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These definitions have been compiled from various sources, including the USGS's Water Resources Data, Weather Service Operations Manual, Chapter E-90, and Glossary of Meteorology (AMS, 1959).

daily flood peak

The maximum mean daily discharge occuring in a stream during a given flood event.

dam

Any artificial barrier which impounds or diverts water. A dam is generally considered hydrologically significant if it is

1. One and one quarter feet (0.4 meters) or more in height from the natural bed of the stream and has a storage of at least 15 acre-feet, or
2. Or has an impounding capacity of 50 acre-feet or more and is at least six feet (2 meters) above the natural bed of the stream.

dam failure

Catastrophic event characterized by the sudden, rapid, and uncontrolled release of impounded water.

data collection platform (DCP)

An electronic device which connects to a river gage or rainfall gage which records data from the gage. At predetermined times it transmits that data via satellite to a remote computer.

datum

see gage datum.

day-second foot (dsf)

A volume equivalent to a flow of one cubic foot per second (cfs) maintained for one day.

dead storage

The volume in a reservoir below the lowest controllable level.

deep percolation loss

Water which percolates downward through the soil beyond the reach of plant roots.

deep seepage

Infiltration which reaches the water table.

design criteria

The hypothetical flood used in the sizing of the dam and the associated structures to prevent dam failure by overtopping, especially for the spillway and outlet works.

delta

An alluvial deposit, often in the shape of the Greek letter "delta," which is formed where a stream drops its debris load on entering a body of quieter water.

dendritic

The form of the drainage pattern of a stream and its tributaries when it follows a treelike shape, with the main trunk, branches, and twigs corresponding to the main stream, tributaries, and subtributaries, respectively, of the stream.

density current

A flow of water maintained by gravity through a large body of water, such as a reservoir or lake, and retaining its unmixed identity because of a difference in density.

density of snow

The ratio, expressed as a percentage, of the volume which a given quantity of snow would occupy if it were reduced to water, to the volume of the snow. When a snow sampler is used, it is the ratio expressed as percentage of the scale reading on the sampler to the length of the snow core or sample.

depletion curve

That part of the hydrograph extending from the point of termination of the recession curve to the subsequent rise or alternation of inflow due to additional water becomming available for stream flow.

depression storage

The volume of water contained in natural depressions in the land surface, such as puddles.

depth of runoff

The total runoff from a drainage basin divided by its area. For convenience in comparing runoff with precipitation, the term is usually expressed in inches of depth during a given period of time over the drainage area or acre-feet per square mile.

design criteria

The hypothetical flood used in the sizing of the dam and the associated structures to prevent dam failure by overtopping, especially for the spillway and outlet works.

detention basins

Structures which are built upstream from a populated area so that precipitation flows do not flood and cause the loss of life or property. They are normally dry, but are designed to detain surface water temporarily during and immediately after a runoff event. Their primary function is to attenuate the storm flows by releasing flows at a lower flow rate. There are no gates or valves allowed on the outlet so that water can never be stored on a long-term basis. Typical detention times in such a basin would be on the order of 24 to 72 hours although some are as long as 5 to 10 days.

detention storage

The volume of water, other than depression storage, existing on the land surface as flowing water which has not yet reached the channel.

direct flood damage

The damage done to property, structures, goods, etc., by a flood as measured by the cost of replacement and repairs.

direct runoff

The runoff entering stream channels promptly after rainfall or snowmelt. Superposed on base runoff, it forms the bulk of the hydrograph of a flood.

discharge

The rate at which a volume of water passes a given point in a given amount of time. Common units are cubic feet per second (cfs), second-day feet (sdf), and cubic meter per second (cms). Two types of discharges are often measured and recorded:

1. instantaneous discharge: the discharge at a particular instant of time.
2. mean discharge: the arithmetic mean of individual discharges during a period of time.

discharge curve

A curve that expresses the relation between the discharge of a stream or open conduit at a given location and the stage or elevation of the liquid surface at or near that location. Also called rating curve and discharge rating curve.

discharge table

1. A table showing the relation between two mutually dependent quantities or variable over a given range of magnitude.
2. A table showing the relation between the gage height and the discharge of a stream or conduit at a given gaging station. Also called a rating table.

distribution hydrograph

A unit hydrograph of direct runoff modified to show the proportions of the volume of runoff which occur during successive equal units of time.

diversion

The taking of water from a stream or other body of water into a canal, pipe, or other conduit.

divide

The high ground that forms the boundary of a watershed. A divide is also called a ridge and a drainage divide.

downstream slope

The slope or face of the dam away from the reservoir water. This slope requires some kind of protection (e.g.; grass) from the erosive effects of rain and surface flow.

drainage area

An area having a common outlet for its surface runoff. Also known as watershed, catchment area, and drainage basin.

drainage basin

A part of the earth's surface which is occupied by a drainage system which consists of a surface stream with all its tributaries and impounded bodies of water. Also known as watershed, catchment area, and drainage area.

drainage divide

The boundary line, along a topographic ridge or along a subsurface formation, separating two adjacent drainage basins.

drains (relief wells)

A vertical well or borehole, usually downstream of impervious cores, grout curtains or cutoffs, designed to collect and direct seepage through or under a dam to reduce uplift pressure under or within a dam. A line of such wells forms a "drainage curtain".

drawdown

The lowering of the surface elevation of a body of water, the water surface of a well, the water table, or the piezometric surface adjacent to the well, resulting from the withdrawl of water therefrom.

dredging

The scooping, or suction of underwater material from a harbor, or waterway. Dredging is one form of channel modification. It is often too expensive to be practical because the dredged material must be disposed of somewhere and the stream will usually fill back up with sediment in a few years. Dredging is usually undertaken only on large rivers to maintain a navigation channel.

drought

A period of abnormally dry weather sufficiently prolonged from the lack of precipitation to cause a serious hydrological imbalance.

drought index

Computed value which is related to some of the cumulative effects of a prolonged and abnormal moisture deficiency. (An index of hydrological drought corresponding to levels below the mean in streams, lakes, and reservoir.)

dry floodproofing

A dry floodproofed building is one which is sealed against flood waters. All areas below the flood protection level are made watertight. Walls are coated with waterproofing compounds or plastic sheeting. Openings like doors windows, sewer lines and vents are closed, whether permanently, with removable shields, or with sandbags. The flood protection level should be no more than 2 or 3 feet (0.5 to 1.0 meter) above the top of the foundation because the buildings walls and floors cannot withstand the pressure of deeper water.