true. BUt it's a homemade problem. It's what comes from LA and SF draining the last little droplet of water from the mountains and their foothills. If they didn't, there'd be forrests and shrubs with wide roots that hold the soil in place like a net. As it is, in these semi-arid regions plants need carrot-like long roots to reach what little groundwater is left and these can't hold the soil. They are rather like a grid through which the mud slides easily.
Have you ever been in the Santa Ana Mountains above LA? It is a desert now with dead lakes with mummified fish. Only a century ago the area was inhabited by native tribes that lived on fishing and farming. Then came the big city with her insatiable thirst and the whole area got dried out. The water pipes for LA alone reach over 120 miles into the mountains. Those for SF are even longer.
The water supply isn't endless. There are already shortages. But if you hear that the average German uses 8 gallons of water a day and the average US-American 40, there is a lot of potential for saving water and rescuing your environment there.