Outdoor Glossary: D

dabbling ducks
also referred to as puddle ducks. Ducks that feed on surface or in shallow water and do not dive for their food- e.g., mallard, American black duck, gadwall, American wigeon, northern pintail, northern shoveler, and teal.

daily retardation of tides
The amount of time by which corresponding tidal phases grow later day by day (about 50 minutes).

dam
Structure built in rivers or estuaries, basically to separate water at both sides and/or to retain water at one side.

dapping
The art of dangling rather than casting your fly

data collection platform (DCP
A microprocessorbased system that collects data from sensors, processes the data, stores the data in random access memory (RAM), and provides communication links for the retrieval or transmission of the data.

datum
Any position or element in relation to which others are determined, as datum point, datum line, datum plane.

datum (vertical)
For marine applications, a base elevation used as a reference from which to reckon heights or depths. It is called a tidal datum when defined in terms of a certain phase of the tide. Tidal datums are local datums and should not be extended into areas which have differing hydrographic characteristics without substantiating measurements. In order that they may be recovered when needed, such datums are referenced to fixed points known as bench marks.

datum of tabulation
A permanent base elevation at a tide station to which all tide gauge measurements are referred. The datum is unique to each station and is established at a lower elevation than the water is ever expected to reach. It is referenced to the primary bench mark at the station and is held constant regardless if changes to the tide gauge or tide staff. The datum of tabulation is most often at the zero of the first tide staff installed.

datum plane
A horizontal plane used as a reference from which to determine heights or depths. The plane is called a tidal datum when defined by a certain phase of the tide. Datum planes are referenced to fixed points known as bench marks, so that they can be recovered when needed. See also reference plane.

Davidson Current
A North Pacific Ocean countercurrent setting northward between the California Current and the coasts of California, Oregon, and Washington during the winter months.

day
The period of rotation of the Earth. There are several kinds of days depending on whether the Sun, Moon, or other object or location is used as the reference for the rotation. See constituent day, lunar day, sidereal day, and solar day.

daylight saving time
A time used during the summer months, in some localities, in which clocks are advanced I hour from the usual standard time.

DBH (diameter at breast height)
The diameter of a tree 4 and 1/2 feet above the ground on the uphill side of the tree.

debris line
A line near the limit of storm wave UPRUSH marking the landward limit of debris deposits.

decay area
Area of relative calm through which waves travel after emerging from the generating area.

decay distance
The distance through which waves travel after leaving the generating area.

decay of waves
The change which occurs in waves when they leave a generating area and pass through a calm(or region of lighter winds). In the process of decay the significant wave length increases and the significant wave height decreases.

decibar
The practical unit for pressure in the ocean, equal to 10 centibars.

decision criteria
The rules and standards used to evaluate alternatives to a proposed action on National Forest land. Decision criteria are designed to help a decisionmaker identify a preferred choice from the array of alternatives.

decking area
A site where logs are collected after they are cut and before they are taken to the landing area where they are loaded for transport.

declination
Angular distance north or south of the celestial equator, taken as positive when north of the equator and negative when south. The Sun passes through its declinational cycle once a year, reaching its maximum north declination of approximately 23- about June 21 and its maximum south declination of approximately 23- about December 21. The Moon has an average declinational cycle of 27-1/3 days which is called a tropical month. Tides or tidal currents occurring near the times of maximum north or south declination of the Moon are called tropic tides or tropic currents, and those occurring when the Moon is over the Equator are called equatorial tides or equatorial currents. The maximum declination reached by the Moon in successive months depends upon the longitude of the Moon's node, and varies from 28- when the longitude of the ascending node is 0, to 18- when the longitude of the node is 180. The node cycle, or time required for the node to complete a circuit of 360 of longitude, is approximately 18.6 years.

declinational inequality
Same as diurnal inequality.

declinational reduction
A processing of observed high and low waters or flood and ebb tidal currents to obtain quantities depending upon changes in the declination of the Moon; such as tropic ranges or speeds, height or speed inequalities, and tropic intervals.

deep water
In regard to waves, where depth is greater than one-half the wave length. Deep-water conditions are said to exist when the surf waves are not affected by conditions on the bottom.

deep water waves
A wave in water the depth of which is greater than one-half the wave length.

deflation
The removal of loose material from a beach or other land surface by wind action.

degradation
The geologic process by means of which various parts of the surface of the earth are worn away and their general level lowered, by the action of wind and water.

