[3] The main inflows are Cuddy Creek and small intermittent streams originating in Bear and Crane Canyons, draining a total of 56 square miles (150 km2) into the lake.

Although Castac Lake itself is saline, or salty, the abundance of freshwater springs nearby made it an attractive area for human settlement.

[8] The lake was first seen by Spanish explorer Pedro Fages who in 1772 led the first European expedition to cross the Tehachapis via Tejon Pass into the San Joaquin Valley.

[11] The Rancho Castac was eventually acquired by Edward Fitzgerald Beale, who founded Tejon Ranch (at one point the largest private landholding in California).

[16] Due to the local geology, Castac Lake contains naturally elevated levels of arsenic, boron, selenium, and aluminum, which concentrate during drought conditions and are flushed out in occasional floods.