The Merced River originates on the western slope of the Sierra Nevada and flows through Yosemite National Park. The river drains 1,276 square miles before joining the San Joaquin River near Stevinson, Calif. Data were collected to permit descriptions of the river's quality during a hydrologic cycle, the natural and manmade factors in the basin that affect the quality, and the trends that might be used in developing procedures to reduce degradation in the future.