Sort of. It's true for the urban and coastal areas - in fact, there's a landslide & flash flood watch for the CZU burn scar and most of the North Bay right now

Sort of. It's true for the urban and coastal areas - in fact, there's a landslide & flash flood watch for the CZU burn scar and most of the North Bay right now. Large parts of the Bay Area mountains drain into captured reservoirs, though. The eastern slope of the SF peninsula all drains into Crystal Springs reservoir. Most of the inner Berkeley Hills & Briones Regional Park drains into the San Pablo reservoir. The mountains between Oakland and Danville drain into Upper San Leandro reservoir. Sunol and the lower Tri-Valley drains into San Antonio reservoir. Henry Coe State Park and the surrounding ranchlands (which is huge, bigger than all of the urbanized Bay Area put together) drains into the Calaveras and San Luis reservoirs, as well as Anderson & Coyote Lakes. Anderson Lake, last month, was at 3.5% capacity. So yes, this is a very welcome rainstorm, and much of it will be captured for municipal water. If it came on the heels of a very wet season (like the 2017 atmospheric river that flooded Coyote Creek and threatened to overtop Anderson Lake), they yeah, it'd be wasted, but that's not the situation today.