The confluence of the Middle and North Forks marks the start of the Stanislaus River proper. It flows southwest through a canyon to the New Melones Lake reservoir in the Sierra Nevada foothills, forming the boundary between Calaveras County (west) and Tuolumne County (east). At the reservoir, it is joined by the smaller South Fork, which descends for from the Sierra Nevada to the east. Much of State Route 108 (the Sonora Pass Highway) runs parallel to the South Fork, as well as the upper part of the Middle Fork, linking a number of small communities in the upper Stanislaus basin. At the lower end of New Melones Lake is the tall New Melones Dam, the sixth tallest dam in the U.S., completed in 1979 for flood control, irrigation, hydropower generation and fisheries management. Below New Melones, the river flows through the smaller Tulloch Reservoir before reaching Goodwin Dam, the oldest dam on the river (completed 1913) where large volumes of water are diverted for irrigation.