DEIS (Draft Environmental Impact Statement)
The draft version of the Environmental Impact Statement that is released to the public and other agencies for review and comment

delta
(1) An alluvial deposit, usually triangular, at the mouth of a river of other stream. It is normally built up only where there is no tidal or current action capable of removing the sediment as fast as it is deposited, and hence the delta builds forward from the coastline. (2) A tidal delta is a similar deposit at the mouth of a tidal inlet, put there by tidal currents. (3) A wave delta is a deposit made by large waves which run over the top of a spit or bar beach and down the landward side.

density
(1) a measure of animal abundance in which the number of animals is given per unit area (i.e., 100 elk/km2). (2) independent factors- not related to the density of animals, cause changes in population parameters (primarily survival rates).

density
A line's buoyancy, which is determined by the relationship of its mass to that of water. If the density of line is greater than water the line will sink; if it's less, it will float.

density stratification
The lateral expansion of a sediment plume as it moves out of the distributary mouth, where salt and fresh water mix. This is most likely to occur where the speed of the river flow is moderate to low and the distributary mouth is relatively deep.

density, in situ ( s,t,p)
Mass per unit volume. The reciprocal of specific volume. In oceanography, the density of sea water is numerically equivalent to specific gravity and is a function of salinity, temperature, and pressure. See specific volume anomaly, thermosteric anomaly, sigma-t, and sigma-zero.

density-driven circulation
Variations in salinity create variations in density in estuaries. These variations in density create horizontal pressure gradients, which drive estuarine circulation.

depression
A general term signifying any depressed or lower area in the ocean floor.

depth
A vertical measurement, most often employed for the maximum height of the body excluding the fins; also refers to the distance below the sea surface in which the fish lives; also the vertical distance from a specified sea level to the sea floor.

depth, controlling
See contolling depth.

depthfinder
- An electric device that is used to detect the geography beneath the waters surface. Also used as a fishfinder.

design storm
Coastal protection structures will often be designed to withstand wave attack by the extreme design storm. The severity of the storm (i.e. return period) is chosen in view of the acceptable level of risk of damage or failure. A design storm consists of a design wave condition, a design water level and a duration.

design wave
In the design of harbors, harbor works, etc., the type or types of waves selected as having the characteristics against which protection is desired.

desired future condition
Land or resource conditions that are expected to result if goals and objectives are fully achieved.

detached breakwater
A breakwater without any constructed connection to the shore.

detritus
Small fragments of rock which have been worn or broken away from a mass by the action of water or waves.

developed recreation
Recreation that requires facilities that, in turn, result in concentrated use of the area. For example, skiiing requires ski lifts, parking lots, buildings, and roads. Campgrounds require roads, picnic tables, and toilet facilities.

deviation (of compass)
The deflection of the needle of a magnetic compass due to masses of magnetic metal within a ship on which the compass is located. This deflection varies with different headings of the ship. The deviation is called easterly and marked plus if the deflection is to the right of magnetic north, and is called westerly and marked minus if it is to the left of magnetic north. A deviation table is a tabular arrangement showing the amount of deviation for different headings of the ship. Each compass requires a separate deviation table.

diapositive
A positive photograph on a transparent medium.

differential erosion / weathering
These features develop in rocks which have varying resistance to the agencies of EROSION and/or weathering so that parts of the rock are removed at greater rates than others. A typical example is the removal of soft beds from between harder beds in a series of sedimentary rocks. The term may be applied to any size of feature, from small-scale etching to the regional development of hills and valleys controlled by hard and soft rocks.

