The importance of Sierra Nevada water to California as a whole cannot be understated.  One statistic says it all -- About 60 percent of the water used by Californians today comes from the Sierra Nevada.

The rivers and streams of the Sierra Nevada are included in five of the nine major hydrologic, or water delivery, systems of California. Three of these, the Sacramento, San Joaquin, and Tulare Lake systems, capture all of the stream flow of the western Sierra Nevada. These systems are heavily influenced by the orographic, or rain shadow, effect of the range, which causes the warm and moist Pacific winds to shed their moisture content in the form of rain and snow as the wind currents sweep up the western side of the range. The combined runoff from snowpack, rain, and the base flow from the Sierra Nevada watersheds creates many streams that run year round. Because of the rain shadow effect, most streams of the range are found on its western slope. There are fifteen major rivers and watershed systems.  Most of these have numerous tributaries and forks higher up slope. All of the major rivers and many of their tributaries have been significantly altered in historic times by human impact. The rivers of the Sierra Nevada eventually all join the Sacramento and flow into the Delta. A significant portion of this water is diverted and transported through canals to Southern California.

The major river systems are:
American River
Cosumnes River
Feather River
Kaweah River
Kern River
Kings River
Merced River
Mokelumne River
Owens River
Sacramento River
San Joaquin River
Stanislaus River
Truckee River
Tuolumne River
Yuba River

The Sierra Nevada contains thousands of lakes. Many originate in cirques, or massive rock piles, created by melting glaciers. Other lakes and ponds result from rain and melting snow. These lakes vary in size from surface areas of 300 square miles to very small glacial ponds. As with its rivers, the western Sierra Nevada has the more numerous natural lakes.  The eastern Sierra Nevada has fewer, but generally larger lakes, with the most magnificent being Lake Tahoe.

There is evidence that droughts that lasting more than a 100 years have occurred.  In fact, recent studies indicate that long period of drought are the norm in Sierra Nevada history, and that we are presently in an unusually wet pattern.