Higher mountains and plateaus in the Central Arizona Highlands generally support southwestern mixed conifer forests, associated aspen and spruce-fir forests, and a small acreage of grasslands interspersed among the forested areas. Most of the major rivers in the region originate on headwater watersheds that support mixed conifer forests where annual precipitation, particularly snow, is high and evapotranspiration demands are lower than other vegetation types in the Central Arizona Highlands. Rich and Thompson (1974) reported that 6% of Arizona's surface water originates from the portion of the state occupied by mixed conifer watersheds (less than 0.5%).