The state of California has a large number of reservoirs. Many, though not all, of these are publicly accessible in parks and recreation areas, and these lakes contribute significantly to the state's beauty, as many of them are set in the Diablo Range and the Sierra Nevada.
California's climate is Mediterranean, with most rain and snow falling in winter across much of the state; and in many populated regions such as Sacramento and the Bay Area, summer precipitation is almost unheard of, while winter precipitation is abundant. Therefore reservoirs are necessary to support California's large and rapidly growing population throughout the year.
Winter precipitation, which falls as snow in the Sierra Nevada and as rain in the Diablo Range, accumulates into rivers and creeks in canyons and valleys. Rivers flowing from the Sierra Nevada ultimately lead to the San Joaquin Valley and from there to the Sacramento Delta, while those from the Diablo Range drain into the San Francisco Bay via the Alameda and Coyote Creeks. City governments such as those in San Francisco and Los Angeles recognized the need for water and, despite environmental concerns, built dams (therefore forming reservoirs) on these rivers and creeks for the purpose of water storage. Complex underground canals, such as the Hetch Hetchy Aqueduct, along with canals in the Central Valley, brought water from the new rural reservoirs to major cities. One of the most beautiful valleys in the U.S., Yosemite Valley's neighbor and former twin Hetch Hetchy was one of the most significant casualties of this surge in development, and today the valley has been replaced by a reservoir, although the scenery around the lake remains picturesque.
Increased urbanization of the south and western portions of the state, along with suburban sprawl surrounding the state's capital, caused the construction and expansion of existing reservoirs to continue, and there are few signs that the trend will end as the state's population continues to expand rapidly. Unfortunately, many canyons and valleys are no longer to be found, but in many places this has been compensated with the creation of recreation areas surrounding the reservoirs, and activities such as boating and fishing have become possible in previously dry riverbeds and temporary streams. (In the case of Los Vaqueros Reservoir, for example, water supply is not from the nearby dry canyons, but instead from the Sacramento Delta, which has a varying level of salt content. The lake/reservoir itself is ironically in a semi-desert.)
Since the depth of canyons and mountain valleys makes these places ideal locations for reservoirs, California's reservoirs are found in mountain ranges or the foothills of them. The abundance of remote canyons within the Diablo Range, along with the great depth of the Sierra's canyons, valleys, along with more canyons within the foothills, has concentrated reservoirs in these areas. It would be an impossible task to list all California's reservoirs (there are so many of them), but some of the large and particularly beautiful of them are included here. Some of them are additionally significant for historical and/or meteorological context, such as Lake Oroville, which surpassed its maximum capacity due to high precipitation during a wet season, while others during the 2010s drought ran almost completely dry. Despite the fact that droughts and floods must have caused headaches for engineers and government officials throughout the many natural disasters of the state's history, many of these events' impacts and the stories of such events have been largely forgotten (with the exception of the Lake Oroville incident), and incidents that ought to be better-known may now be marked with nothing more than an inconspicuous plaque beside a flooded building.
- 1 Folsom Lake is near — as its name implies — the city of Folsom, which is in turn northeast of Sacramento, the state capital; the dam can be seen from one of the local roads.
- 2 Hetch Hetchy is north of Yosemite Valley, in Yosemite National Park. Once a valley not so different from its southern neighbor, it's now the location of a long, fairly narrow reservoir alongside which there's a hiking trail. The Hetch Hetchy Aqueduct runs underground toward the San Francisco Bay Area.
- 3 Lake Clementine near Auburn is a moderately sized reservoir in that region's deep, narrow canyons. Upstream from an extremely high bridge, a trail and road go to the reservoir, the trail going directly underneath the bridge, although the dam (pictured in banner) is spectacular in itself.
- 4 Lake Del Valle is surrounded by Del Valle Regional Park, which is dominated by hills in the north — which overlook cliffs, golf courses, vineyards, and of course the lake — and mountains covered with pine woodlands to the south. There are two entrances to the park, one in the north, from which you must hike to the top of the ridge to the see the lake, and another in the south, which goes directly to the lake's southern end. Arroyo Del Valle flows into the lake from this end, though the stream's water level is inconsistent. There are some campsites at the southern end of the park.
- 5 Lake McClure and 6 Don Pedro Reservoir are in the Sierra Nevada foothills.
- 7 Lake Nacimiento is close to its neighbor the 8 Lake San Antonio, not to be confused with the other San Antonio Reservoir, also a reservoir, in the East Bay. Lake Nacimiento is large and beautiful, yet in a fairly remote location not too far from Paso Robles. Apart from its nearby twin, which went almost completely dry during the 2010s drought, there are few important points of interest in the area; it's really one of California's "hidden gems." The lake's shape, when viewed on a map, resembles that of a dragon.
- 9 Lake Oroville is famous for exceeding its maximum capacity during flooding the 2010s as a result of a particularly wet rainy season. The emergency spillway was used and thousands of residents of nearby Oroville were evacuated. The lake itself is one of the larger reservoirs in the Sierra foothills.
- 10 Los Vaqueros Reservoir entered the "big league" of reservoirs in the early 2010s when its dam was raised to accommodate a larger reservoir, and the marina and some trails were moved for the sake of these changes, which were put into practice to bring more water to Contra Costa County. The west side of the lake, and the region to the north of it, are part of a preserve, and there are miles of hiking, though some trails away from the lake are rather remote. To the south is the giant Altamont Pass Wind Farm, while to the east of the lake are the Vasco Caves, which contrary to their name consist more of rocky terrain than actual caves.
- 11 San Luis Reservoir in the eastern foothills of the Diablo Range is a large lake surrounded by the golden hills of the western edge of the Central Valley, southeast of the Bay Area and east of Gilroy.
- 12 Shasta Lake is near Mount Shasta, one of the state's highest mountains, at the northern end of the Central Valley were the Coast Ranges meet the Cascades.
Reservoirs in remote (or even relatively remote) wilderness areas are popular with bears, and mountain lions are known to live in wilderness regions of the state. Reservoir water levels, which when critically low can cause fines for excessive water use and other measures, can be found at the California Department of Water Resources website.