Course
The original headwaters of Oso Creek were in a small canyon in the south-central part of the Santa Ana Mountains. A dam was built across this canyon, flooding it to create Upper Oso Reservoir, which now forms the headwaters of the creek, fed by three small seasonal streams. After leaving the reservoir, Oso Creek crosses under California State Route 241 and for the next mile (1.6 km) of its course, it flows through a narrow riparian corridor surrounded by residential areas in the city of Mission Viejo. The creek enters an underground culvert, bends east and south, and enters Lake Mission Viejo, 12 miles (19 km) from the mouth.
Downstream of the artificial lake, Oso Creek flows through a golf course, then after flowing through another narrow canyon, receives an unnamed tributary from the left, 10.5 miles (16.9 km) from the mouth. It then bends slightly to flow southwest and enters a culvert under Marguerite Parkway, emptying into another golf course, 8 miles (13 km) from the mouth. The creek then crosses under Interstate 5 and flows between the highway and Camino Capistrano through a series of freshwater marshes. The La Paz Channel, its largest tributary, joins here on the right. It passes the Galivan Basin, which functions to capture floodwaters from Oso Creek, on the right bank, and receives from the right a second unnamed tributary, 7.5 miles (12.1 km) from the mouth. This tributary actually flows through the Galivan Basin before meeting Oso Creek.
The creek then bends southwest around several shopping centers near the convergence of Interstate 5 and California State Route 73, within the city limits of Laguna Niguel. Near this area, Oso Creek enters a concrete flood control channel with sloping sides, 5 miles (8.0 km) from the mouth. It then flows through a riprap lined channel before entering a concrete box culvert, which runs south under the state route. The creek then spills out into a natural channel, 1.2 miles (1.9 km) from the mouth, which it has incised to depths of 50 feet (15 m) or more, as the interstate bends southeast. Flowing through an agricultural area, the creek bends east to join Trabuco Creek inside San Juan Capistrano.
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