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Unformatted text preview: e is any dissolved substance; in natural environments it is generally rock, mineral, soil, or
organic matter dissolved in water.
Sorting is the process by which sediment particles of similar characteristics, mostly size but
shape and specific gravity as well, are selectively separated from other particles, concentrated,
and deposited as a sedimentary mass by an entraining fluid, generally water and wind but to a
lesser degree by ice and slurries (mass movement).
Sorting index is a numerical measure of the extent to which the process of sorting has caused
the concentration certain particle sizes within a sediment deposit; prominent is a sorting index,
S, based on a standard deviation, of 0.5 (d50/d16 + d84/d50), in which d16, d50, and d84,
respectively, are the particle diameters, as determined from a size-distribution analysis, in
which 16, 50, and 84 percent of a sediment sample by weight is finer than the total sample
Source area is, as related to the process of channel initiation, the area of a watershed supplying
water and sediment to the area of stored sediment above the highest point of an identifiable
channel; thus, a source area supplies sediment to a drainage network, and some or most of that
sediment may be temporarily stored before entrainment as fluvial sediment.
Specimen, relative to the needs and activities of the natural sciences, is an individual item, part,
or quantity typical of a larger mass, volume, group, or population, which has apparent or
obvious characteristics that categorize it as representative of part of a larger feature, element,
or landscape process, and which, owing to susceptibility to deterioration, lacks permanence. In
general, a specimen (such as a bacteria culture) has properties that prevent it, without
undergoing some form of preservation or reduction, from being suitable for long-term storage.
Stability, as a descriptor of geomorphic processes and landforms, refers to a condition of
approximate balance between inflows and outflows of matter through or over a landscape
feature. As a geomorphic concept, stability generally is regarded as being an integration of
processes affecting a system and thus has time-independence; the term often is used
synonymously with (dynamic or quasi) equilibrium.
Stage, or gage height, is the height of a water surface above an established datum plane,
generally at a gaging station. 42 Stage-discharge curve, or rating curve is a graph showing the relation between the gage
height, usually plotted as the ordinate, and the amount of water flowing in a channel,
expressed as volume per unit time, plotted as abscissa.
Stage-discharge relation is the relation between stage and discharge expressed by the stagedischarge curve.
Standard deviation, as applied to sediment studies, technically is the square root of the
average of the squares of the deviations of specified particle sizes, typically d16, and d84, about
the mean, d50, of a particle-size analysis. Practically, the standard deviation, as applied to
sediment studies, is a measure of data scatter, and thus provides also measures of the range of
particle sizes from the mean and sediment sorting; the computation causes about 68 percent of
data populating a set to be within one standard deviation of the mean, which is the reason for
selecting values of d16, and d84.
Stemflow, a hydrologic term that also pertains to sedimentology (erosion) and plant ecology, is
an effect of interception that results in the gravitational movement of water down the limbs,
stems, or trunk of plants, especially trees. Particularly for trees, stemflow concentrates
intercepted rainfall by the crown of the tree at the surfaces of stems or trunks, causing
enhanced wetting and soil moisture in the ground areas beside the trunks. This process set
minimizes the erosive impact of raindrop impact beneath the tree, but especially during
intense storms may cause overland flow and rill erosion at the base of the plant owing to the
concentration of water.
Stream is a general term for a body of flowing water; in hydrology the term is generally applied
to the water flowing in a natural channel as opposed to a canal or a drainage ditch.
Streamflow is the discharge (m3 s-1) that occurs in (and, during floods, adjacent to) a natural
channel. The term streamflow is more general than runoff and can be applied to discharge
regardless of whether it is affected by diversion or regulation; streamflow is the water
remaining after losses of precipitation or snowmelt to evaporation or sublimation and after
available water has satisfied the needs of vegetation and replenishment of soil moisture.
Ground water, as inputs from springs and seeps, may be a significant component of streamflow
in some channels.
Stream order is a designation indicating the position that a stream-channel segment has within
the hierarchy of channels of a drainage network; the uppermost, headwater channels of a
drainage network are typically assigned a stream order of 1 and the most downstream channel
segment has the highest stream-order designation, perhaps 6 or 8. Owing t...
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