LA does have levees along a number of (usually) dry river basins. When they are dry they are very very dry, but when they are wet they are HORRID! LA has not yet figured out how to deal with water that doesn't come from a pipe, but they're trying. Very Trying.

I remember oh, 45 years ago standing alongside a weir on Eagle Creek, just off the Columbia near Bonneville Dam, watching hatchery employees harvesting the raw material for the salmon hatchery. The weir was truly full of salmon and the routine was for the workers to club the females and slit their eggs into 5 gallon buckets. Then they'd grab the males and milk them over the buckets. Stir the resulting mess and Bob's your uncle. Didn't save the salmon runs, though.