Concentrations of soluble and total phosphorus in runoff from 16 grassed and cropped watersheds were measured over the past decade. Six watersheds drained into two flood detention reservoirs, where phosphorus concentrations were also measured at the inflow, within the lake, and at the outflow over a two-year period, to assess the effects of agricultural management practices on water quality. Soluble and total phosphorus concentrations in runoff from the native grass (133 and 343 mgP m−3, respectively) and cropped watersheds (215 and 2423 mgP m−3, respectively) were similar to phosphorus concentrations at the inflows to the reservoirs. Adsorption, biological uptake, and suspended sediment settling reduced soluble and total phosphorus levels 75 and 93 percent, respectively, in outflow from the reservoir with the grassed drainage basin and 72 and 96 percent, respectively, for the cropped drainage basin reservoir. Mean annual chlorophyllaand Secchi depths classified both reservoirs as eutrophic, even though only small amounts of fertilizer phosphorus were applied in their watersheds. Chlorophyll a and Secchi depths predicted from flushing-corrected inflow phosphorus concentrations were within the 95 percent confidence intervals of trophic response curves. Although flushing-corrected inflow phosphorus concentrations from native grass were appreciably lower than from cropped land, all management practices contributed runoff capable of accelerating eutrophication of the reservoirs.