Rivergages.com - Glossary of Terms

Glossary Terms
Glossary Term	Definition
100-year flood	The term "100-year flood" is misleading. It is not the flood that will occur once every 100 years. Rather, it is the flood elevation that has a 1- percent chance of being equaled or exceeded each year. Thus, the 100-year flood could occur more than once in a relatively short period of time. The 100-year flood, which is the standard used by most Federal and state agencies, is used by the NFIP (National Flood Insurance Program) as the standard for floodplain management and to determine the need for flood insurance. A structure located within a special flood hazard area shown on an NFIP map has a 26 percent chance of suffering flood damage during the term of a 30 year mortgage.
200-year flood	The flood elevation that has a 0.5 - percent chance of being equaled or exceeded each year.
500-year flood	The flood elevation that has a 0.2 - percent chance of being equaled or exceeded each year.
7Q10 (also called Q7,10)	Seven-day, 10-year low flow (7Q10) is the discharge below which the annual 7-day minimum flow falls in 1 year out of 10 on the long-term average. The recurrence interval of the 7Q10 is 10 years; the chance that the annual 7-day minimum flow will be less than the 7Q10 is 10 percent in any given year.
Evaporation	Phase change of liquid water to water vapor.
Evapotranspiration	The combined process of evaporation and transpiration.
Flow	An estimate of the volume of water that crosses a certain location over a specified length of time. Typically expressed in units of cubic feet per second (cfs).
Gage Zero	The reference elevation of a stream gage. The gage zero plus the stage reading give the approximate elevation of the water surface in a known datum. All zeros are based on a particular datum (msl 1912, NGVD 1929, NAVD88, etc.) which is not strictly tied to "sea level". These reference standards are established and maintained by the National Geodetic Survey - http://www.ngs.noaa.gov/
Hydrograph	A plot of stage or flow vs. time.
Hydrologic Cycle	The continuous process of water movement near the earth's surface. The cycle experienced by water in its travel from the ocean, through evapotranspiration and precipitation, infiltration, runoff, and return to the ocean.
Infiltration	Movement of water from the surface into the soil.
LWRP	A hydraulic based reference plane established from long-term observations of the river's stage, discharge rates, and flow duration periods developed about the 97-percent flow duration line.
Lower Gage	The river stage measured on the downstream side of a Lock and Dam.
Pool Level	The stage or elevation of water measured on the upstream side of a dam.
Precipitation	Water that falls to the earth in the form of rain, snow, hail, or sleet.
Rating Curve	Relationship between depth and the amount of flow in a channel, or storage in a reservoir.
Stage	A measurement of the depth of water above an arbitrary reference point in a stream channel. The reference elevation is shown as the "Gage Zero" on the RiverGages page for each station.
Transpiration	The conversion of water to water vapor through plant tissue.
Upper Gage	The river stage measured on the upstream side of a Lock and Dam.
Water Surface Profile	Plot of the depth of water in a channel along the length of the channel.
Watershed	An area of land that drains to a single outlet and is separated from other watersheds by a divide.