The San Gabriel River originates in the San Gabriel Mountains of Southern California, flowing south through Los Angeles County before emptying into the Pacific Ocean. Its two main tributaries, the East Fork and West Fork, converge near Azusa to form the main stem. The river stretches approximately 60 miles in total length.

For paddlers, the San Gabriel offers a range of whitewater opportunities in its upper reaches. The East Fork provides the most reliable paddling, with a popular 4-mile Class III-IV+ run from East Fork Station to Camp Oak Grove. This section features a constant gradient averaging 105 feet per mile through cobble bars and numerous bends. The West Fork contains more remote and challenging sections, particularly above Cogswell Reservoir, where a stunning 10-mile wilderness run exists for expert paddlers during high water years. Below Morris Dam, a short 1.5-mile Class III-IV section gives a glimpse of the river's pre-dam character.

As the river exits the mountains, it becomes increasingly channelized and urbanized, with flatwater paddling opportunities in the lower reaches near Long Beach. The river's flow is highly seasonal, with the best paddling typically occurring from late winter through spring during snowmelt and rainy periods. El Niño years can provide exceptional conditions, opening up rarely run creeks and tributaries.

The upper watershed supports diverse plant communities, including chaparral species like Arctostaphylos glauca and Artemisia californica. Wildlife includes coyotes and various bird species. The river's course passes through the Angeles National Forest, offering scenic views of the San Gabriel Mountains.

Key access points include East Fork Station, Camp Oak Grove, and various spots along Highway 39. The nearby city of Azusa serves as a gateway to the upper river sections.

Historically, the San Gabriel River played a crucial role in the development of Los Angeles County, providing water for agriculture and later urban growth. Today, it remains an important water source and recreational asset for the region.

Paddlers should be aware of potential flash flood dangers, especially in narrow canyon sections. Some areas may require permits or have seasonal closures. Multi-day trips are possible in the more remote upper sections, but most runs are typically done as day trips due to the river's proximity to urban areas.