This website
Water Education gives a decent explanation of why the levels have dropped in recent years.
We also have to remember these are manmade structures. When the dams were built they served small cities with water by capturing the spring flow and holding it to be available later. Now they are serving many more households with water. People keep moving into areas with limited natural water flow. Regardless of the yearly variable water inflow into those reservoirs, when you get to the point of average water use being higher than the average water inflow into the reservoir, water levels will go down and stay there. Eventually the demand will be so high it will be back to the river it once was with no extra to fill the reservoir. At that point the suburbs will start to fade away because the cities will be getting all of the water. Eventually the desert will win and go back to the inhospitable place it once was.