Runoff from the Ventura River gave Lake Casitas some much-needed relief over the past several weeks until about five feet of muck got in the way. With no imported water, the lake depends on local rainfall and river runoff, including through a diversion canal above the lake. In recent years, however, diverting water happened a lot less as a years-long drought dragged on. Rainstorms got fewer and much further between. Lake levels dropped and the local water supply shrank.