The Stanislaus River watershed drains and is divided into two distinct sections – the mountainous upper watershed, where the vast majority of its flow originates, and the narrow, heavily developed lower watershed where it flows across the San Joaquin Valley. Goodwin Dam has traditionally been considered the dividing line between the upper and lower watersheds. Elevations in the watershed range from less than at the confluence with the San Joaquin River to over in the high Sierra Nevada. Annual precipitation varies from in the valley regions to or more at higher elevations; at elevations above most precipitation falls as snow.