Water glossary

100-year flood: A flood that experts believe has a 1 percent, or one out of 100, chance of being equaled or exceeded in any given year. It also is referred to as the 1 percent annual chance of exceedance (1 percent ACE) flood. The term refers to the flood's size; not how often it occurs. Several 100-year floods can occur within the same year or within a few short years. Experts designate the size of a 100-year flood for specific areas; the actual size will vary from one place to another. Other measures include 10-year floods, or 10 percent ACE; 50-year floods, or 2 percent ACE; and 500-year floods, or 0.2 percent ACE.

Acre-foot: The amount of water required to cover an area of 1 acre to a depth of 1 foot. One acre-foot of water is equal to 325,851 gallons.

Action Stage: The level of a rising stream at which local communities need to take some type of protective action to prepare for possible flooding. Action stage is determined by the National Weather Service and the local communities that may be affected by flooding. Action stage is sometimes used interchangeably with Bankfull Stage. See also Flood Stage.

Bankfull Stage: The highest level a river can reach at a given location without overflowing the riverbanks or causing any significant damage. Bankfull Stage is determined by the National Weather Service and the local communities that may be affected by flooding. Bankfull Stage is sometimes used interchangeably with Action Stage. See also Flood Stage.

Cubic feet per second (cfs): The rate at which water is flowing. One cfs is equal to about 450 gallons per minute.

Conservation pool: The area of lakes Travis and Buchanan dedicated to water storage for municipal, domestic, industrial, agricultural and recreational purposes. The conservation pool of Lake Travis is the area below 681 feet above mean sea level (feet msl). It holds about 1,135,000 acre-feet of water. The conservation pool of Lake Buchanan is the area below 1,020 feet msl. It holds about 876,000 acre-feet of water.

Constant level: Not a correct term when referring to any of the Highland Lakes. Lakes Buchanan and Travis fluctuate more than the other lakes, but they all change daily. Levels can change rapidly during floods. See also Pass-through lakes.

Critical period: The time period in which the driest conditions occurred from a water supply perspective. The inflows during that period define the hydrology LCRA uses to calculate how much water the lower Colorado River system can supply.

Dissolved oxygen (DO): The amount of oxygen available to fish and other aquatic organisms. Fish begin to show negative effects when DO falls below 2 milligrams per liter (mg/L) for an extended time. Aquatic plants and algae are important contributors of dissolved oxygen.

Drought Worse Than Drought of Record (DWDR): Drought condition under LCRA’s 2015 Water Management Plan based on inflows, drought duration and combined storage, where an ongoing drought has a real likelihood of becoming a new Drought of Record. A DWDR declaration by LCRA’s Board of Directors would trigger action to cut off interruptible stored water and implement mandatory pro rata curtailment of water for firm demands.

Elevation: The height or level of a lake. It is measured in feet above mean sea level. See also mean sea level.

Fecal coliform bacteria: An indicator of the possible presence of harmful organisms in water. Fecal coliform bacteria levels in rivers and lakes usually rise for a few days following rain. The U.S. Environmental Protection Agency recommends no contact with water containing more than 200 colonies of fecal coliform bacteria per 100 milliliters.

Flash flood: A flood caused by heavy or excessive rain falling within a few minutes or a few hours time. Flash floods are usually characterized by raging torrents after heavy rains that rapidly flood low-elevation areas.

Flash Flood Warning: Issued by the National Weather Service to warn the public that flash flooding is in progress, imminent or highly likely, and to take necessary precautions to protect lives and property.

Flash Flood Watch: Issued by the National Weather Service to alert the public that current or developing conditions are favorable for flash flooding within a designated area, but the occurrence is neither certain nor imminent.

Floodgates: Solid gates in a dam that can be opened to release floodwaters downstream.

Floodplain: An area at risk for flooding. A 100-year flood would affect a 100-year floodplain.

Flood pool: A specific area in Lake Travis and on the surrounding land that is used when necessary to store floodwaters until they can be released safely. The flood pool of Lake Travis is the area between 681 feet above mean sea level (feet msl) to the crest of the emergency spillway at 714 feet msl. The Lake Travis flood pool can store about 787,000 acre-feet.

Flood Stage: An established gauge height (or stage) for a given location above which a rise in water surface level begins to create a hazard to lives, property or commerce. Flood Stage is determined by the National Weather Service and the local communities that may be affected by flooding. See also Action Stage and Bankfull Stage.

Flow: The volume of water passing a given point during a unit of time. Typically, flow is expressed as cubic feet per second (cfs). Also known as flow rate or discharge.

Gauge height: The level of a river measured at a gauge. Often used interchangeably with the more general term stage.

Groundwater: Water below the surface of the ground, usually in an aquifer or underground stream or lake. Wells are used to draw up groundwater for drinking and other purposes.

Head level: The elevation of a lake at the upstream face of a dam.

