Pictures from around western Colorado and eastern Utah
Common Hydrologic Terms and Definitions
ACRE-FOOT: The volume of water required to cover one acre to a depth of one foot; approximately 43,560 square feet.

BANKFULL: An established river stage/elevation at a given location along a river which is intended to represent the maximum safe water level that will not overflow the river banks or cause any significant damage within the river reach.

BASIN: Whole geographic area having a common outlet (such as a river, stream, or lake) for its surface runoff. Also known as drainage area or watershed.

CREST: The highest stage or level of water at a specific point before the water begins to recede.

FLOOD FREQUENCY: The chances that a particular flood event will occur during any given year. The greater the flood, the higher the "Year Flood" Value, but the less chance of occurrence. (i.e. a "100 Year Flood" has a 1% chance of occurring each year; a "2 Year Flood" has a % chance of occurring each year).

FLOOD STAGE: The elevation at which overflow of the natural banks of a waterway begins to cause damage or presents a flood-damage hazard near the gage where the elevation is measured (can also be expressed in terms of flood flow).

FLOW: The volume of water that passes through a point of a river during a given time. Usually expressed in cubic feet per second (cfs).

HYDROGRAPH: A graph showing stage, discharge, or other property of a river with respect to time.

REACH: The length of area between two specific points along a river, stream, or channel for which measurements of a river gage are representative.

RUNOFF: The part of precipitation that flows toward a river or stream on the surface of the ground, or through the soil before returning to the surface.

STAGE: The level of the surface of a river or lake above a predetermined base elevation (or "datum"). The stage is not a measurement of the actual water depth.

WATER YEAR: Hydrologists dealing with surface-water supply use the 12-month period, October 1 through September 30. The water year is designated by the calendar year in which it ends and which includes 9 of the 12 months. Thus, the year ending September 30, 2005, is called the "2005 water year." (There are a number of reasons for dividing the months in this fashion. The primary one is that it allows all of the seasonal snowfall to be included into the same group).


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.