Until the opening of the Los Angeles Aqueduct in 1913, the Los Angeles River was the primary water source for the Los Angeles Basin, but much of its channel had extremely low discharge except during the winter rains. Unpredictable and devastating floods continued to plague it well into the 1930s (the most notable one being the catastrophic 1938 flood that precipitated the recall of then-mayor of Los Angeles Frank L. Shaw), leading to calls for flood control measures. The Army Corps of Engineers duly began an ambitious project of completely encasing the river's bed and banks in concrete, with only a trickle of water usually flowing down its middle. The only portions of the river that are not completely paved over are in the flood-control basin behind the Sepulveda Dam near Van Nuys; an 11-mile (17.7-km) stretch east of Griffith Park known as the Glendale Narrows; and along its last few miles in Long Beach.