Merced River - Watershed

Watershed

The drainage basin of the Merced River is located in the central Sierra Nevada, spreading across 1,726 square miles (4,470 km2). It can be characterized as the slightly smaller southern companion of the Tuolumne River, the major Sierran river just north of the Merced. On the south, the basin borders on the headwaters of the San Joaquin River itself. The Merced River watershed includes the subwatersheds of Illilouette, Echo, Tenaya, Yosemite, Bridalveil, Cascade, and Dry Creeks, as well as the South and North forks of the river, of which the South Fork, at 43 miles (69 km) long, is the largest. Tributaries to the South Fork include Bishop, Rail, Alder and Chilnualna Creeks. There are also many lakes in the watershed of the Merced River, including Merced Lake, Tenaya Lake, the Chain Lakes, May Lake, Lake McClure, and Lake McSwain.

Much of the basin of the Merced River is characterized by an alpine climate. The upper basin of the river receives heavy snowfall in the winter, which is usually enough to feed the river and its tributaries year round. However, most of the snowpack melts by the late autumn, reducing flows in the main stem and its tributaries significantly, and causing some smaller tributaries to dry up altogether. Snowmelt constitutes a majority of the river's springtime and early summer flow, and provides up to 85% of the flow above Happy Isles. The dry season depends more on groundwater to supply the river's flow. The middle and lower basin are dominated by an arid, Mediterranean or even semi-desert climate.

The Merced River is the third largest tributary of the San Joaquin River. Before irrigation started in the Central Valley and dams were constructed, the river's natural flow at the mouth was much higher than the current average of 661 cubic feet per second (18.7 m3/s), or about 479,000 acre feet (591,000,000 m3) per year. Upper on the river, at Happy Isles, the average flow is 355 cubic feet per second (10.1 m3/s). The United States Geological Survey has river gauges at three locations along the Merced River: at Happy Isles, above Lake McClure, and at the mouth. The first two record flows unaffected by dams and human intervention, but discharge at the mouth is chiefly controlled by New Exchequer Dam. The Lake McClure gauge, located at the former mining town of Bagby, is probably the most accurate gauge for flows overall. The average annual flow recorded there was 1,185 cubic feet per second (33.6 m3/s) from 1923 to 1966. A peak of 92,500 cubic feet per second (2,620 m3/s) was reported there on December 23, 1955. For the mouth gauge, the highest flow was only 13,600 cubic feet per second (390 m3/s) in 1950. Finally, for the gauge at Happy Isles, the largest flow ever recorded was 10,100 cubic feet per second (290 m3/s) in the 1997 Yosemite floods, which destroyed many campgrounds, roads, paths, and bridges in the valley.

The Merced River's primary human use is irrigation. The Merced Irrigation District (MID) operates most of the irrigation infrastructure, which supplies water to 154,394 acres (624.81 km2) of farmland. As a whole, the system includes about 4,000 sets of control gates and 793 miles (1.276 km2) of canals. Irrigation has taken most of the water out of the lower river, which now rarely reaches the sea. Water that does return to the river does so in for form of irrigation return flows, which carry pesticides, fertilizer and other pollutants. The MID is federally required to allow at least 15,000 acre feet (19,000,000 m3) of water annually to flow continually down the river, not including flooding. The water has allowed the San Joaquin River below the Merced River confluence to contain water, while above the confluence all of its water is usually diverted for irrigation.