Feel free to look up Terminus Dam at Lake Kaweah and Schafer Dam at Lake Success. Both runoff rivers flow AWAY from Los Angeles and basically drain into other lakes or into the soil (at least as far as I can tell from the maps). There was no plan to capture any of this water, and Trump is demanding the release of water from several more dams shortly.
So not only has he wasted water which won't be used to help fight the fires (and wouldn't have been of any use anyway), but he has greatly depleted the reservoirs used to grow crops this coming Summer. He's not trying to fight fires, he's just depleting the food that the US will have available this year.
Trump -- too dumb to know that water doesn't flow uphill.