Differences in primary production and periphyton and macroinvertebrate biomass between pairs of constrained (valley floor width of less than twice the active channel width) and unconstrained reaches were investigated in Elk River and four tributaries in southwest Oregon. In August 1991, macroinvertebrates were sampled from individual cobbles, and rocks were collected to determine periphyton biomass. In August 1992, gross primary production was estimated in the four tributaries using closed, non-circulating chambers. Unconstrained reaches received approximately twice as much direct solar radiation (measured with a Solar Pathfinder) as constrained reaches. Gross primary production in unconstrained reaches was double that of constrained reaches. Periphyton biomass did not differ between reach types. Total macroinvertebrate biomass was 38% greater in unconstrained reaches, where scraper biomass was 2.4 times that of constrained reaches. When regressed across streams, gross primary production, macroinvertebrate biomass, and scraper biomass were positively associated with solar radiation. Greater solar radiation in unconstrained reaches is the most apparent causal mechanism for greater gross primary production, which in turn may cause greater macroinvertebrate biomass in unconstrained reaches. Greater invertebrate consumption in unconstrained reaches may limit periphyton accrual, keeping standing crops at levels similar to those found in constrained reaches. When identifying variables that shape biotic communities and determine productive potential, channel constraint is important. / Graduation date: 1994