Many of our big lakes are man made and used primarily for water storage. Those lakes are mostly located in the foothills and the dams that hold the water back are built in canyons. At full pool, some of these lakes can get pretty deep...400'+ at New Melones for example. It's not uncommon for us to have no rainfall of any significance between mid-June and the beginning of November, hence the drop in water levels. It effects us anglers in trying to launch boats on ramps that are high and dry, or have a 5mph speed limit because of unmarked hazards. The fish are still there, however, but now it's a matter of fishing structure you haven't seen before.
The natural waters, Tahoe, Clear Lake, and the Delta aren't affected quite as much as the others. For example, an 8' drop on Clear Lake is a big deal.