Fed primarily by rainwater and snowmelt (in winter and spring), the Donald C. Tillman Water Reclamation Plant in Van Nuys (in summer and fall), and urban discharge, it is one of the few low-elevation perennial rivers in Southern California.

The Los Angeles River's official beginning is at the confluence of two channelized streams – Bell Creek and Arroyo Calabasas – in the Canoga Park section of the city of Los Angeles, just east of California State Route 27 (Topanga Canyon Boulevard), at 34°11′43″N 118°36′07″W / 34.1952°N 118.601838°W / 34.1952; -118.601838 (the east side of Canoga Park High School).

The river then rounds a bend to the northeast, now in a concrete box culvert, and crosses under the Hollywood (170) Freeway and Highway 101, and receives Burbank Western Channel on the left bank, 39 miles (63 km) from the mouth.

Paralleling the Golden State Freeway for the next few miles, the river runs by the eastern side of Griffith Park and the Harding-Wilson Golf Course.

[4][5][12][13] Paralleling the Long Beach Freeway south-southwest, the river then crosses under former California State Route 42 and the interstate as it receives the Rio Hondo from the left, 9 miles (14 km) from the mouth.

Crespi chose that name, because August 15 is the date of the yearly Catholic feast day in honor of the Virgin Mary's Assumption into Heaven and assuming the role as Queen of the Angels, to which the small Portiuncula chapel, where the Franciscan Order began in Assisi, Italy, is dedicated.

[21] In the early 19th century, the river turned southwest after leaving the Glendale Narrows, where it joined Ballona Creek and discharged into Santa Monica Bay in present Marina del Rey.

[22] However, this account is challenged by Col. J. J. Warner, in his Historical Sketch of Los Angeles County:"...until 1825 it was seldom, if in any year, that the river discharged even during the rainy season its waters into the sea.

These groups were turbulent in their efforts; resorting to violence, trying to get the attention of state and national press, and calling for politician interference to support their demands for more compensation.

"[27][28] The St. Francis Dam was built starting in 1924 (and through 1926), leading to the creation of a large reservoir in San Francisquito Canyon, and provided water for Los Angeles.

[19] Despite smaller mitigation efforts than the St. Francis Dam, unpredictable and devastating floods continued to plague it well into the 1930s, including: The incidents resulted in the US Army Corps of Engineers beginning an ambitious project of encasing the river's bed and banks in concrete.

[29] The only portions of the river that are not paved over are in the flood-control basin behind the Sepulveda Dam near Van Nuys; an 11-mile (17.7-km) stretch east of Griffith Park known as the Glendale Narrows; and along its last few miles in Long Beach.

According to an August 2013 article in the Los Angeles Times, the water in the river today is largely "industrial and residential discharge," which originates from the "two giant pipes that collect the sewage from the homes of 800,000 San Fernando Valley residents" that lead to the Tillman Water Reclamation Plant, "before crashing over a man-made waterfall into Lake Balboa.

[citation needed] Remediation efforts in order to tame the river along with the construction of highways in the 1950s and 1960s displaced hundreds of thousands of individuals.

Except for infrequent but dramatic flood episodes, this dry-land flood control basin, most of which is leased from the Corps by the City of Los Angeles Department of Recreation and Parks, plays host to diverse uses today including athletic fields, agriculture, golf courses, a fishing lake, parklands, a sewage treatment facility, and a wildlife reserve.

[33][34] The bike path runs parallel to the 5 freeway for the majority of its length and has mile markers and call boxes for information and safety purposes.

In this area, the busy Long Beach Freeway (I-710) and several high-voltage power lines run within a few hundred feet of the riverbed.

[39] There is an abundance today of non-native fish species in the Los Angeles River which include common carp, largemouth bass, tilapia, green sunfish, Amazon sailfin catfish, bluegill, black bullhead, brown bullhead, channel catfish, fathead minnow, crayfish, and mosquito fish.

There is also a large variety of bird species in the Los Angeles River which include snowy egret,[40] great egret,[41] black-necked stilt,[40] great blue heron,[40][41] green heron,[40] mallard,[40] cinnamon teal,[40] American coot,[40][41] Muscovy duck,[40] white pelican, Canada goose, osprey, California high desert mourning dove, black-chinned hummingbird, barn owl, and red tailed hawk.

There is indirect evidence that North American beaver (Castor canadensis) were native to the river, as the Beñemé (Mojave) and Jeniguechi (San Jacinto branch of the Cahuilla) Indians of the San Gabriel Mission were described by Father Pedro Font on the second de Anza Expedition in 1776, "The costume of the men in heathendom is total nakedness, while the women wear a bit of deer skin with which they cover themselves, and likewise an occasional cloak of beaver or rabbit skin, although the fathers endeavor to clothe the converted Indians with something as best they can.

[48][49] The Los Angeles City Council formed an ad hoc committee in 2002 to focus on accessing and revitalizing the river that was chaired by Councilmember Ed Reyes.

Developer Mort La Kretz provided the seed financing for the bridge, which was built next to the 5 freeway in the Elysian Valley by the Los Angeles Bureau of Engineering and completed in 2020.

On July 23, 2013, the LARRC announced a goal of completing a continuous 51-mile (82 km) greenway and bike path along the river by the end of the decade.

[54] The Los Angeles Times first reported in 2015 that the renowned architect Frank Gehry was collaborating with the LARRC (whose name was changed to River LA).

In 2019, Gehry revealed to Alta Magazine that he is working on designing several projects along the River, including a cultural center in Southgate.

[60] As a result, in 2018, Los Angeles and its LARiverWorks policy team had a project underway for "every mile of the river within city limits, aiming to improve connectivity, increase open space, and restore habitat.

"[61] Plans to cleanup 11 miles of river north of downtown Los Angeles, in the hopes of attracting steelhead trout, will cost $1.1 billion.

[66] One FoLAR's more prominent ongoing projects is the annual Great LA River Cleanup, which began in 1988 with a group of less than twenty people.

In 2012, FoLAR raised a $1 million donation to support the completion of the ARBOR study by the United States Army Corps of Engineers (USACE).

[71] This study ultimately resulted in the adoption of Alternative 20, a restoration plan focused on 719 acres of habitat and pedestrian-friendly improvements, among other things.