The term "watershed" refers to an area or region of land where water drains from higher elevation points into a larger body of water such as an ocean, river or lake or seeps into the soil to help replenish the groundwater supply. Watersheds, therefore, have a direct relationship with the hydrologic cycle by managing the amount of water they are able to drain.
The "hydrologic cycle" or "water cycle" is a three-step process by which our planet is continually able to recycle its water. The first step of this process is called precipitation. During precipitation, water falls to Earth as rain, sleet, snow or hail. The second step is called evaporation. During evaporation, water that has fallen to Earth is warmed by the sun. It then turns into a vapor or gas and rises into the atmosphere to begin its third and final process called condensation. During condensation, water vapor that has risen into the atmosphere eventually forms clouds. Once those clouds become too heavy with water, precipitation occurs, and the whole water cycle repeats itself as it has for millions of years.
Just as the hydrologic cycle has a direct impact on our watershed, so does the management and use of the land within the watershed. Clearing forests, planting trees, urban sprawl, agricultural uses and making decisions solely based upon immediate economic need have all contributed to decreasing the efficiency of our nation's watersheds. Valuable topsoil where plants anchor themselves is lost, causing flooding to occur more rapidly as water runs immediately into surrounding bodies of water that are often heavily populated by human residences and businesses.
In the South Bay and southeastern parts of Los Angeles County, there are three watersheds that empty into the Pacific Ocean: Santa Monica Bay, Los Angeles River and San Gabriel River. These three watersheds often overlap each other's boundaries and can be characterized as densely urban environments, which contain both point and non-point pollution sources.
Point pollution sources can include a pipe discharging into a river, a leaking underground storage tank, a boat or another single source. This type of pollution is easy to fix because its source can be quickly identified. Non-point pollution sources, however, are more difficult to fix because the source cannot be easily identified. Typically, non-point pollution sources can be traced to various sources such as industries, agriculture, mining and other large-scale human activities. Point and non-point pollution sources become particularly troublesome to a watershed during times of extreme precipitation where flooding can cause pollution runoff to be carried throughout a watershed. This, in turn, affects the quality of water that eventually seeps into the ground, a river or lake and becomes a part of the drinking water supply.
Nationally, flood control management includes all or a combination of the following:
- Construction of dams and other measures to reduce the level of peak flows
- Construction of levees, dikes and diversion canals to keep water within a confined channel
- Land-use controls and flood-proofing of buildings to minimize property damage
- Flood protection to provide early warning of a flood
- Evacuation plans and sand-bagging
- Disaster relief
- Insurance and victim assistance programs
The building of large dams has been a very popular flood control method because of the multipurpose uses: flood control, irrigation water, electric power, water for domestic and industrial usage, and recreation. However, dams can also have negative aspects. Dams can be very expensive to build and maintain. A dam's reservoir may cause flooding of the surrounding land, evaporation of water, silt build-up or loss at riverbanks, and destruction of fisheries.
Within Los Angeles County, flood control and water conservation is the responsibility of the Los Angeles County Department of Public Works (LACDPW), which operates, maintains and constructs flood control facilities. The LACDPW works closely with the United States Army Corps of Engineers (COE), which is responsible for the supervision of the Los Angeles and San Gabriel Rivers. Both of these rivers have their origins to the north of Los Angeles in the Angeles National Forest before making their long journey south to ultimately pass through the City of Long Beach and become a part of the Pacific Ocean. Along the Los Angeles and San Gabriel Rivers, the COE has constructed numerous drainage facilities to alleviate potential property damage and loss of life during a flood.
In addition to county and federal government agencies, environmental organizations are also very concerned about our local watersheds. Environmental organizations such as Heal the Bay, Friends of the Los Angeles and San Gabriel Rivers, and the Watershed Council are at the forefront of watershed management issues. These and other local environmental groups monitor water quality, educate the public and work with government agencies to safeguard the efficient use of watershed management facilities without compromising the high quality of life that we all enjoy.