The Los Angeles River, historically known as Paayme Paxaayt by the Tongva and the Río Porciúncula by the Spanish, is a major river in Los Angeles County, California. Its headwaters are in the Simi Hills and Santa Susana Mountains, and it flows nearly 51 miles (82 km) from Canoga Park through the San Fernando Valley, Downtown Los Angeles, and the Gateway Cities to its mouth in Long Beach, where it flows into San Pedro Bay. While the river was once free-flowing and frequently flooding, forming alluvial flood plains along its banks, it is currently notable for flowing through a concrete channel on a fixed course, which was built after a series of devastating floods in the early 20th century.

Los Angeles County, officially the County of Los Angeles, and sometimes abbreviated as L.A. County, is the most populous county in the United States, with 9,861,224 residents estimated in 2022. Its population is greater than that of 40 individual U.S. states. Comprising 88 incorporated cities and many unincorporated areas within a total area of 4,083 square miles (10,570 km2), it is home to more than a quarter of Californians and is one of the most ethnically diverse U.S. counties. The county's seat, Los Angeles, is the second most populous city in the United States and the most populous city in California, with 3,822,238 residents estimated in 2022. It is also known for being the home of the American film and television industry, a distinction it has held since the early 20th century which has given the county global prominence.