Chapter 1: Introduction

1.3 The Tujunga/Pacoima Watershed

For many of us, water simply flows from a faucet, and we think little about it beyond this point of contact. We have lost a sense of respect for the wild river, for the complex workings of a wetland, for the intricate web of life that water supports.

—Sandra Postel

The Tujunga/Pacoima Watershed drains to the Los Angeles River and is the largest subwatershed of the Los Angeles River Watershed, which drains to the Pacific Ocean in Long Beach. This 225-square-mile subwatershed comprises both remote open space of the Angeles National Forest, and the highly urbanized lands of the cities of Los Angeles & San Fernando. The watershed has a very steep slope—the high elevations of the San Gabriel Mountains in the upper watershed drop rapidly to the valley floor at an average rate of 41 feet/mile. Dozens of streams feed the three main tributaries—the Big Tujunga, Little Tujunga, and Pacoima Washes.

The watershed has a population of approximately 525,000, is roughly 61 percent Latino with 32 percent of the population under the age of 17 and 19 percent living in poverty. While the upper watershed includes more than 165 square miles of the Angeles National Forest and a large regional recreation area behind Hansen Dam, the lower watershed is extremely park-poor.

Our watershed’s Mediterranean ecosystem, the California Floristic Province, is one of the world’s top ten “hotspots” of biodiversity and is considered more threatened than the rainforest. Our region was once alive with native plants and animals that evolved and adapted over millions of years to be perfectly adapted to our cycles of drought and inundation. Of the 4,426 vascular plants found here, 48 percent are found nowhere else in the world. There are more plant species native to this ecosystem than in the whole central and northeastern United States combined. More bird species breed in our region than anywhere else in the country.

Although Los Angeles averages only 15 inches of annual rainfall, the higher elevations of this watershed receive some of the most concentrated rainfall in the United States. Historically, the Tujunga/Pacoima Watershed was a major contributor of groundwater supply. It sits atop the San Fernando Groundwater Basin—a natural underground reservoir that has become depleted over the years as most of the valley floor became impervious. Most of the rain that used to soak into the ground now runs off of concrete and asphalt and directly into the stormdrains, channelized washes, and the Los Angeles River. Prior to the channelization of our river systems and the subsequent intense development, it was estimated by Los Angeles County flood control engineers that 80 percent of stormwater percolated to groundwater. Current estimates are that around 8 percent of rainfall in urbanized areas percolates, the rest being lost to the ocean via the channelized system, carrying contaminants from urbanized land uses.

The San Fernando Groundwater Basin currently provides nearly 15 percent of Los Angeles’s drinking water but has the potential to provide much more.

Although watersheds share similar form and functions, each has unique characteristics that reflect the geography, geology, and topography of each. The essential characteristics of Tujunga/Pacoima Watershed— concentrated rainfall, pervious soils, and a capacious underground reservoir—offer our strongest regional opportunity to secure a sustainable local water supply. Enabling these attributes to work in concert again is the central objective of watershed management.