite of urban characteristics that influence wood dynamics.

Across streams, most large wood was less than 5 m in length, had a diameter less than 20 cm, and was oriented parallel to flow, with no apparent trends with watershed characteristics. Wood jams were absent in some of the more urbanized reaches, but where present they stored most of the wood. Pool spacing increased with jam spacing, in reaches where jams occurred. Deposits associated with wood tended to have finer sediments than the streambed average, but the four reaches with the least wood had no wood-associated sediment deposits. Our results suggest that easily calculable urban forest-stream connectivity metrics can identify urban stream reaches where wood may be mobile and hazardous and reaches where stable wood may be providing geomorphic and ecologic benefits, suggesting opportunities for targeted management.