The Westminster Watershed covers 74.1 square miles in the southwestern corner of Orange County. It includes portions of the cities of Anaheim, Cypress, Fountain Valley, Garden Grove, Huntington Beach, Los Alamitos, Santa Ana, Seal Beach, Stanton, and Westminster. Three main tributaries drain this watershed. The Los Alamitos Channel drains into the San Gabriel River. The Bolsa Chica Channel empties into the Anaheim Bay-Huntington Harbour complex. The East Garden Grove-Wintersburg Channel drains through Bolsa Bay into Huntington Harbour.

In 2001, the U.S. Army Corps of Engineers initiated a comprehensive watershed study. A Reconnaissance Study was completed in June 2001. Although it is titled the "Westminster Watershed Reconnaissance Study", it covers three Orange County watersheds: Coyote Creek, Carbon Creek, and Westminster. In fall 2002, the Corps of Engineers is scheduled to begin the Feasibility Phase of the watershed study on the Westminster Watershed while a separate Feasibility Study to be conducted for Coyote Creek and Carbon Creek watersheds.