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We studied several streams spanning a steep dissolved phosphorus (PO4-P) gradient to test the hypothesis that faster stream velocity would reduce alkaline phosphatase activity (APA) and carbon:phosphorus (C:P) of benthic periphyton because higher velocities should increase the supply rate of dissolved phosphorus at the community-water interface. We tested the hypothesis that the differences in APA and C:P between fast and slow velocity locations within a stream reach would decline as stream PO4-P concentrations increased, and, therefore, velocity effects should be the greatest at low levels of PO4-P. APA declined in response to both the increased water velocity and PO4-P, but the effect of velocity on APA was negligible at the highest levels of PO4-P. Further, we found a strong, negative relationship between periphyton C:P and PO4-P levels as hypothesized, but did not detect significant relationship between C:P and velocity after accounting for the effects of PO4-P. The lack of an effect of velocity on C:P is probably due to the higher levels of APA in low-velocity, low PO4-P reaches, as the higher APA rates reflect an alternative pathway for acquiring sufficient PO4-P to sustain periphytic growth and metabolism. These results have important implications for stream ecosystem function because of the increasing frequency of extreme weather events associated with the climate change, particularly droughts that reduce or eliminate perennial stream flow, and further illustrate the important effects of stream flow on biogeochemical processes.