Water-quality study of a reach of the Merced River in Yosemite National Park and vicinity, California, April 1973 through September 1974

A 25-mile reach of the Merced River was studied to provide the U.S. National Park Service with water-quality data prior to the operation of a new sewage treatment plant. Water-quality variables investigated were discharge, temperature, major chemical constituents, selected trace metals, specific conductance, suspended sediment, turbidity, dissolved oxygen, pH , total alkalinity, carbon dioxide, total organic carbon, nitrogen, phosphorus, algal growth potential, coliform bacteria, periphyton, and benthic invertebrates. Results indicate that water in the reach of the Merced River studied is a dilute system of good quality. Mean concentrations of nitrogen and phosphorus were highest in the Merced River at Rancheria Flat. Algal growth potential tests revealed that phosphorus was limiting algal growth at Happy Isles Bridge, El Capitan Bridge, and South Fork Merced River, which nitrogen was limiting it at Rancheria Flat. Input of excessive nitrogen and phosphorus concentrations in the Merced River during the summer low-flow period, and mixing with warm tributary inflow could cause excessive algal growth and production. (Woodard-USGS)