Water-quality assessment of the Merced River, California

The Merced River and its major tributaries have been subject of water-quality and water-quantity studies by local, State, and Federal agencies since before 1900. Data have been compiled and analyzed, and even though water-quality problems exist in the basin, the water generally is of good quality for most of the beneficial uses defined by the California State Water Resources Control Board. Water-quality objectives for dissolved oxygen, pesticides, and pH were violated in some parts of the basin. The most likely cause of the dissolved-oxygen and pesticide violations is the return of agricultural irrigation water to the river in the lower 30 miles of the river. Violations of pH objectives occurred only in the upper drainage and were likely due to naturally occurring, poorly buffered water. Water quality is currently being monitored at three stations in the basin by California Department of Water Resources, at one station by the U.S. Geological Survey, and at several sites by the National Park Service. Modifications to the current water-quality monitoring program are proposed to gain further information on dissolved-oxygen fluctuations and pesticide concentrations in the lower river and to investigate nutrient input to Lake McClue. (USGS)