Abrasion
The physical wearing of sediment grains by frictional contact and
impact during fluvial transport; Abrasion is not a part of WILSIM
but is an important fluvial process.
Aggradation
The accumulation of sediment on a flood plain or on an alluvial fan
that is sufficient to cause an increase in elevation of that surface;
Alluvial Fan
A cone shaped accumulation of sediment formed by fluvial deposition
just beyond the mount of a canyon; usually forms along fault block
mountain escarpments where the decrease in fluvial channel slope
from the canyon onto the adjacent plain facilitates deposition;
Alluvium
Detrital sediment deposited on the floors of fluvial channels during
low discharge or on adjacent flood plains during flood events;
the sediment transported by streams;
Angle of Repose
The maximum slope angle at which unconsolidated sediment can accumulate
and still remain stable;
Annular Drainage Pattern
Circular or ring like drainage pattern, usually associated with structural
domes of bedrock or eroded sedimentary basins with strata of variable
erodability
Antecedent Stream
Drainage system that existed on the landscape prior to tectonic deformation
and surviving through the deformation event;
Aquiclude
A rock formation of low permeability and transmissivity that absorbs
or transmits water slowly
Aquifer
A rock formation that contains sufficient porosity or fractures to
hold substantial water
Artesian Water
Groundwater within a confined unit or layer that is under sufficient
hydrostatic pressures such that it rises above its level of confinement
when breached by a well or natural spring
Azimuth
Horizontal direction to the horizon from a point on the landscape
or an observer, measured in 360 degrees of arc clockwise from the
north (north=00, east=900, south=1800, west=2700)
Barbed Tributary
A tributary that enters another stream in an upstream direction
Base Level
The lowest elevation to which a stream is capable of eroding the
landscape; the lowest elevation or mouth of a drainage system;
usually refers to sea level, or localized on a lake
Basin
An enclosed area or topographic depression that facilitates the accumulation
of water and/or sediment
Bed Load
Detrital sediment transported by flowing water along a channel bed
or sides either through rolling, sliding, or saltation; Usually
of coarser grain size as compared to the suspended load
Beheaded Stream
A stream segment that has been abandoned from connection to a drainage
network by stream piracy
Bifurcation
The observed branching of stream channels into more numerous and
smaller tributaries in the upstream direction.
Bifurcation Ratio
The ratio of the number of streams in a given stream order to the number of
streams in the next highest order;
Blind Valley
A stream segment that terminates downstream in a bedrock cliff or
obstruction; water is diverted underground into a karst system
of groundwater flow
Bolson
A topographically enclosed depression or basin that has no stream
outlet
Braidplain
The floodplain of a braided stream system
Braided Stream
A stream channel morphological type consisting of multiple channels
that diverge and rejoin across the floodplain (braidplain); the
channel cross section is typically wide relative to its depth and
the predominant sediment transported is bedload
Climate
The mean or average state of the atmosphere or seasonal variation
in meteorological conditions that characterize a region of the
Earth’s surface over an extended period of time (decades,
centuries, or millennia)
Collapse Sink
Subterranean karst caverns that become enlarged by dissolution to
the point where the roof collapses to create a closed depression
at the land surface
Coluvium
Unconsolidated sediments residing on a slope as the result of weathering
process; they are generally susceptible to creep or other gravity
driven mass wasting processes
Consequent Stream
A stream that develops as a consequence of the formation of an original
slope, generally related to tectonic tilting of the land surface
Constructional Surface
An area or region of the landscape where sediments are accumulating
(aggradation) to the point where the elevation increases; generally
associated with aggrading floodplains or allluvial fans
Contour
A line projected onto the land surface such that every point on it
lies at the same elevation
Contour Interval
The difference in elevation between successive contour lines
Corrasion
A general term describing physical or mechanical erosion by diverse
processes (wind, glacial, coastal, or fluvial)
Creep
The slow downslope movement of unconsolidated sediment or soil that
results from natural weathering of surficial materials; creep can
be either seasonal or continuous with downslope transport rates
measured in mm. or cm./year
Cuesta
An asymmetrical landform associated with tectonic tilting of sedimentary
rock; includes a short steep slope generated as a fault escarpment
and a longer gentler slope corresponding to the dip of the original
bedrock surface after tilting (dipslope)
Degradation
The lowering of the land surface due to erosion, or the process of
fluvial erosion and transport of floodplain sediment such that
through time the elevation of the floodplain decreases
Debris Cone
A cone shaped deposit of sediment; usually associated with the accumulation
of talus or rock fall debris but also with alluvial fans
Debris Flow
Mass transport process involving a slurry of sediment (of a variety
of different grain sizes from clays to boulders) and water; debris
flows can be a common process by which sediments are transported
onto alluvial fans in desert regions
Delta
The fluvial sediment (generally bedload) that accumulates where a
stream enters a lake or ocean
Dendritic Drainage Pattern
A generally uniform branching pattern for streams where lower order
streams enter larger streams at acute angles forming an overall
pattern similar to that observed in the vein structure of leaves
Detritus
