the lower Sacramento River is rejoined by the Deep Water Ship Channel and the Yolo Bypass and curves southwest along the base of the Montezuma Hills, forming the border of Solano and Sacramento Counties. This part of the river is dredged for navigation by large oceangoing vessels and averages three-quarters of a mile (1.2 km) across. North of Antioch and Pittsburg, the Sacramento and San Joaquin Rivers join at the head of Suisun Bay, marking the official end of both rivers. The combined waters flow west through Suisun Bay and the Carquinez Strait into San Pablo Bay and San Francisco Bay, joining the Pacific at the Golden Gate. Discharge Following the Columbia River, the Sacramento is the largest river by discharge on the Pacific coast of the continental United States. The natural runoff of the river is 22 million acre feet (27 km3) per year, or about . Before dams were built on its tributaries, the river flooded up to during the rainy season, equal to the flow of the Mississippi River. Late summers of particularly dry years could see flows drop below . Large volumes of water are withdrawn from the Sacramento River for irrigation, industry and urban supplies. Annual depletions (water not returned to the river after use) are about 4.72 million acre feet (5.83 km3) for irrigation and for urban use. An additional 7.61 million acre feet (9.39 km3) is reserved for environmental uses, primarily to maintain a minimum fresh water outflow in the Delta to combat salinity. The U.S. Geological Survey (USGS) has stream gauges on 25 locations along the Sacramento River, although not all of them are currently operational. The ones currently in operation are at Delta, California (near the source at Mount Shasta), at Keswick (near Redding), Colusa (about halfway down the river), Verona, and Freeport. The Freeport gauge, which sits just downstream of Sacramento, provides a relatively good metric of the annual outflow from the Sacramento River Basin. The average flow between 1949 and 2013 was . The maximum recorded flow was on February 19, 1986; the lowest was on October 15, 1977. Flow in the Yolo Bypass, a relief channel designed to carry a portion of the flood waters in order to protect the Sacramento area, is not measured by the Freeport gauge. A separate gauge on the bypass recorded an average throughput of between 1939 and 2013, mostly from December–March. The