With seismic uplift, the ocean receded, leaving the Santa Susana, Santa Monica, and San Gabriel mountain ranges in its place. The LA River traversed the lowest passages. In the following millennia, the continued erosion of soils from these mountains created massive alluvial plains, into which vast quantities of snowmelt and runoff from the mountains were stored, creating the groundwater basins that would become an essential resource of future ecosystems and human societies.¹Gumprecht, Blake. The Los Angeles River: Its Life, Death, and Possible Rebirth. Baltimore: Johns Hopkins University Press, 2001. 12. The steep mountains surrounding the LA River create a very “flashy” river system, meaning that as precipitation falls, the amount of water in streams and channels swells far beyond the amount of water in dry conditions. These streams and channels bring water to the LA River, which drops almost 800 feet in elevation over its 51-mile course. Although the LA River today looks very different than it did prior to development, the tendency for flash flooding always existed due to these geophysical characteristics.

In the earliest accounts, the river flowed above ground—either through the Elysian Valley, where shallow bedrock forced groundwater toward the surface, providing a year-round base flow, or in other locations that, during rains, fed a continuous flow draining into the ocean. Where bedrock lay much deeper and soils were most conducive to drainage, such as the rockier, more porous soils that erode from the San Gabriel Mountains, above-ground flow was more ephemeral. Below the surface, the river guided runoff from the mountains to the basin, where the water then percolated into aquifers. When they did appear, visible channels in these areas were often shallow and poorly defined; only during extreme rain events would streams materialize above saturated topsoils.²Gumprecht, Blake. The Los Angeles River: Its Life, Death, and Possible Rebirth. Baltimore: Johns Hopkins University Press, 2001. 16. Periodically, massive floods converted the wide, flat floodplains of the lower LA River into raging torrents.³Gumprecht, Blake. The Los Angeles River: Its Life, Death, and Possible Rebirth. Baltimore: Johns Hopkins University Press, 2001. 12. The 1916 soil map of LA County shows floodplain soils across a vast territory of the Los Angeles plain, indicating the extent of this flooding historically, even before the urban sprawl of the 20th century.⁴United States Geological Survey. Los Angeles County Soils Map. 1916.

The river system frequently migrated. Some years, south of present-day Downtown LA, the main channel headed west and entered Santa Monica Bay through what is now Ballona Creek. Other years, the river stretched towards the ocean between present-day Downtown LA and Long Beach as a broad floodplain of intermittent streams, with dense trees and wetlands.⁵Rairdan, Charles. “Regional Restoration Goals for Wetland Resources in the Greater Los Angeles Drainage Area: A Landscape-Level Comparison of Recent Historic and Current Conditions using Geographical Information Systems.” Dissertation. UCLA, 1998. Early Spanish settlers noted that it could be difficult to discern the location of the mouth of the river. In this area, the river was “a small gentle stream flowing through a broad, sandy bed most of the year and a large, turbulent, unpredictable river for a few days every winter.”⁶Gumprecht, Blake. The Los Angeles River: Its Life, Death, and Possible Rebirth. Baltimore: Johns Hopkins University Press, 2001. 12.

The general course of the LA River as it is known today, starting in the San Fernando Valley and discharging into the Pacific Ocean at Long Beach, emerged in 1825 when a massive flood cut a channel across the existing plain of wetlands and forests.⁷Crandell, John. “The L.A. River’s ‘Natural’ History: Until 1825, the Los Angeles Basin was vastly different from the current desert. What was the area’s environment in the distant past?” Los Angeles Times (August 14, 1994).

Before development, the LA basin was likely characterized by a mix of coastal sage scrub, valley grasslands, swaths of Southern California oak, and seasonal wetlands.⁸The historic ecology of the LA River is not well studied of documented. The most conclusive mapping, while likely imperfect, is referenced here: Kuechler, A.W. Natural Vegetation of California [map]. 1977. 1:1,000,000. “David Rumsey Map Collection”. Accessed October 14, 2019. These habitats hosted abundant wildlife such as deer, antelope, coyotes, gray foxes, mountain lions, grizzly bears, steelhead, countless birds and rodents, turtles, gophers, badgers, shrews, moles, cuckoos, owls, vireos, woodpeckers, and Pacific lamprey.⁹Gumprecht, Blake. The Los Angeles River: Its Life, Death, and Possible Rebirth. Baltimore: Johns Hopkins University Press, 2001. 25. To better understand the historical ecology of the LA basin, studies are now underway at the University of Southern California and other institutions, where researchers are assembling a more holistic interpretation of written diaries, images, and other early narratives that describe the native plant communities, wetlands, and riparian areas of the river. Similar studies have been completed for adjacent watersheds such as Ballona Creek and the San Gabriel River.¹⁰Dark, Shawna, Eric D. Stein, Danielle Bram, Joel Osuna, Joseph Monteferante, Travis Longcore, Robin Grossinger, and Erin Beller. Historic Ecology of the Ballona Creek Watershed. Accessed October 14, 2019. ¹¹Stein, Eric D., Shawna Dark, Travis Longcore, Nicholas Hall, Michael Beland, Robin Grossinger, Jason Casanova, and Martha Sutula. Historical Ecology and Landscape Change of the San Gabriel River and Floodplain. Accessed October 14, 2019.