Influence of Channel Geomorphology on Retention of Dissolved and Particulate Matter in a Cascade Mountain Stream

2 Assistant Professor of Fisheries, Associate Professor of Fisheries, Department of Fisheries and Wildlife, Oregon State University, Corvallis, Oregon; and Postdoctoral Research Associate, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. Abstract: Retention of particulate and dissolved nutrients in streams is a major determinant of food availability to stream biota. Retention of particulate matter (leaves) and dissolved nutrients (nitrogen) was studied experimentally during summer 1987 in four 300-500 m reaches of Lookout Creek, a fifth-order stream in the Cascade Mountains of Oregon. Constrained (narrow valley floor) and unconstrained (broad valley floor) reaches were selected within old-growth and second-growth riparian zones. Gingko leaves and ammonium were released into the channel and retention rates were measured. Retention of leaves and nutrients was 2-4 times higher in unconstrained reaches than in constrained reaches, in both old-growth and second-growth riparian zones. Retention was enhanced by increased geomorphic complexity of channels, diversity of riparian vegetation, presence of woody debris, and heterogeneity in stream hydraulics, sediments, and lateral habitats. Unconstrained reaches express these qualities and thus are critical areas for retention of particulate and dissolved nutrients in stream ecosystems.