The Colorado River is the single largest source of water in the southwestern United States and northwest Mexico; however, before massive dam projects tamed the river in the 20th century, its flow was far from dependable. Annual discharge from the Colorado River and its tributaries ranges from , and 10-year averages may fluctuate as much as . Flooding, and the river's enormous silt or sediment load, created problems for settlements in the Lower Colorado River Valley and navigation on the lower portion of the river. During droughts, there was too little water available for irrigation. In 1904, the Colorado River was accidentally redirected after it damaged a canal gate in Mexico, causing the river to flood part of California's Imperial Valley and create the Salton Sea. After this catastrophe, California and Arizona began to call for a dam to control the tempestuous river.