Why would it change it forever, Death Valley gets torrential rains and flash floods a few times a year already.
> The 1.46 inches of rain recorded at Furnace Creek is still preliminary data, and mere drops away from the all-time record of 1.47 inches. The rain caused widespread damage and closure of all park roads.
https://www.nps.gov/deva/learn/news/death-valley-experiences...
If Hillary is expected to drop 8-15 inches in Death Valley, that's going to be quite a bit more water.