The Santa Ana River is the largest watershed in Southern California. The river originates in the San Bernadino Mountains and flows roughly 100 miles west/southwest to the coast near Huntington Beach. It is fed by more than 50 tributaries. The upper river is a relatively natural area, while the lower river has been dammed and channeled through highly urbanized areas.

An article in the San Bernadino Guardian in October 1870 reported that a group of fishermen caught 140 trout on the south fork of the Santa Ana, most weighing four pounds. Many similar reports from that era exist. Over the years though, the trout population has declined due to human activity such as logging and water diversion for farming.

Just south of Big Bear Lake, the south fork and main fork of the Santa Ana join. Here, the California Department of Fish and Wildlife plants hatchery rainbow trout in spring and throughout fall. Both of these forks benefit from these stockings. There are also wild trout in the Santa Ana, although many are reported to be in the 4-8 inch range.

Flows are often very low due to prolonged California droughts. Much of the river is no more than 15 feet wide and can be walked across with ease. The water itself is typically clear and the shoreline is bordered by oaks and pines. Conservation groups are working hard to protect native species in the lower river, mainly the Santa Ana Sucker and Arroyo Chub. These species are listed as threatened under federal legislation.

Much of the upper river can be accessed via Seven Oaks Road. A few campgrounds and lodges are also present along the river.