Santa Ana River, CA

The Santa Ana River is the largest river in Southern California of the United States. Its drainage basin spans four counties. It rises in the San Bernardino Mountains and flows past the cities of San Bernardino and Riverside, before rounding the northern tip of the Santa Ana Mountains and flowing southwest past Santa Ana to spill into the Pacific Ocean. The Santa Ana River is over 110 miles (180 km) long, and drains a watershed of 2,650 square miles (6,900 km2).

For its size the ecology of the Santa Ana drainage basin is quite diverse. It ranges from high peaks of the inland mountains to the hot, dry interior and semi-desert basin to the flat coastal plains. Its climates range from dry alpine to chaparral and desert, and the watershed as a whole is very arid. Relatively little water actually flows in the river or most of its tributaries. One of its largest tributaries, the San Jacinto River, rarely reaches the Santa Ana except in extremely wet years. The relative lack of vegetation also makes the river prone to flash flooding. Even so, a wide variety of animal and plant life has always been dependent on the river.

The Santa Ana River rises in Santa Ana Canyon in the southern San Bernardino Mountains, at the confluence of two tiny creeks, Heart Bar Creek and Coon Creek, at an elevation of 6,991 feet (2,131 m). Its highest sources are Dollar Lake, at 9,288 feet (2,831 m), and Dry Lake, at 9,068 feet (2,764 m), both on the northern flank of San Gorgonio Mountain, at the headwaters of a small left tributary, the South Fork Santa Ana River. The river initially flows west through this broad and deep gorge, and about 18 miles (29 km) from its headwaters, receives its first major tributary from the right: Bear Creek, flowing southwest from well-known Big Bear Lake. The river turns south, passing through the Seven Oaks Dam, flowing out of its canyon into the arid interior basin of San Bernardino County and Riverside County, and receives Mill Creek from the left as it winds westwards towards the city of San Bernardino. As it passes through the urban area, it receives City Creek from the right and enters a flood control channel with levees on either side.

Not long after the confluence with City Creek, Lytle Creek enters from the right. Lytle Creek is one of the largest tributaries of the Santa Ana river, rising in three forks in the middle San Bernardino Mountains and flowing southwest, becoming the Lytle Creek Wash before discharging into the main stem. From there, the Santa Ana flows southwest, and after passing through the city of Riverside, it discharges into the normally dry flood control reservoir formed by Prado Dam. Three major tributaries of the river join in the reservoir area: Cucamonga Creek and Chino Creek from the right, and Temescal Creek from the left. Temescal Creek drains the largest area of all the tributaries, because it provides the outflow from Lake Elsinore, which the San Jacinto River flows into. It is also one of the longest, at 32 miles (51 km) in length. Except during the wettest years, Temescal Creek contains little or no water because Lake Elsinore is not high enough to overflow.

After flowing out of the Prado Dam, the Santa Ana River cuts a second Santa Ana Canyon, a water gap, between the northern Santa Ana Mountains and the Puente Hills and Chino Hills, crossing into Orange County. The river roughly bisects the county as it flows southwest towards the ocean. The river is then entirely diverted into spreading grounds to recharge the aquifer of north Orange County, providing about half of the entire county’s municipal water. Downstream of there, the river serves only for flood control and waste drainage purposes, and typically has no more than a trickle of water. Passing the cities of Orange and Tustin, it receives Santiago Creek from the left as it enters the city of Anaheim. Here, the river is entirely confined to a concrete flood control channel between earthen levees. After crossing under Interstate 5 near Santa Ana the riverbed again becomes earthen as it flows to its mouth between Huntington Beach and Newport Beach. The river accretes in a small lagoon before flowing out to sea at the northern end of Santa Ana River County Beach.

Historically, the Santa Ana was named "the best stream in Southern California ". The steelhead is an anadromous fish, similar to salmon, that migrates up rivers and streams to spawn. Unlike salmon, which usually only reproduce once, steelhead may reproduce multiple times and have a much longer life span. Steelhead was once found along the entire main stem of the Santa Ana River, as well as on some of its main tributaries—Santiago Creek, San Antonio and Chino Creeks, Cucamonga Creek, Lytle Creek, City Creek, and Mill Creek. Few, if any, steelhead were present in Temescal Creek (although one of its tributaries was stocked in the 1930s) and none inhabited the San Jacinto River, because it is disconnected from most of the Santa Ana River system. Up to the 1950s, steelhead trout still migrated in from the ocean. Because of pollution and modifications to the river, it is now unlikely that steelhead use the river at all as a habitat, instead using smaller streams north and south of the Santa Ana to spawn. The exception is the presence of landlocked rainbow trout—the freshwater phase of steelhead—upstream of Seven Oaks Dam and in the upper reaches of a few tributaries.