Too bad they keep taking out those dams so they could have stored most of that water for this summer/fall.
good thing we still have Sepulveda Dam (thank you Army Corps of Engineers) last reading of rainfall there overnight was 9.5 inches.
Too bad LA basin is right on the coast and there is a mountain range on the east side of the basin with the ocean to the west. There is no way to store the water without flooding a portion of the city with has built out over the entire LA basin floor and up into the hills yes those Hollywood hills. You would have to pump billions of gallons up over the basin divide and into the central valleys area. Edwards AFB sits in the bottom of the closest valley to the LA basin...The other valleys are the yucca valley that flows down to the salton sea you would flood Palm Springs and some very rich unhappy people. Look at Google maps in terrain mode then zoom out to the LA basin it’s a bathbowl against an ocean surrounded by other housing devs.
The best LA can hope for is to build floating ring dikes offshore at the river outlets a few miles in diameter and touching bottom at shore and out to 100 feet depth or so let the deeper parts float. Fresh water is less dense than seawater so when it flows out into the inside of the ring dikes it displaces the seawater downward and out the open bottom end.
What you end up with is a pinned in area of floating fresh water offshore that you can then pump from the top down over a few weeks until it mixes naturally back to seawater salt levels. The LA basin has a large aquifer under it pump that fresh water into that aquifer as underground storage like a lake underground but with zero evaporation losses. Added benefit is all the sediments sink to the open bottom end and flow down dip offshore to the basin depths just like they would.naturally had you not trapped the outflows this is important from environmental protection.