The Los Angeles River Watershed is approximately 834 square miles and home to a whopping 9 million people living among 43 cities (LACDPW). The Los Angeles River itself runs 52 miles from its headwaters in Canoga Park to the Pacific Ocean. As the river winds, it crosses multiple habitats and urban areas, meeting people and picking up pollutants on its way. It is imperative we understand the land use, climate, and geology of the whole LA River Watershed system as we consider how to make it more sustainable and healthy in the future.
Topography of the LA River Watershed
The topography of the Los Angeles River Watershed is unique because it encompassess both mountain ranges and the coast. The river runs from the mountains, down the rich, nutritious alluvial plain of the LA Basin, and out into the Pacific Ocean. The highest point in the LA River Watershed is over 7,000 feet in the northwest San Gabriel Mountains, dropping down to sea level at the Pacific Ocean. This drastic change in elevation over a relatively short distance makes the LA River speedy! The river drops an average of 31 feet per mile! Compare this to the 2,348-mile long Mississippi which only drops 1 foot per mile, and you’ll quickly understand how speedy the LA River is.
There are four mountain ranges in the Watershed embracing the northern region: the San Gabriel Mountains, the Verdugo Mountains, the Santa Susana Mountains, and the Santa Monica Mountains. All of these mountains are a part of the Transverse Ranges, an entire network of mountain ranges running east to west, formed by tectonic activity. The San Gabriel Mountains are mostly composed of old rock from the Mesozoic period. The rock found in the Santa Monica Mountains, ranging from the Cretaceous to the Miocene, and the Santa Susana Mountains, ranging from the Miocene to Pleistocene, is younger (CWH, 2012). As the streams and River run quickly down these mountains to the ocean, they carry away thousands of pieces of rock. Overtime, water is very erosive, so it can break down rock into sediment and sand. It is this process that forms the alluvial fan of rich sediments in the coastal area of the watershed.
Los Angeles experiences warm, dry summers and cool, wet winters because it is a “Mediterranean climate.” Mediterranean climates only really experience two seasons, summer and winter, which are greatly influenced by ocean currents and water temperature. Other places in the world with a similar climate include Southern Australia, Central Chile, the Mediterranean Basin, and Western Cape, South Africa. The climate of the LA River Watershed is also shaped by our topography. The mountain to coast topography of the Watershed explains why there is so much spatial variation in temperature and rainfall across all of the cities in the LA River Watershed (CWH, 2012).
Water Body Profiles
The Los Angeles area is often portrayed as a dry, water-desolate region, but there are actually many bodies of water in the watershed. Some are natural streams which flow in the LA River, and other have come about because of humans installing dams. Nevertheless, wetland features provide important natural habitat for many creatures. Get to know some of your local wetlands with this slides!
The predominant land uses in the LA River watershed are (LACDPW):
Most of the land in the watershed is urbanized and highly developed. About one third the the watershed is covered in impervious materials like concrete, asphalt, and buildings (LA Stormwater). Water can’t infiltrate impermeable materials like these, so any water that falls on these surfaces runs off into the storm drains and the River. Another third of the LA River Watershed falls within the Los Angeles National Forest boundary up in the mountains.
(1) 2010 Census Data
“About the LA River.” Los Angeles River Revitalization. City of LA.
“Los Angeles River Watershed” California State Water Board.
“Los Angeles River Watershed.” LA Stormwater.
“Los Angeles River Watershed.” LACDPW
“Trash Total Maximum Daily Loads for the Los Angeles River Watershed.” California Regional Water Quality Control Board. 2007.
Urban Waters and the Los Angeles River Watershed. EPA.
“Los Angeles River 2012 State of the Watershed Report” Council for Watershed Health. 2012.
“Wetlands of the Los Angeles River Watershed” Coastal Conservancy. 2000.