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Unformatted text preview: d suspended-sediment load is the suspended-sediment load that can be computed
from water discharge and the concentration of depth-integrated samples.
Mechanical (or physical) weathering is the reduction of rock fragments and rock surfaces by
physical processes including abrasion, shattering by particle impact, expansion of crevices by
roots, frost action, or salt-crystal growth, and gravitational effects such as slope failure and
other forms of mass movement. Mechanical weathering affects the physical condition of the
rock or rock fragment; the chemistry of the rock is unaltered.
Mesic refers to an environment (habitat) that is characterized by moist conditions, neither
markedly wet (hydric) or dry (xeric).
Mesophyte is a plant that is adapted to a habitat of moderately moist conditions.
Monsoon is a seasonal wind or shift in wind direction resulting from greater variation in air
temperatures over land areas than over oceans. Commonly monsoonal shifts in air flow are
accompanied by changes in precipitation patterns owing to changed moisture sources; in
southwestern North America, for example, influxes of moist marine air from the south in July,
August, and September often cause high-intensity convectional monsoonal thunderstorms. N
Normal distribution, a statistical concept, is a probability distribution of (hydrologic) data that
exhibits symmetry relative to the mean, median, and modal values of the data. A normal
distribution assumes a “bell-shaped” curve when the data are plotted as magnitude relative to
frequency and is known also as a Guassian or Laplacean distribution. A principal application of
probability distributions to hydrology is flood-frequency analysis; flood records of some stream
exhibit a normal distribution but most do not, necessitating the use of a skew coefficient as a
means of describing the distribution. 29 O
Ordinary High Water Mark is a legal term with numerous definitions generated by court
decisions. According to Title 33 Code of Federal Regulations 329.11(a)(1), the “ordinary high
water mark” on non-tidal rivers is the line on the shore established by the fluctuations of water
and indicated by physical characteristics such as a clear, natural line impressed on the bank,
shelving, changes in the character of soil, destruction of terrestrial vegetation, the presence of
litter and debris, or other appropriate means that consider the characteristics of the
surrounding areas. Though open to other interpretations, this definition implies a level similar
to that of the stage of mean discharge and thus a flow duration of generally 5 to 20 percent.
Outlier, relative to a list or group of credible hydrologic data, is a value within the data set that
departs from the trend defined by the rest of the data; an example could be a flood discharge
five-fold greater than that of any other flood in a lengthy record of annual flood discharges.
Outwash in a general sense is soil particles or sediment that moves down an upland surface
with overland flow to a rill or gully and is re-deposited on areas of lesser slope; more commonly
outwash is used as a geomorphic term referring to rock debris that is removed from a glacier by
meltwater and is re-deposited in the stream channel as glaciofluvial sediment.
Overland flow is that part of precipitation or snowmelt that moves over the land surface, often
in small rivulets owing to micro-topography, toward a rill, gully, or channel before becoming
runoff as concentrated flow within the rill, gully, or channel.
Oxbow, as a hydrologic feature, is a horseshoe-shaped length of stream channel, a nearly
closed meander loop. As a fluvial-geomorphic feature, an oxbow is an abandoned meander
loop of an alluvial channel on a flood plain or alluvial terrace; evidence of an oxbow that has
largely filled with sediment may remain as a meander scar on the flood plain or alluvial terrace.
An oxbow lake is a horseshoe-shaped body of water, sometimes ephemeral, that occupies a
geomorphic oxbow. P
Partial-duration flood series is a list of all flood peaks that exceed a chosen base stage or
discharge, regardless of the number of peaks occurring in a year.
Particle size is the diameter (mm), as measured along the intermediate axis of a sediment
particle (thus, the maximum particle size that can pass through a screen or sieve of that mesh
size); sediment types are typically specified using a phi scale, by which particle sizes of less than
0.002 mm are clay, those ranging from 0.002 to 0.062 mm are silt, those of 0.062 to 2.0 mm are
sand, and particles of 2.0 to 64 mm, 64 to 256 mm, and more than 264 mm, respectively, are
designated gravel, cobbles, and boulders. Sediment of wash-load size, less than 0.062 mm, is
determined by hydrometer analysis.
Particle-size distribution curve is a graph, or curve, of specified particle sizes of a fluvialsediment sample plotted against values of percent weight of incremental portions of the
sediment sample finer than the total dry weight of the sediment sample....
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This document was uploaded on 02/26/2014 for the course ES 322 at Western Oregon University.
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