Whittier Narrows, the natural water gap between the Puente and Montebello Hills that forms the southern entrance to the San Gabriel Valley. Here it is impounded by the Whittier Narrows Dam which also serves primarily for flood control. The Rio Hondo also flows through the Whittier Narrows, to the west of the San Gabriel. The Rio Hondo drains most of the western half of the San Gabriel Valley, approaching the San Gabriel River at the Whittier Narrows; south of there, it swings to the southwest and joins the Los Angeles River. In the Whittier Narrows they are connected by a short channel through which water can flow in both directions. Today the Rio Hondo is generally considered a separate stream and tributary to the Los Angeles River, but historically the two rivers sometimes joined the other, flowing to various outlets. The Rio Hondo sometimes changed course to join the San Gabriel River; alternatively, the San Gabriel sometimes shifted course into the Rio Hondo, merging into a single watershed with the Los Angeles River. Today, Whittier Narrows Dam controls the outflow from both rivers into their artificially fixed channels. During storms water is distributed based on the availability of space in the downstream channels. Lower river Below the Whittier Narrows Dam the river flows south-southwest across the coastal plain roughly defining the border of Los Angeles County and Orange County. It flows through Whittier and Pico Rivera and under the Interstate 5 to Downey, where the river becomes a concrete channel. It turns due south, crossing under Interstate 105 and the Metro Green Line, then crossing under SR 91 at Bellflower. The San Gabriel River Bike Trail parallels the river starting at Whittier Narrows for to the Pacific Ocean at Seal Beach. From Cerritos the river flows south-southeast until reaching its confluence with Coyote Creek, the largest tributary of the lower river, which drains much of northwest Orange County. A short distance below Coyote Creek the river bed reverts from concrete to earth. It passes under Interstate 405 and SR 22, past Leisure World and Joint Forces Training Base - Los Alamitos (Los Alamitos Army Airfield), and under the Pacific Coast Highway. It empties into the Pacific Ocean between Alamitos Bay and Anaheim Bay (to the south), on the boundary of Long Beach in Los Angeles County, and Seal Beach in Orange County. Geology The San Gabriel River, its canyons and