Water glossary

water_gauge.jpgDo you know the difference between a floodplain, a flood pool and a flood stage? Check the list below for explanations of these and other water terms.
Acre-foot: The amount of water required to cover an area of one acre to a depth of one foot. One acre-foot of water is equal to almost 326,000 gallons.

Bankfull stage: The highest level a river can reach at a given location without overflowing the riverbanks or causing any significant damage. Compare with 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.

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 pass-through lakes.

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 of record: The decade-long drought that affected Central Texas from the late 1940s through the late 1950s. (See LCRA history.) No other drought in recent history was as severe or as sustained. LCRA and other organizations use it as a benchmark to compare recent droughts and to prepare for future droughts.

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

Firm water: The amount of water lakes Buchanan and Travis could supply during a repeat of the most severe drought on record. Most of this water is committed for use by cities, industries, power plants and protection of aquatic life.

Flash flood: A flood that follows within a few hours of heavy rainfall.

Flash flood warning: Issued by the National Weather Service to warn citizens that flash flooding has been reported or is imminent, and to take necessary precautions to protect lives and property.

Flash flood watch: Issued by the National Weather Service to alert citizens that flash flooding is possible within a designated area.

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

Floodplain: An area officially determined to be at risk for flooding in the event of severe weather. In most cases, these areas lie along lakes and rivers. A 100-year floodplain would be affected by a 100-year flood.

Flood pool: A specific area in Lake Travis and on the surrounding land that is used when necessary to store flood waters until they can be released safely.

Flood stage: The height (or stage) of a river or stream at which point the water flows out of its banks and could result in flooding or damage of adjacent land or property.

Flow: The volume of water passing a given point during a unit of time. Typically expressed as cubic feet per second, or 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 that lies 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 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.

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 generation units.

Hydromet: A system of electronic rainfall and water-level equipment installed at remote sites along the Colorado River and its major tributaries, and rainfall sensors clustered around the Highland Lakes. The system uses LCRA's 900-MHz radio system to transmit data to the River Operations Center. See lakes and river information from LCRA's Hydromet system.

Hydroelectric Operations Control Center (HOCC): The location at Buchanan Dam where LCRA staff controls hydroelectric generators and dispatches crews to open floodgates when the River Operations Center issues orders to do so.

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.

Lower Colorado Region (LCR): See Region K.

Lower Colorado Regional Water Planning Group (LCRWPG): A group of individuals 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 of streets, parking lots, yards, construction sites and agricultural lands sweep these pollutants into bodies of water when it rains. About 70 percent of pollution in the nation's waters come from nonpoint sources.

100-year flood: A flood so large that experts believe it has a 1 percent, or one out of 100, chance of occurring in any given year. Nevertheless, 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, with a 10 percent chance of occurring in any given year; 50-year floods, with a 2 percent chance of occurring in any given year; and 500-year floods, with a two-tenths of 1 percent chance.

Operating range: The levels between which the lakes are operated during normal weather conditions. Under extreme weather conditions, lake levels can fluctuate outside of these ranges.

Pass-through lakes: Lakes designed to allow water to pass through as opposed to being stored. Inks Lake, Lake LBJ and Lake Marble Falls pass water from Lake Buchanan to Lake Travis. Lake Austin passes water to Town Lake to flow down the Colorado River to Matagorda Bay.

Power pool: The amount of water in a lake that may be used to operate the hydroelectric generators at the dam's powerhouse.

Probable maximum flood (PMF): A flood so severe and statistically remote that the probability of its occurrence cannot be measured. Its magnitude far exceeds 100-year and 500-year floods and would result from a combination of the worst hydrological and weather conditions. For example, a PMF would occur if 36 inches of rain fell in a 72-hour period when the Highland Lakes were full. Although a PMF has not occurred in the lower Colorado River basin since LCRA was created, a $50 million project is in its final phase to ensure that all the Highland Lakes dams can withstand the greatest possible water load.

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 Center (ROC): The location in Austin where LCRA staff analyzes weather radar reports and data from the Hydromet. During floods, the staff communicates with the National Weather Service and U.S. Army Corps of Engineers and, if necessary, issues orders for floodgate operations.

Safe Drinking Water Act: The main federal law to ensure that drinking water is safe. The U.S. Environmental Protection Agency sets required standards for drinking water quality and oversees their implementation. The Texas Commission on Environmental Quality sets and enforces standards at the state level that are in harmony with federal law.

Spillway: Dams without floodgates are designed with an area called a spillway that allows water to flow freely over it during floods. A controlled spillway has floodgates.

Stage: A measure of a river's height, though it usually doesn't correspond with the natural bottom of the channel since the riverbed changes over time. Instead, the stage is determined from a benchmark where all the measurements are taken. Typically, these readings are set below the natural bottom of a channel.

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 at the head level, usually measured in acre-feet.

Warning stage: The point at which a river's depth triggers the National Weather Service to review conditions for potential flooding.