The San Diego Creek Watershed covers 112.2 square miles in central Orange County. In includes portions of the cities of Costa Mesa, Irvine, Laguna Woods, Lake Forest, Newport Beach, Orange, Santa Ana, and Tustin. Its main tributary, San Diego Creek, drains into Upper Newport Bay. Smaller tributaries include Serrano Creek, Borrego Canyon Wash, Agua Chinon Wash, Bee Canyon Wash, Peters Canyon Wash, Sand Canyon Wash, Bonita Canyon Creek, and the Santa Ana Delhi Channel.

The U.S. Army Corps of Engineers completed a Watershed Reconnaissance Study in 1998 and initiated the Feasibility Study phase in 1999. In addition, the San Diego Creek watershed is the only one in Orange County with TMDL limits adopted or in development. These regulations will limit the "Total Maximum Daily Load" of sediments, nutrients, pathogens and toxics entering waters of the creek and bay.