information:filtering
Discharge

With an average flow at the mouth of about 265,000 cu ft/s (7,500 m /s), the Columbia is the largest river by discharge flowing into the Pacific from the Americas and is the fourth-largest by volume in the U.S. The average flow where the river crosses the international border between Canada and the United States is 2,790 m /s (99,000 cu ft/s) from a drainage basin of 102,800 km (39,700 sq mi). This amounts to about 15 percent of the entire Columbia watershed. The Columbia's highest recorded flow, measured at The Dalles, was 1,240,000 cu ft/s (35,000 m /s) in June 1894, before the river was dammed. The lowest flow recorded at The Dalles was 12,100 cu ft/s (340 m /s) on April 16, 1968, and was caused by the initial closure of the John Day Dam, 28 mi (45 km) upstream. The Dalles is about 190 mi (310 km) from the mouth; the river at this point drains about 237,000 sq mi (610,000 km) or about 91 percent of the total watershed. Flow rates on the Columbia are affected by many large upstream reservoirs, many diversions for irrigation, and, on the lower stretches, reverse flow from the tides of the Pacific Ocean. The National Ocean Service observes water levels at six tide gauges and issues tide forecasts for twenty-two additional locations along the river between the entrance at the North Jetty and the base of Bonneville Dam, its head of tide.