At Caspers Wilderness Park, the San Juan Canyon opens up into a fairly wide valley in the foothills of the Santa Ana Mountains. This reach of San Juan Creek is underlain by thick alluvial deposits and is still used for farming and grazing. It is joined by Bell Canyon from the right, and Verdugo Canyon Creek from the left. Trampas Canyon enters from the left and Cañada Gobernadora and Cañada Chiquita enter from the right. The creek flows through residential Rancho Mission Viejo, crosses under Interstate 5, and enters a concrete flood control channel, turning south and receiving El Horno Creek on the right. It receives its largest tributary, Arroyo Trabuco, from the right, then flows south toward the Pacific Ocean. The creek forms a fresh water lagoon at the northern end of Doheny State Beach, which overflows into Capistrano Bay during periods of high flow.