San Juan Creek is a 29-mile stream draining a 133.9 square mile watershed that originates in the southern Santa Ana Mountains within Cleveland National Forest in Riverside County and flows west and south through San Juan Canyon before joining with Arroyo Trabuco in Orange County near San Juan Capistrano, ultimately reaching the Pacific Ocean at Doheny State Beach. State Route 74, the Ortega Highway, parallels portions of the upper watershed. This watershed encompasses approximately 456 square kilometers in southern Orange County and represents designated Critical Habitat for the Southern California Steelhead Evolutionarily Significant Unit. The creek system historically supported diverse fish populations, though pollution, habitat loss, and introduced species have significantly impacted native fish communities. The lower 2.7 miles of San Juan Creek and much of its tributaries have experienced channelization, with concrete-lined sections in the lower reaches, though the upper portions within Cleveland National Forest maintain more natural character.