Unformatted text preview: um of the processes that result in the wearing away or the progressive
lowering of the earth’s surface by various natural agencies that include weathering, erosion,
mass wasting, and transportation.
Deposition is the constructive process of accumulation into beds or irregular masses of loose
sediment or other rock material by any natural agent; it is especially the mechanical settling of
sediment from suspension or tractive movement in water. 15 Depth-integrated sample is a sample of sediment that is accumulated continuously in a
sampler that moves vertically at a constant rate and that admits water and sediment mixture at
a velocity about equal to the stream velocity at every point of the sampler’s travel. Depthintegrating samplers normally collect water and sediment mixture only from the stream surface
to about 0.1 meter above the stream bed. The part of the stream traversed by depthintegrating samplers is the “sampling zone” or the “sampled zone”.
Desert is a generic term describing an area of low precipitation relative to evaporation;
typically, the term desert applies to areas receiving less than 250 mm mean annual
precipitation, but areas of higher precipitation are also considered deserts if evaporation rates
are high. Owing to low precipitation, very cold areas, such as Antarctica, may be classed as
deserts. Where subtle differences in climatic variables result in different vegetation
communities, deserts are given specific names such as the Mojave, Sonoran, and Chihuahuan
Deserts of southwestern North America.
Detachment is the process of separation of transportable particles from a soil or soft-rock layer,
usually by running water, raindrop impact, or wind.
Discharge, as a hydrologic term of streamflow, is expressed as the movement downstream per
unit length of channel of a volume of water; water discharge is given in volume per unit time,
typically cubic meters per second (m3 s-1). As a sedimentology term, discharge is the movement
of a mass of sediment per unit length of channel in a specified time interval; technically it is
expressed in watts per meter (W m-1), but informally it is viewed as mass per unit time. Owing
to theoretical considerations, the term sediment-transport rate is preferred to that of
Dissolved load is the part of the total stream load that is carried in solution, such as chemical
ions yielded by erosion of the landmass during the return of precipitation to the oceans; also
called dissolved solids and solution load.
Dissolved solids refers to the sum of all mineral material in solution in surface or ground
waters; the amount of dissolved solids is a measure of the quality of the water and is generally
expressed as milligrams per liter. The regulatory acronym for dissolved solids is TDS (total
Distributary, as a fluvial-geomorphic term, typically refers to the spitting of a stream channel
into two or more segments that leave the main channel and do rejoin it, as generally occurs on
deltas; less commonly the term is used to characterize the individual channels of an alluvial fan
that split from a main, up-slope, channel and again coalesce downslope.
Distribution (or partition) coefficient, Kd, is the extent to which a solute is sorbed to a given
sediment particle at equilibrium, and is quantified as Cs/Ce, where Cs is the concentration of a
contaminant sorbed to a specific weight of sediment, and Ce is the concentration of the same
solute dissolved in an equal amount of water. This definition, for use in sediment studies,
differs from that used by chemists, which is the ratio of concentrations of a solute in two
immiscible solvents (for geologic situations, generally oil and water). 16 Disturbance is any short-term alteration, natural or imposed, of the land surface that results in
a change of geomorphic, hydrologic, or biological processes from a state of approximate
equilibrium to one of relative instability; time scales of disturbance generally vary from years to
Divergence, when used in a geomorphic context, is a term to acknowledge that some
landforms with characteristics measurably different among each other are the end, or final,
results of a narrow range of similar process sets causing the landforms. When used in a
hydrologic context, divergence, or divergent flow, refers to a broadening or expansion of flow
paths as surface runoff traverses a convex slope or as ground water moves from a restricted
flow path to one of less restriction.
Dominant discharge is described as the discharge of a stream that is associated with the
maximum sediment-transport rate for specified magnitude and frequency of flow; as such it is
a theoretical discharge representing the single flow rate of a stream that accomplishes the most
geomorphic work during an extended period of time. The term is an extension of the bankfulldischarge concept and is commonly inferred to be the maximum flow that the channel of an
adjusted perennial stream can convey without causing spillage onto the flood plain. When...
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