Find us at: 570 W. Ave 26 #250 • Los Angeles, CA 90065

Los Angeles River Environment

b and w river

Natural History

Much of what the River looked like hundreds of years ago exists only in tiny fragments along our mostly channelized River. The history of the River has been ever changing as topography; weather and human settlement have all had their influence.

Thousands of years ago, the LA Basin was largely grassland with the Los Angeles River meandering its way through basin to the Pacific Ocean in Long Beach. Naturally shallow, the River flooded and changed course throughout the year. Before it was channelized, the L.A. River shifted course several times across the coastal plain. Flooding deposited rich soil and created marshes and small ponds throughout the LA Basin turning it into a rich alluvial plain that in turn created ecologically diverse habitats for wildlife.

Not a Desert

Southern California falls within a Mediterranean biome. Only 5% of the world’s biomes are Mediterranean, including Central Chile, Cape of South Africa, Mediterranean Sea borderlands, and southwest Australia. Located on the western coasts of continents and influenced by cold offshore ocean currents, the climate of a Mediterranean biome is characterized by 6 months of cool, wet winters and 6 months of hot, dry summers.

This type of climate makes for an incredible biodiversity. It was this biodiversity that provided the Native Americans with everything that they needed to survive for hundreds of years. The Los Angeles River was a vitalresource, providing much of the needed plants and animals for every day aspects of their lives, including a waterway for travel.

In the days before channelization, the Los Angeles River was known to flood its banks periodically, meander along, and shift its course to the ocean. The resulting erosion and widespread deposit of sediments created flat strips of land called floodplains.


These deposits resulted in the growth of willows, cottonwoods, and other aquatic and semi-aquatic plants. At one time these lowland forests formed one of the most biologically rich habitats of the River watershed. Since channelization, these areas are harder to find, but reestablished areas and remnants still exist. The best examples are behind damns including Hansen, Sepulveda, well as in the Glendale Narrows.

History of Floods

The first major flood to affect the city occurred in 1815. The pueblo was washed away, and a new one was built. Since that time the River changed its course several times and ran into what is now Ballona of river

In 1825, another flood caused the River to change its course again. This time it flowed south to Long Beach. The River was known to flow gently for many years of low rainfall, and then rage wildly during occasional years of flooding.

Because of the ever-changing nature of the River, settlements could be safe for years, then be washed away overnight. During the recorded history of Los Angeles, the River changed course at least nine times.

Unique Topography


Farms along the L.A. River in Elysian valley circa 1900. Department of Special collections, University of Southern California Library/Title Insurance & Trust Company Collections.

The topography of the Los Angeles River watershed is highly unusual. It ranges from 3,000 feet in the San Gabriel Mountains to sea level in a very short distance. The average grade of the mountain slopes is 65-70%. As an example, the Los Angeles River is 51 miles long and drops 30’ per mile. The Mississippi is 2,348 miles long and drops about 1’ per mile.

It is this topography that created to the once, rich alluvial soils that attracted farmers in the first place. The San Gabriel Mountains are young mountains, geologically speaking, and are still rising at a rate of about 3/4” per year. This means they are also eroding rapidly, and would naturally be depositing rich soils in the valleys and replenishing our beaches with sand via the natural transportation of the River systems. With channelization, these processes have been impaired.



Thousands of years ago the Los Angeles River created and flowed through several fresh water marshes. Marshes can form in almost any shallow depression that is kept wet by streams or ground water. Along the Los Angeles River, these marshes formed in places where the water table was high year round. Fresh water marshes contain layers of low, non-woody vegetation in soil saturated with moisture.

These wetlands serve important hydrologic, biological, and habitat functions. Hydrologic functions include long term and short term water storage, subsurface water storage for ground water recharge, energy dissipation, and moderation of groundwater flow or discharge. Fresh water marshes also convert water, sunlight and minerals to biomass at rates much higher than in dry ecosystems. They also provide a lush and safe environment for a wide range of life, from planktonic and filamentous algae to animals such as frogs and water fowl. Cattails are often a symbol for these wetland habitats and found with other plants such as duckweed that float along the surface and sedges and grasses that are found around the edges of the marsh.
These plants stabilize sediments and add organic matter.

While many of these original wetlands along the Los Angeles River have disappeared as a result of changing topography and human development and growth, efforts are underway to restore portions of important wetland habitats along the Los Angeles River.

There are however a few coastal wetlands remaining in Southern California that are being protected and preserved through the efforts of the Friends of the Ballona Wetlands and Los Cerritos Wetlands.

River Estuary

estuaryAccording to the Environmental Protection Agency, estuaries are places where rivers meet the sea. They are highly productive ecosystems and distinct from all other places on earth. The tidal, sheltered waters of estuaries also support an assortment of specialized plants, animals, and micro organisms especially adapted for life in unique waters.

Estuaries, like the LA River’s Willow Street Estuary in Long Beach, are among the most productive ecosystems on earth, creating more organic matter each year. Thousands of species of birds, mammals, fish, and other wildlife depend on estuarine habitats as places to live, feed, and reproduce. They are called the nurseries of the seas as many species of fish spawn in their sheltered and protected waters. In addition, estuaries also filter out sediments and pollutants before it reaches the ocean. The plants and soils can act as a natural buffer between the land and ocean. However, most estuaries are at risk due to human activities, both past and present. River Wildlife

Learn more with FoLAR’s Watershed Wonders