farms and towns along its course. By the early 20th century engineers had realized not all the floodplains could be safely reclaimed, leading to the intentional creation of flood bypasses where development is limited to annual crops and recreational uses. Further south, much of the Delta region is actually below sea level: subsidence caused by wind erosion and intensive farming have caused the land in the delta to gradually sink since the late 19th century. Many of the delta islands would be underwater if not for the maintenance of the levees and pumps that keep them dry. Some of the "islands" are now up to below the adjacent channels and sloughs. Land use The Sacramento River watershed is home to about 2.8 million people; more than two-thirds live within the Sacramento metropolitan area. Other important cities are Chico, Redding, Davis and Woodland. The Sacramento River watershed covers all or most of Shasta, Tehama, Glenn, Butte, Plumas, Yuba, Sutter, Lake and Yolo Counties. It also extends into portions of Siskiyou, Modoc, Lassen, Lake (in Oregon), Sierra, Nevada, Placer, El Dorado, Sacramento, Solano and Contra Costa Counties. The river itself flows through Siskiyou, Shasta, Tehama, Butte, Glenn, Colusa, Sutter, Yolo, Sacramento, Solano and Contra Costa, often forming boundaries between the counties. Many of the mountainous regions of the watershed are administered by the U.S. Forest Service. The Sacramento River watershed includes large areas of coniferous forests in the Mendocino and Trinity National Forests in the Coast Ranges, Shasta and Lassen National Forests in the southern Cascades and the Plumas, Tahoe and Eldorado National Forests on the western slopes of the Sierra Nevada. The watershed also has Lassen Volcanic National Park, which covers centered on Lassen Peak, the southernmost Cascade volcano. Whiskeytown-Shasta-Trinity National Recreation Area, which is over in size, straddles much of the upper Sacramento and Trinity Rivers, and is named for the three local reservoirs (Shasta Lake, Trinity Lake and Whiskeytown Lake) which are popular tourist areas. Many other state parks and recreation areas lie within the watershed. Geology By geologic standards, the Sacramento is a fairly young river; the borders of its watershed began to form only a few million years ago as magma welling up below the Earth's crust pushed up by the Pacific Plate colliding with the North American Plate caused the formation of the Sierra Nevada. Although mountains had existed as early as 100 million years ago