If a tree falls in an urban stream, does it stick around? Mobility, characteristics, and geomorphic influence of large wood in urban streams in northeastern Ohio, USA

Abstract

Large wood influences geomorphological, ecological, and biogeochemical functions of streams, but in urban areas it is also considered an unaesthetic hazard. Quantifying the abundance and mobility of in-stream wood along an urban intensity gradient, and understanding the relationship between wood dynamics and stream reach, network, and watershed characteristics allows for a stronger understanding of the potential benefits and hazards of wood in urban streams. Repeat wood surveys were conducted at 11 stream reaches in the Cleveland, Ohio area, where wood removal is not practiced and forested riparian zones exist. Study reaches span a gradient of impervious surface cover and extent of the stream network with a forest buffer. Channel morphology and sediment size were also measured. Stream hydrology does not change when imperviousness is 12–40% in this region, so quantification of impervious surfaces stands in for a broader suite of urban impacts.Wood abundance decreases – and mobility increases – as urban intensity in the watershed and along the stream network increases. Jams become smaller and less stable, but the size and orientation of individual wood pieces do not change along an urban gradient. Over the 6-month study period, wood mobility exceeded 100% in some of the most urbanized streams and transport distances are inferred to be on the order of 100s of meters. There was a positive correlation between jam spacing and pool spacing, and finer sediment sizes occurred in some wood-associated deposits, but overall geomorphic effects are limited. Where reaches are confined, wood abundance is suppressed, but impervious surfaces and the extent of forest along the stream network are stronger predictors of wood dynamics than reach scale characteristics. These results emphasize the importance of situating understanding of reach-scale wood dynamics in the context of larger scale phenomena.