Rainfall that stays in the liquid state becomes runoff that makes up streams and rivers.  Water that is not used for some purpose eventually flows to the ocean.  Water that evaporates from the oceans then condenses and eventually falls back to Earth in the form of rain. This is also known as the hydrologic cycle. 

The water cycle is the continuous movement of water in the atmosphere, over the land, and in the ocean. Through precipitation, water condenses, forming a liquid and falling to the Earth as rain, snow, hail or fog.  Once on the ground, water either remains in its liquid state, freezes, becoming ice or evaporates, becoming a gas. For a full-szed chart of the water cycle, click here.  

A rivers speed is measured by cubic feet per second (cfs) or how quickly a cubic foot of water passes by a set point for a period of one second.  The set point is usually a gauge managed by the United States Geological Survey (USGS) set at intervals along a water course. 

An acre-foot is commonly used to measure water volume. It is the amount of water needed to cover one acre (43,560 square feet) with one foot of water.  One acre foot is equal to 325,851 gallons of water. 

Streamflow is the water discharge in a natural channel.  Streamflow is measured in cubic feet per second (cfs) and monitored by the United States Geological Survey.  You may view the stream gages in the Brazos River basin by clicking here. 

A drought is generally considered to be a prolonged period of less-than-normal precipitation such that the lack of water causes below average streamflow or lake levels, lowered soil moisture, crop damage, or economic losses.   

A flood plain is any normally dry land area susceptible to inundation by water. This area is usually low, flat and next to a stream or other body of water.

Floodgates are solid metal barriers in a dam that can be opened to release flood waters downstream.

The National Weather Service issues a flash flood warning to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.  For additional information on flood warning and watches, click here

Firm water is a supply term referring to the specific amount necessary to fully supply an area with water during a repeat of the most severe drought of record.

The National Flood Insurance Program is a federal insurance program under which flood-prone areas are identified and flood insurance is made available to residents of participating communities that agree to adopt and enforce floodplain management ordinances to reduce future flood damage.  For additional information and qualification requirements, click here 

Constant level is a term given to the practice of maintaining a body of water at a specific elevation.  No natural body of water has a constant level.  While some fluctuate more than others, all natural bodies of water change on a daily basis, some rapidly during floods.  Few man-made reservoirs have the ability to be maintained at a constant level as evaporation, drought, water use and/or flooding affect elevation levels. 

The measurement cubic foot per second (cfs or ft3/s) is the rate of water movement representing a volume of 1 cubic foot passing a given point during 1 second.  This measurement is equivalent to approximately 7.48 gallons per second or 448.8 gallons per minute.

Bank-full stage is an established river stage at a given location along a river that represents the maximum safe water level that will not overflow the river banks or cause any significant damage within the river reach.

The United States Geological Survey (USGS) is a federal scientific agency that works in biology, geography, geology and hydrology. USGS is the nation’s primary civilian mapping agency. The agency also monitors streams and other bodies of water across the country for flow and depth. Among its other activities, USGS also monitors earthquakes and volcanic activity.

The USGS web site may be found by clicking here. 

A measure of water’s alkalinity or acidity is called pH. Water with a pH of 7 is neutral; lower pH levels indicate increasing acidity and higher pH levels indicate increasingly basic solutions.

A 500-year flood is a high streamflow that overflows the natural or artifical banks of a water way and has a two-tenths of one percent chance of occurring in any given year.

A 50-year flood is a high streamflow that overflows the natural or artifical banks of a water way and has a two percent chance of occurring in any given year.

A 10-year flood is a high streamflow that overflows the natural or artifical banks of a water way and has a ten percent chance of occurring in any given year.

Datum is the basis for relating a river's stage to mean sea level. (datum + stage = elevation in feet above mean sea level)

A gaging station is a site on a stream, lake, reservoir or other body of water where observations and hydrologic data such as gage height and stream discharge are obtained.

Percolation is the movement of water through soil and its layers by force of gravity. 

Surface water consists of all water that is naturally exposed to the atmosphere including creeks, ponds, rivers, lakes, reservoirs, seas and oceans. Also in this category are springs and wells that are directly influenced by surface water sources.

Groundwater is water found beneath the Earth’s surface that gradually seeped down by saturating soil or rock. This water is stored in underground crevices and in the pores of rocks and other materials beneath the surface.

Natural lakes have been a rare commodity in Texas. In fact, the state had only one natural lake, Caddo Lake in East Texas, that was formed by a log jam.  A permanent dam was installed at the lake in the early 20th century.

A large number of the state’s remaining lakes were made in response to the occasional propensity of Texas’ usually tranquil rivers to flood during heavy rains.

Texas’ early history is filled with accounts of devastating floods causing loss of human life and destroying livestock and property, particularly along the Brazos. As a result, in the 1930s and 40s officials began building dams along Texas rivers to create flood control reservoirs that would absorb the floodwaters and alleviate damage and loss of life.  

 Possum Kingdom Lake (left) was built during this period. Momentum for such flood control projects picked up during the 1950s and the US Army Corps of Engineers was tasked with building several more lakes for flood control over the next several decades. Ironically, the worst drought of record for Texas took place during the 1950s, prompting emphasis on a second priority for these lakes: water supply . 

Mean sea level is the average height of the ocean’s surface, between high and low tide. It is used as a standard in reckoning land and other elevations such as lake levels. A lake’s conservation pool will be measured as a certain number of feet above mean sea level. 

Texas streams and rivers are in constant motion and the waters they pour into our lakes carry with them a continuous but varying amount of sediment.

When the water is slowed or stopped as it runs into a lake or by a dam, the sediment drops to the lake’s bottom. This sediment builds up year after year and at some point, fills the lake to a point it can no longer continue to serve its purpose in flood control or water supply. This would be the end of the lake’s effective “lifetime.”  The average life of a lake in Texas is considered to be about 100 years. 

The conservation pool of a reservoir is a specified amount of water dedicated to water storage. This water is to be used to meet water needs, including municipal, domestic, agricultural, industrial, and recreational.

To overdraft a lake, aquifer or basin is to draw water out faster than the water can be replenished by rain, runoff or percolation.  Overdraft may also refer to an use of the permitted yield of a reservoir.

Yield refers to the amount of water produced by a water treatment process or the quantity of water that can be collected for a given use from surface or groundwater sources. The yield may vary depending on the proposed use, the development plan, location of the water source and economic considerations.

A flood pool is a specified area within a flood control lake and the surrounding land that may only be inundated during periods of flooding. This allows the flow of waters to be regulated and released in a safe manner.

Evaporation is the process in the water cycle in which liquid water becomes water vapor.

A 100-year flood is a flood event that it has a 1 percent chance of happening in any given year. The term “100-year” is a measure of a flood’s size, not how often it occurs. Although statistically unlikely, several 100-year floods can occur within the same year or a few short years. It is also called the base flood.

 A drought of record is the worst recorded drought since compilation of meteorologic and hydrologic data began.  In terms of severity and duration, the devastating drought of the 1950s is considered the drought of record for many areas in the Brazos River basin. This drought lasted a decade in many places and covered much of the nation, including all of Texas.  In 2008-2009, some parts of the state recorded a new drought of record.

Xeriscaping is a form of landscaping using native plants and those adapted to survive in areas with periods of low precipitation. Using such techniques can be especially helpful in conserving water or in dealing with periods of drought.