highest recorded flow was on February 20, 1986. During the dry season of July through September, the bypass carries low to zero flow. Although the Sacramento River nominally begins near Mount Shasta, the true hydrological source of the Sacramento River system is the Pit River, which is by far the largest of the three rivers flowing into Shasta Lake. At the USGS Montgomery Creek gauge, the average flow of the Pit River was for the period 1966–2013. By comparison, the Sacramento River at Delta gauge, a few miles above Shasta Lake, recorded an average of for the period 1945–2013. The McCloud River had an average discharge of for the 1967–2013 period. Since the 1960s, the McCloud River flow has been reduced and the Pit River flow increased due to diversion of water for hydropower generation; however the total volume of water entering Shasta Lake remains the same. Before Shasta Dam was built, the volcanic springs feeding the Pit and McCloud Rivers provided the majority of river flow in dry summers when the Upper Sacramento and other tributaries slowed to a trickle. Monthly combined discharge of Sacramento River at Freeport and Yolo Bypass near Woodland (cfs) Watershed The Sacramento River's watershed is the largest entirely in California, covering much of the northern part of the state. The endorheic (closed) Goose Lake drainage basin in southern Oregon, however, has been known to overflow into the Sacramento River system during particularly wet years. The Sacramento River basin generally lies between the Sierra Nevada and Cascade Range on the east and the Coast Ranges and Klamath Mountains in the west, although the part of the basin drained by the Pit River extends east of the Cascades. The Pit River, has the distinction of being one of three rivers that cut through the main crest of the Cascades; its headwaters rise on the western extreme of the Basin and Range Province, east of major Cascade volcanoes such as Mount Shasta and Lassen Peak. The other two are the Klamath River and Columbia River. By discharge, it is the second-largest contiguous U.S. river draining into the Pacific, after only the Columbia River, which has almost ten times the flow of the Sacramento. The Colorado River, which reaches the Gulf of California just south of the US-Mexico border near the southeast part of the state, is far larger than the Sacramento in terms of length and drainage