014-052_Green_Ch 01 5/7/07 1:19 PM Page 14 Chapter 1 1 LOS ANGELES AREA WATER SUPPLIES When the well is dry, we learn the worth of water. , Poor Richard’s Almanac, The almost total development of the Los Angeles Area was made possible only because of three giant aqueduct systems, built at public expense, that bring water from hundreds of miles away. The area continues to be dependent also on local rainwater and groundwater for about a third of its water supply. This chapter quickly reviews local surface water, ground- water, and the three aqueduct systems that import water to the Los Ange- les Area, the reliability of each source, and the environmental and/or water quality constraints that apply to each. These are the sources on which the Los Angeles Area depends for its drinking water supply. LOCAL SURFACE WATER: RIVERS, STREAMS, LAKES, AND RESERVOIRS Surface water in the Los Angeles Area comes from the skies, in the form of rain or snow. This is the water that runs off into rivers and creeks, evap- orates, is used by plants in their growing cycle, or infiltrates into the soil, replenishing our groundwater resources. The Los Angeles Area has two major river systems, the Los Angeles and San Gabriel rivers, and several large creek systems, Ballona and Malibu creeks being the largest. These rivers and creeks rise in the local mountains, where rainfall is much heavier than on the coastal plain. The average rainfall at the Los Angeles Civic Center is inches a 14 014-052_Green_Ch 01 5/7/07 1:19 PM Page 15 Figure . The hydrologic cycle. year, whereas it rains as much as inches a year on some of the peaks of the San Gabriel Mountains. The Storm Drain System The single-purpose storm drain system was built to contain and pass a cap- ital storm. The system consists of an extensive network of underground storm drains connected to an above-ground system of concrete-lined channels that have replaced our native rivers and creeks. Reservoirs, check dams, detention and debris basins, and other facilities are also part of the storm drain system. They capture water for flood control purposes (hold- ing peak flows until after the storm passes), or they capture water for later infiltration into the groundwater basins. With peak flows captured, the water can be released after the storm has passed and when there is more capac- ity in the storm channels. Then, water left in storage can be released slowly into spreading grounds or basins until it gradually percolates down into the ground and into the groundwater. A capital storm or flood is defined by the Los Angeles County Depart- ment of Public Works as runoff from a -year-frequency-design storm falling on a saturated watershed over a four-day period. After it has rained LOS ANGELES AREA WATER SUPPLIES 15 014-052_Green_Ch 01 5/7/07 1:19 PM Page 16 for three days and the ground is fully saturated, the heavens open up and dump, in a -hour period, the amount of water that would be contained in a storm that could happen only once in years. For example, in the foothills of the San Gabriel Mountains, where Pasadena, Azusa, and Glen- dora are located, the capital flood for which the storm drain system is designed would generate the following amounts of rainfall: . inches on the first day, . inches on the second, . inches on the third, and . inches on the fourth. (For more definitions, see the Glossary.) In the s, roughly % of the rain falling on Los Angeles either infil- trated into the ground or evaporated. Only % ran off to the sea. Today, with the extensive development and the paving over of our urban environ- ment (as much as % of the land is now covered with roofs, roads, park- ing lots, patios, etc.) and the construction of the massive storm channel system, about % of stormwater runs off in the Los Angeles River drainage area, while % either infiltrates or evaporates. About % of the San Gabriel River’s flow is captured for recharge into the groundwater sup- ply. On average, only about % of the upper Los Angeles River native runoff is captured, due to a lack of sufficient spreading capacity and the prevalence of clay soils. The concrete-lined channels that cross the Los Angeles Area carry rain- water to the Santa Monica Bay, the Los Angeles/Long Beach Harbor, or Seal Beach. During dry months, the flow is composed of urban runoff (the excess water that flows in our streets), effluent from water reclamation plants (sometimes called sewage treatment plants), and groundwater seepage. Dur- ing a heavy rainfall, these channels become dangerous torrents flushing large quantities of stormwater, trash, debris, and pollutants to the ocean. They accomplish the single-purpose job they were designed to do well. Spreading Basins Spreading basins are large, shallow man-made pits or ponds where water is slowly allowed to percolate into the soil in order to recharge or replen- ish the underlying groundwater basins. Spreading basins are filled with stormwater that has been diverted