diffraction
The phenomenon occurring when water waves are propagated into a sheltered region formed by a breakwater or similar barrier that interrupts a portion of the otherwise regular train of waves, resulting in the multi-directional spreading of the waves.

digital tide gauge
See automatic tide gauge.

dihedral
Wings of a flying bird held at an angle appearing to form a "V". No picture yet.

dike
Sometimes written as dyke; earth structure along a sea or river in order to protect littoral lands from flooding by high water; dikes along rivers are sometimes called levees.

dink
A very small fish; also known as a "guppy" or "fry guy."

direction of current
Direction toward which current is flowing.

direction of waves
Direction from which waves are coming.

direction of wind
Direction from which the wind is blowing.

disc drag
Allows for line to go out under pressure to avoid line breaks

disorger
An implement that removes hooks embedded in the throats and mouths of fish

dispersal
the movement of an animal from it natal area (place where it was born) to a new area where it lives and reproduces (if it survives that long).

disperse
(1) To spread or distribute from a fixed or constant source. (2) To cause to become widely separated.

dispersed recreation
Recreation that does not occur in a developed recreation site, such as hunting, backpacking, and scenic driving.

dispersion
(1) Act of dispersing, or state of being dispersed. (2) The separation of waves by virtue of their differing rates of travel.

disturbance
Any event, such as forest fire or insect infestations that alter the structure, composition, or functions of an ecosystem.

diurnal
Having a period or cycle of approximately one tidal day. Thus, the tide is said to be diurnal when only one high water and one low water occur during a tidal day, and the tidal current is said to be diurnal when there is a single flood and a single ebb period of a reversing current in the tidal day. A rotary current is diurnal if it changes its direction through all points of the compass once each tidal day. A diurnal constituent is one which has a single period in the constituent day. The symbol for such a constituent is the subscript 1. See stationary wave theory and type of tide.

diurnal current
The type of tidal current having only one flood and one ebb period in the tidal day. A rotary current is diurnal if it changes its direction through all points of the compass once each tidal day.

diurnal inequality
The difference in height of the two high waters or of the two low waters of each tidal day; also, the difference in speed between the two flood tidal currents or the two ebb currents of each tidal day. The difference changes with the declination of the Moon and, to a lesser extent, with the declination of the Sun. In general, the inequality tends to increase with increasing declination, either north or south, and to diminish as the Moon approaches the Equator. Mean diurnal high water inequality (DHQ) is one-half the average difference between the two high waters of each tidal day observed over the National Tidal Datum Epoch. It is obtained by subtracting the mean of all the high waters from the mean of the higher high waters. Mean diurnal low water inequality (DLQ) is one-half the average difference between the two low waters of each tidal day observed over the National Tidal Datum Epoch. It is obtained by subtracting the mean of the lower low waters from the mean of all the low waters. Tropic high water inequality (HWQ) is the average difference between the two high waters of each tidal day at the times of tropic tides. Tropic low water inequality (LWQ) is the average difference between the two low waters of each tidal day at the times of tropic tides. Mean and tropic inequalities, as defined above, are applicable only when the type of tide is either semidiurnal or mixed. Diurnal inequality is sometimes called declinational inequality.

diurnal range
Same as great diurnal range.

diurnal tide level
A tidal datum midway between mean higher high water and mean lower low water.

diving ducks
synonymous with bay-diving ducks.

dock
The slip or waterway between two piers, or cut into the land, for the reception of ships.

dorsal fin
The top fin of a fish. A median fin along the back, which is supported by rays. There may be two or more dorsal fins, in which case the most anterior one is designated the first.

double ebb (tidal)
An ebb current having two maxima of velocity separated by a smaller ebb velocity.

double flood (tidal)
A flood current having two maxima of velocity separated by a smaller flood velocity.

double tide
A double-headed tide; that is, a high water consisting of two maxima of nearly the same height separated by a relatively small depression, or a low water consisting of two minima separated by a relatively small elevation.

downbound
toward the Atlantic Ocean.

