Surface Water

(definition -- surface water--water that is on the Earth's surface, such as in a stream, river, lake, or reservoir)

In watershed assessment and planning, the basic characteristics about surface water are flow (volume of water per unit of time), timing (when this flow occurs), storage (volume of water in groundwater, reservoirs, lakes, or snowpack at a particular time), and quality (what’s in the water besides just water). The hydrologic cycle is a useful framework for thinking about how water moves through your watershed. The hydrologic cycle is a conceptual description of the ways in which water moves around the world. Water is generally moving in the hydrologic cycle, although some of it may be in temporary storage on the surface or underground for a wide range of time periods.

Surface Water Management

The concept encompasses the diversion, storage, and impedence of surface water moving in artificial or natural channels. People manage surface water to reduce flooding, irrigate agricultural lands, supply domestic and commercial needs, recharge ground-water and aquifers, and to maintain natural processes.

Dams are an obvious feature of surface water management. They serve to retain water to reduce risk of flooding downstream, to supply water during dry periods, to divert water out of the natural channel and to generate hydro-electric power. They also severely impact many natural processes dependent on the natural flow and fluctuations in flow of water (e.g., sediment transport) and serve as barriers to migrating fish and other animals.

Artificial channels are another feature of surface water management. They can be lined (e.g., with concrete) or unlined and are usually used to transfer water from one waterbody to another. In the lower Tujunga Wash watershed, many channels have taken the place of natural streams and are basically storm-water conveyance devices, taking the water from a reservoir to the LA River channel.

Dams and diversions (any structure that facilitates removing water from a stream for the purpose of transporting the water to another location) alter streamflow in a variety of ways. A stream’s hydrograph (a graph of how flow changes over time) is very different above and below a dam or major diversion.

Dams are constructed to alter streamflow timing. Water generated during the rainy or snowmelt season is captured behind the dam in a reservoir and released later to meet downstream needs (irrigation, municipal supply, hydroelectric generation, or instream flow, for example).

Depending on a dam’s size and flood reservation (management guidelines that keep part of the reservoir unfilled at different times of the year as related to the flood risk), peak flows may be entirely captured behind the dam and slowly released later in the year at a controlled rate. Smaller dams lack the capacity to have much effect on the hydrograph of large floods, whereas downstream of a big dam, there may be no indication of the floodwaters pouring in upstream. During the portions of the year when flows would be low under natural conditions, water releases from a reservoir may increase streamflow several fold above its natural level.

(See the California Watershed Assessment Manual, Chapter 3)