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

Retention of particulate and dissolved nutri- ents in streams is a major determinant of food avail- ability 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 val- ley floor) and unconstrained (broad valley floor) reaches were selected within old-growth and second-growth ri- parian zones. Gingko leaves and ammonium were re- leased into the channel and retention rates were mea- sured. 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 geomor- phic complexity of channels, diversity of riparian vege- tation, presence of woody debris, and heterogeneity in stream hydraulics, sediments, and lateral habitats. Un- constrained reaches express these qualities and thus are critical areas for retention of particulate and dissolved nutrients in stream ecosystems.