downdrift
The direction of predominant movement of littoral materials.

downrigger
A mechanical device that uses larger weights to get fishing lures or bait down to a specific depth and keep it there.

downwelling
A downward movement (sinking) of surface water caused by onshore Ekman transport, converging currents or when a water mass becomes more dense than the surrounding water.

draft (or draught)
depth of water needed to float a ship.

drag
The mechanical system in a reel that applies friction to the spool; also the force of water against a fly line or leader that causes the fly to move in an unnatural manner.

drainage basin
Total area drained by a stream and its tributaries.

dredging
Excavation or displacement of the bottom or shoreline of a water body. Dredging can be accomplished with mechanical or hydraulic machines. Most is done to maintain channel depths or berths for navigational purposes; other dredging is for shellfish harvesting or for cleanup of polluted sediments.

dressed fish
Fish with viscera, head and tail removed (some fish may have the tail on) but with skin and bone retained.

dressing weight
The weight of a fish after the viscera and sometimes the head and other body parts have been removed.

drift (of current)
The speed of the current.

drift current
A broad, shallow, slow-moving ocean or lake current. Opposite of stream current.

drift sector
A particular reach of marine shore in which littoral drift may occur without significant interruption, and which contains any and all natural sources of such drift, and also any accretion shoreforms accreted by such drift.

dropline
A deepwater fishing method involving the use of a vertical line bearing rows of baited hooks.

dry fly
An artificial fly that floats on the surface of the water; designed to replicate the adult stage of an insect

drying beach
That part of the beach uncovered by water (e.g. at low tide). Sometimes referred to as subaerial beach.

dunes
(1) Accumulations of windblown sand on the BACKSHORE, usually in the form of small hills or ridges, stabilized by vegetation or control structures. (2) A type of bed form indicating significant sediment transport over a sandy seabed.

durable water-repellent finish
(DWR) - Chemically treated fabrics that are water-repellent

duration
In forecasting waves, the length of time the wind blows in essentially the same direction over the FETCH (GENERATING AREA).

duration of ebb
The interval of time in which a tidal current is ebbing, determined from the middle of the slack waters.

duration of fall
The interval from high water to low water.

duration of flood
The interval of time in which a tidal current is flooding, determined from the middle of slack waters.

duration of flood and duration of ebb
Duration of flood is the interval of time in which a tidal current is flooding, and duration of ebb is the interval in which it is ebbing, these intervals being reckoned from the middle of the intervening slack waters or minimum currents. Together they cover, on an average, a period of 12.42 hours for a semidiurnal tidal current or a period of 24.84 hours for a diurnal current. In a normal semidiurnal tidal current, the duration of flood and duration of ebb each will be approximately equal to 6.21 hours, but the times may be modified greatly by the presence of nontidal flow. In a river the duration of ebb is usually longer than the duration of flood because of fresh water discharge, especially during spring months when snow and ice melt are predominant influences.

duration of rise and duration of fall
Duration of rise is the interval from low water to high water, and duration of fall is the interval from high water to low water. Together they cover, on an average, a period of 12.42 hours for a semidiurnal tide or a period of 24.84 hours for a diurnal tide. In a normal semidiurnal tide, duration of rise and duration of fall each will be approximately equal to 6.21 hours, but in shallow waters and in rivers there is a tendency for a decrease in duration of rise and a corresponding increase in duration of fall.

duration, minimum
The time necessary for steady-state wave conditions to develop for a given wind velocity over a fetch.

dynamic decimeter
See geopotential as preferred term.

dynamic depth (height)
See geopotential difference as preferred term.

dynamic depth (height) anomaly
See geopotential anomaly as preferred term.

dynamic equilibrium
Short term morphological changes that do not affect the morphology over a long period.

dynamic meter (D)
The former practical unit for geopotential difference (dynamic depth), equal to 10 geopotentials (dynamic decimeters). See geopotential (dynamic depth) anomaly.

dynamic topography
See geopotential topography as preferred term.