Most of the erosion that shaped the modern Stanislaus River basin is believed to have occurred during glacial and interglacial periods in the Pleistocene, starting about 1 million years ago. During the ice ages California had a much wetter climate; average river flows in the past may have been as high as what is considered "flood stage" today. The climate was also cold enough to support large glaciers in the Sierra Nevada at elevations above . These glaciers carved large U-shaped valleys in the high elevations, and supplied vast volumes of meltwater which accelerated erosion along the foothill canyons of the Stanislaus River. During the last glacial period the main Stanislaus glacier was up to long; during previous ice ages it may have extended as far as . Although glaciation did not have as dramatic an impact on the Stanislaus River watershed as it did further south (such as at Hetch Hetchy and Yosemite), many major features of the upper Stanislaus were sculpted by ice, such as the Clark Fork valley and the Middle Fork valley at Donnell Lake.