One thing to keep in mind that a natural wetland area in the LA basin in 1800's is going to be full of mosquitoes that carry malaria. People were highly motivat

One thing to keep in mind that a natural wetland area in the LA basin in 1800's is going to be full of mosquitoes that carry malaria. People were highly motivated to eliminate all standing water that mosquito larva could breed in. Thus, fill in the marshes/wet meadows to create dry land and/or dredge to make lakes/ponds. Most current First World inhabitants greatly underestimate the scourge of malaria, especially before modern medicine allowed for a cure. The herd immunity like situation of modern California to endemic malaria is due to a wide assortment of factors and is maintained with relatively small cost today, but I imagine malaria could tip back to being widespread pretty quickly.