into the spreading basins or held in reser- voirs or lakes for future spreading. On average, nearly , acre-feet of water in Los Angeles County are returned to the groundwater supply in the county each year through spreading operations. Like surface water, the quantity of water diverted to spreading basins each year is affected by 16 LOS ANGELES AREA WATER SUPPLIES 014-052_Green_Ch 01 5/7/07 1:19 PM Page 17 weather conditions. In water year –, a wet year, , acre-feet of water were spread. That number dropped to , acre-feet in water year –. Imported water and reclaimed water are also spread. Reservoirs/Lakes A large volume of surface water is held in reservoirs or lakes throughout the Los Angeles Area. Though the terms may be confused and inter- changeable, a reservoir is always associated with a dam. Lakes, on the other hand, exist naturally, but many reservoirs have “lake” in their name. All major lakes within the Los Angeles Area are actually reservoirs. The lakes and reservoirs listed in Table were created for a variety of purposes. Cap- tured stormwater is conserved for later release into spreading grounds, where it percolates into the groundwater basin, increasing our drinking water sup- ply. Stormwater runoff is captured to avoid flooding, stored, and later released to the ocean or to a spreading basin. Water imported from out- side the region may be conveyed into reservoirs for the purposes of stor- age and/or regulation of flow to accommodate variability of demand. And any number of recreational activities, including fishing, swimming, and boating, may occur. Role of U.S. Forest Service Land: The Angeles National Forest An estimated % of the Los Angeles Basin’s water supply comes from National Forest lands. The Angeles National Forest was among the first national forests established, and it was established with the prime purpose of protecting the water supply. The role that the forest plays as a major water source and the need to protect this precious water supply were recognized very early. Two-thirds of the land area of the forest is considered to be sensitive water- shed, due to land slopes that are greater than °, or steeper than the angle of repose. The mountains are so steep that rock and debris flows or mud- slides are prevalent whenever the land becomes saturated with rainwater or is shaken by an earthquake. Rainfall in the mountains varies wildly with elevation and location. It averages about inches per year forestwide, with as much as inches a year falling in the higher elevations. The trees and other vegetation in the forest use a significant portion of the total precipitation that falls on the forest. An estimated , acre-feet of water moves directly into the LOS ANGELES AREA WATER SUPPLIES 17 014-052_Green_Ch 015/7/071:19PMPage18 . Major Reservoirs and Lakes in Los Angeles County Design Capacity Name (AF) Management Purpose Water Source Location Big Dalton , LA County Flood control, conservation Stormwater Big Dalton Canyon, miles north of Glendora Big Tujunga , LA County Flood control, conservation Stormwater Big Tujunga Canyon, miles northeast of Sunland Brea , U.S. Army Flood control Stormwater Cogswell , LA County Flood control, conservation Stormwater miles north of Azusa Devil’s Gate , LA County Flood control, conservation Stormwater Arroyo Seco, northeast of La Verne Eaton Wash LA County Conservation, debris storage Stormwater Northeast of Pasadena Encino , LA City Water supply LAAqueduct Santa Monica Mountains Fullerton U.S. Army Flood control Stormwater Hansen , U.S. Army Flood control Stormwater East San Fernando Valley Hollywood , LA City Water supply LAAqueduct Hollywood Hills Live Oak LA County Flood control, conservation Stormwater miles northeast of La Verne Lopez U.S. Army Flood control Stormwater Los Angeles , LA City Water supply LAAqueduct 014-052_Green_Ch 015/7/071:19PMPage19 Morris , LA County Conservation Stormwater San Gabriel Canyon, miles north of Azusa Pacoima , LA County Flood control, conservation Stormwater Pacoima Canyon, mile northeast of San Fernando Puddingstone , LA County Flood control, flow diversion, Stormwater mile south of San Dimas recreation Puddingstone L A County Flood control, conservation Stormwater miles northeast of San Dimas Diversion Santa Anita L A County Flood control, conservation Stormwater . miles north of Arcadia San Dimas , LA County Flood control, conservation Stormwater miles northeast of San Dimas San Gabriel , LA County Flood control, conservation Stormwater San Gabriel Canyon, . miles north of Azusa Santa Fe , U.S. Army Flood Control Stormwater Sawpit LA County Flood control, conservation Stormwater miles north of Monrovia Sepulveda , U.S. Army Flood control Stormwater Stone Canyon , LA City Water supply LAAqueduct Thompson Creek LA County Flood control, conservation Stormwater miles north of Claremont Westlake , Las Virgenes Water supply Colorado River Aqueduct or State MW Water Project Whittier Narrows , U.S.