A study was made to evaluate the effects that treated sewage has on some characteristics of water quality in a reach of the Merced River near Yosemite National Park. Streamflow and water-quality data were collected from July through October 1975 and from July through November 1977 at five stations on the river and at an auxiliary station on the South Fork of the Merced River before and after a sewage treatment plant near El Portal began discharging treated effluent into the river in January 1977. Data collected in 1977 coincided with drought conditions in the Merced River drainage basin.

On-site measurements included streamflow, water temperature, specific conductance, pH, total alkalinity, and dissolved oxygen. Diel measurements were made at selected stations to determine the daily fluctuations of dissolved oxygen, temperature, alkalinity, pH, and specific conductance. Water samples were analyzed for nitrogen, phosphorus, and silica. Periphyton samples were collected from artificial substrates for taxonomic and biomass determinations.

Chemical analyses of water for plant nutrients indicated (1) an increase in the concentration of inorganic nitrogen immediately below the treated sewage effluent, (2) uniformly low phosphorus concentrations above and below the effluent, and (3) silica concentrations above and below the effluent greatly exceeding the minimum concentrations required for diatom growth and production. Diel measurements of dissolved oxygen in the reach below the effluent showed substantial sag during the night with supersaturation during the day, indicating considerable in-stream primary production. Measured and observed periphyton growth suggest that sufficient quantities of plant nutrients were available to support periphytic diatom blooms in the Merced River prior to the operation of the treatment plant during near-normal flow conditions. Nutrient availability was also sufficient to support both periphytic diatom and green-algal blooms above and below the treated sewage effluent during drought conditions. Greatest algal production was observed in early autumn