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The effect of lotic-lentic interactions on the phytobenthic community was studied in the Colorado River below Glen Canyon Dam (GCD) Arizona. Major compositional changes in river phytobenthos occurred following high snowmelt inflows that filled the Lake Powell ell reservoir and resulted in high, steady discharges from GCD. Cladophora glomerata, the previously dominant alga, was partly displaced by a mixed mucilaginous phytobenthic assemblage consisting of miscellaneous algae, macrophytes, and bryophytes (MAMB). Relationships between abiotic parameters (discharge conductivity, nutrient concentration, and season) and biotic responses (composition and biomass of filamentous and epiphytic algae, and invertebrate biomass) were analyzed from 1995 to 1998. In addition, the response of the phytobenthos to an experimental flood (1275 m(3)/s) was examined in the tailwaters below GCD. Significant compositional changes occurred within 3 mo of lake filling and the onset of elevated discharges in June 1995. Relative phytobenthic composition by biomass prior Co June 1995 was: Cladophora glomerata = 915%, MAMB = 35% and cyanobacteria = 5.0% compared to 49.6 47.9, and 2.5%, respectively, after the initiation of high discharges. Cladophila and MAMB had an inverse relationship that fluctuated seasonally. Cladophora was positively associated with increased nutrient concentrations and loner discharge during summer-autumn whereas MAMB was positively correlated with deceased nutrients and higher discharge during winter-spring. Cladophora was positively associated with invertebrate biomass and also had greater epiphyton biomass and density than MAMB. The 1996 experimental flood resulted in a significant loss of MAMB biomass, in contrast to a minimal decrease in Cladophora and invertebrates. interactions between inflow patterns, reservoir chemistry, and discharge regime have an important effect on benthic community structure below dams. Resource managers need to consider these relationships and regard water resources above and below impoundments as a connected system when making management decisions.