Abstract: An experimental diversion of acid mine drainage was set up near an abandoned mine in Saint Kevin Gulch, Colorado. A mass-balance approach using natural tracers was used to estimate flows into Saint Kevin Gulch. The diversion system collected about 85 percent of the mine water during its first year of operation (1994). In the first 2 months after the diversion, benthic algae in an experimental reach (stream reach around which mine drainage was diverted) became more abundant as water quality improved (increase in pH, decrease in zinc concentrations) and substrate quality changed (decrease in rate of metal hydroxide deposition). Further increases in pH to levels above 4.6, however, led to lower algal biomass in subsequent years (1995-97). An increase in deposition of aluminum precipitates at pH greater than 4.6 may account for the suppression of algal biomass. The pH in the experimental reach was lower in 1998 and algal biomass increased. Mine drainage presents a complex, interactive set of stresses on stream ecosystems. These interactions need to be considered in remediation goals and plans.