The Aliso Creek drainage basin lies in the south central part of Orange County, roughly halfway between the Santa Ana River and the border of San Diego County. It is an elongated area of , ranging from about a mile (1.6 km) in width in the north to wide in the south. The watershed is characterized by rolling hills, with very little flat land except in the alluvial valleys along Aliso Creek. The portion of the Santa Ana Mountains in the Aliso Creek watershed top out at approximately , while the San Joaquin Hills rise to at Temple Hill, locally called Top of the World, in Laguna Beach west of Aliso Canyon. Aside from Laguna Niguel Lake, an impoundment of the Sulphur Creek tributary, there are no major bodies of fresh water.