The rainbow trout (Oncorhynchus mykiss) is a species of salmonid native to cold-water tributaries of the Pacific Ocean and are present in the Santa Ana River, which has its headwaters at the foot of the San Bernardino Mountains. The Santa Ana River is the largest river entirely within Southern California. Its drainage basin spans four counties, rising in the San Bernardino Mountains and flowing past the cities of San Bernardino and Riverside before cutting through the northern tip of the Santa Ana Mountains and flowing southwest past Santa Ana to drain into the Pacific Ocean. The Santa Ana River is 96 miles long, draining a watershed of 2,650 square miles.
Resident freshwater rainbow trout adults in riverine environments average between 1 and 5 pounds. more than four pounds. The coloration of the Santa Ana River variety of rainbow trout is a true rainbow, although brown trout are also present in the Santa Ana. The fish have blue-green or olive green hues, some black spotting over the length of the body and a reddish or even orange stripe along the lateral line, from gills to the tail, which is most pronounced in breeding males. The caudal fin is squarish and only mildly forked.  Juvenile rainbow trout display parr marks (dark vertical bars) typical of most salmonid juveniles. In some cases the parr marks are retained into adulthood. There can be confusion over the fish present in the Santa Ana river, since it is stocked with hatchery-bred trout.
Rainbow trout spawn in early to late spring when water temperatures reach at least 42 to 44 °F,  and they prefer to  inhabit and spawn in small to moderately large, well oxygenated, shallow portions of the river with gravel bottoms. The natural rainbow trout population in the Santa Ana has been compromised by the presence of introduced rainbow trout that have established their  own wild and self-sustaining population in the river.
Spawning sites are usually a bed of fine gravel in a riffle above a pool.  A riffle is a short, relatively shallow and coarse-bedded length of stream over which the stream flows at slower velocity but a higher turbulence than it normally does in comparison to a pool. A female trout clears a redd in the gravel by turning on her side and beating her tail up and down. A red is a nest dug into the gravel by a female fish.
Female rainbow trout usually produce 1,000 to 1,400 0.16 to 0.20 inch  eggs per pound of weight. During spawning, the eggs fall into spaces between the gravel, and immediately the female begins digging at the upstream edge of the nest, covering the eggs with the displaced gravel. As eggs are released by the female, a male moves alongside and deposits milt (sperm) over the eggs to fertilize them. The eggs usually hatch in about four to seven weeks although the time of hatching varies greatly with region and habitat. Newly hatched trout are called sac-fry or alevin. In approximately two weeks, the yolk sac is completely consumed and fry commence feeding mainly on zooplankton. The growth rate of rainbow trout is variable with area, habitat, life history and quality and quantity of food. As fry grow, they begin to develop “parr” marks or dark vertical bars on their sides. In this juvenile stage, immature trout are often called “parr” because of the marks. These small juvenile trout are sometimes called fingerlings because they are approximately the size of a human finger.
The maximum recorded lifespan for a rainbow trout is 11 years.
Rainbow trout are predators with a varied diet and will eat nearly anything they can capture. They are not as piscivorous, i.e., likely to eat other fish, or aggressive as brown trout or chars. Rainbow trout routinely feed on larval, pupal and adult forms of aquatic insects (typically caddisflies, stoneflies, mayflies and aquatic diptera). They also eat fish eggs and adult forms of terrestrial insects (typically ants, beetles, grasshoppers and crickets) that fall into the water. Other prey include small fish up to one-third of their length, crayfish and other crustaceans. As rainbow trout grow, the proportion of fish consumed increases in most populations. Rainbows will feed on the decomposing flesh from carcasses of other fish.
Rainbow trout are highly regarded game fish, and a popular target for fly fishers, who use  several angling methods. The use of lures presented via spinning, casting or trolling techniques is common. Rainbow trout can also be caught on various live and dead natural baits.
Because it is stocked with hatchery bred fish, the river is frequented by sport fishers.  The rainbow trout has tender flesh and a mild, somewhat nutty flavor. The wild fish has a stronger, gamier taste than hatchery bred fish. While the taste of wild-caught trout is often promoted as superior, it is illegal to sell or market wild-caught rainbow trout, which are legally classified as game fish, in the United States.
Drought, and the Seven Oaks Dam, which can withhold the amount of water pouring into the Santa Ana River, represent a threat to the wild rainbow trout population in the river.