When Californians talk of lakes, they usually mean reservoirs, the 1500 or so artificial bodies of water behind dams. Alternately, they may be referring to the 4,000 or so natural lakes in the Sierra Nevada or to one of the few large natural lakes in the state, such as Lake Tahoe or Clear Lake. But some of the most interesting lakes in the state draw our attention mainly when demand for water threatens to dry them up. These are terminal lakes, that mostly depend on seasonal rain or snow melt to maintain them as lakes. They are called terminal lakes because water flows into the basins through streams, but leaves mostly by evaporation or sinking into underground aquifers. Each lake has its own unique chemistry and other characteristics, although most are highly productive so are important to migratory waterfowl and invertebrates.