Highland Lakes: The chain of six lakes along the Colorado River, upstream of Austin. Each lake — Buchanan, Inks, LBJ, Marble Falls, Travis and Austin — is created by one of the six dams owned or operated by LCRA.

Historic Drought of Record: The drought that affected Central Texas from 1947 through 1957. (See LCRA history.) LCRA’s existing firm water contracts are based upon supplying water through a repeat of the 1947-1957 drought.

Hydroelectric generation: The creation of electricity by using the force of water to turn a turbine in an enclosed magnetic field. Each of the Highland Lakes dams has a powerhouse with hydroelectric generating units.

Hydromet: A system of electronic rainfall, stream gauge and water-level equipment installed at remote sites across the lower Colorado River watershed, including rain gauges clustered around the Highland Lakes. The system uses LCRA's radio system to transmit data to the River Operations Control Center. See lakes and river information from LCRA's Hydromet system.

Inflows: Water that flows into creeks, rivers, the Highland Lakes and Matagorda Bay.

Interruptible water: Water that is available for use on a year-to-year basis, depending on how much water is stored in lakes Travis and Buchanan. Interruptible water is subject to curtailment during water shortages.

Inventory/firm yield: The amount of water that can be supplied reliably through a repeat of the driest conditions on record.

Lower Colorado Region (LCR): See Region K.

Lower Colorado Regional Water Planning Group (LCRWPG): A group appointed by the Texas Water Development Board (TWDB) in 1998 to develop and update plans to meet long-term water supply needs in the Lower Colorado Region (LCR), or Region K. The group represents various interests: the public, counties, cities, industries, agriculture, the environment, small business, electric generating utilities, river authorities, water districts, water utilities and recreation. TWDB includes the Region K and other regional plans in the State Water Plan.

Mean sea level (msl): A point of reference to measure lake elevation. It refers to the elevation of the ocean halfway between high and low tide. Lake elevations are measured in feet above mean sea level.

Nonpoint-source pollution: Pollutants from a source that is difficult to pinpoint. They include chemicals from lawns and fields, trash, oil and animal and human wastes. Typically, water running off streets, parking lots, yards, construction sites and agricultural lands sweep these pollutants into bodies of water. About 70 percent of pollution in the nation's waters comes from nonpoint sources.

Operating range: The levels between which the lakes typically are operated during normal conditions. Lake levels can fluctuate outside of these ranges during floods or other circumstances.

Pass-through lakes: Lakes designed to allow water to pass through as opposed to being stored. Lakes Inks, LBJ, Marble Falls and Austin are pass-through lakes.

Probable Maximum Flood (PMF): The flood that may be expected from the most severe combination of weather and hydrological conditions that is reasonably possible in a particular drainage area. The magnitude of a PMF far exceeds that of 100-year and 500-year floods. Although a PMF has not occurred in the lower Colorado River basin within the historical period of record, LCRA continually evaluates and makes improvements to all the Highland Lakes dams to ensure they could withstand a PMF.

Region K: The state's designation for the water planning region that includes all or part of 14 counties in Central and South Central Texas. The Texas Water Development Board (TWDB) divided the state into regions and appointed regional planning groups to implement the statewide water planning process required by Senate Bill 1 (SB 1), passed by the Legislature in 1997. See Lower Colorado Regional Water Planning Group.

River Operations Control Center (ROCC): The control center in Austin at which LCRA staff analyzes data from LCRA's Hydromet system, plans daily operations to supply water for customers and environmental needs and controls hydroelectric generation. During floods, the staff prepares lake level forecasts, communicates with the National Weather Service and U.S. Army Corps of Engineers, and dispatches crews to operate floodgates, if necessary.

Safe Drinking Water Act: A federal law to ensure drinking water is safe. The U.S. Environmental Protection Agency (EPA) sets standards for drinking water quality and oversees their implementation. The Texas Commission on Environmental Quality (TCEQ) sets and enforces standards at the state level that work in conjunction with federal law.

Spillway: An area of a dam designed to allow water to flow over it. Spillways can be controlled or uncontrolled. A controlled spillway has floodgates that can hold water above its crest. An uncontrolled spillway has no gates. Water flows freely over the uncontrolled spillway when the lake rises above the spillway crest.

Stage: The level of the water surface of a river or stream above a nearby benchmark. Typically, the benchmark elevations are set below the natural bottom of a channel because the riverbed changes and may become lower. Therefore, stage usually doesn't correspond with the depth of water above the natural bottom of the channel.

Surcharge Pool: A specific area in Lake Travis and on the surrounding land that may be inundated if floodwaters rise above the crest of the uncontrolled spillway. The flood pool of Lake Travis is the area between the crest of the uncontrolled spillway at 714 feet above mean sea level (feet msl) and the top of Mansfield Dam at 750 feet msl. It can temporarily hold about 1,177,000 acre-feet of water. See also Spillway.

Surface water: Water above the surface of the ground, such as a lake or river. The term is used to distinguish it from groundwater.

Volume: The amount of water in the lake, usually measured in acre-feet.