Unconsolidated sediment on the land surface that results from the
natural process of weathering or erosion
Differential Erosion
Variation in the degree and rate of erosion (erodability) of portions
of the landscape because some surficial materials are more susceptible
to disintegration
Differential Weathering
Variation in the rate and degree of disintegration of surficial materials
due to variation in durability or susceptibility to chemical/physical
weathering for differing rock units exposed at the surface
Diffusion
The natural movement of particles from a region of higher concentration
to a lower concentration; in WILSIM, diffusion simulates the movement
of eroded particles from high elevations to lower elevations down
a developing slope
Dip Slope
A natural slope of the land surface that relates primarily to the
tilt (dip) of the underlying bedrock or tilt of the bedding planes
with in a tectonically uplifted block of sedimentary rock
Discharge
The volume measurement per unit time of the water passing through
a specific cross-sectional area of a stream channel
Dissection
The erosion of a landscape by various processes that leads to the
creation of topographic relief; often refers to fluvial erosion
creating a series of gullies, valleys, or canyons
Distributary Channel
The downstream branching of fluvial channels creating multiple channels
from a single larger channel; a common branching pattern observed
on alluvial fans and deltas
Divide
The highest elevation of the land surface (usually a ridge) separating
adjacent drainages
Drainage or Drainage System
A network of streams transporting water and eroded sediment from
a landscape
Drainage Basin
The total area enclosed by a divide that is drained by a stream network
Drowned Valley
The coastal reaches of a river system or valley that has been inundated
by sea level rise
Durability
A qualitative measure of the resistance of rock or soil exposed at
the surface to weathering or erosion;
Entrenched Meander
Downcutting by a stream in which the rate of vertical incision is
greater than the rate of lateral migration of the fluvial channel;
in this case the meander loop is preserved as the stream downcuts
Ephemeral Stream
A stream segment that transports water intermittently, usually after
a rainfall or snowmelt event; it is not fed from spring or groundwater
flow and is dry when precipitation is lacking
Erodibility
The variable in WILSIM that can be adjusted to simulate differing
rates of erosion for differing surface rock types; examples in
the natural landscape would include “soft” sedimentary
rock that is susceptible to weathering and erosion as compared
to more durable rock that erodes slowly;
Escarpment
A cliff, bluff, or steep slope; substantial topographic relief over
a short distance across the land surface; often associated with
tectonic uplift along a fault or intensive fluvial erosion/downcutting
Fall Velocity
The rate at which a particle falls through a column of still water; The rate
is dependent upon particle size, density, and shape.
Fault Scarp
The escarpment or cliff formed from uplift along a fault
Fill Terrace
Remnant of a former floodplain formed by aggradation of fluvial sediment
that is preserved adjacent to the stream after a subsequent period
of incision/downcutting
Floodplain
A low relief, generally planar surface adjacent to larger stream
channels that is inundated during floods or high discharge events;
they are also underlain by alluvium that accumulated during floods
Fluvial
Pertaining to streams, rivers or their processes of water and sediment
transport
Geomophology
The study of various processes, both endogenic (internal - tectonic
or isostatic) and exogenic (external - weathering, erosion, etc.)
that shape the surface of the Earth or the terrestrial planets
Graben
The “downthrown” or down dropped block between two fault
escarpments
Graded Stream
Conceptual hypothesis that a stream’s form and dynamics have
evolved to the point where their slope, discharge, and sediment transport
capabilities are in equilibrium such that the stream can transport
all of the sediment delivered to it by weathering or erosion of the
adjacent valley slopes
Gradient
The slope of a stream channel measured as the change in elevation
from one point to another downstream along a specific distance
along the channel
Groundwater
The zone of subsurface water in which all pore spaces or fractures in the rock
are filled with water (saturated)
Groundwater Divide
The area or line of the groundwater table from which water flows
in different directions
Headward Erosion
The up-valley extension of a stream over time by erosion due to runoff
or sapping at the head of the stream valley or gulley
Hydraulic Gradient
The slope of the water surface in an open stream system, or the change
in elevation of the water table (piezometric surface) in a subsurface
saturated (groundwater) system
Hydrologic Cycle
The cycle of water transfer from evaporation (of ocean, lake, or
stream surfaces) or glacial sublimation - to atmospheric water
vapor - to precipitation - to runoff back to the ocean either through
overland flow of water or by groundwater flow
Hydrostatic Pressure
The pressure exerted on rock or unconsolidated sediment by groundwater
flow through a porous medium; lateral water flow is generated by
a hydrostatic head or variation in the piezometric surface;
Hypsometric Curve
A cumulative measurement of elevation distribution with in a drainage
basin; Specifically it is a measure of the cumulative area that
lies at specific elevations or with in elevation increments for
a drainage basin
Hypsometric Integral
The elevation increments that are measured in a Hypsometric Curve,
or the area beneath the curve corresponding to a specific elevation;
Hypsometry
A measure of the elevation distribution with in a drainage basin
Impermeable
Refers to rock or unconsolidated sediment that does not permit water
to flow into or through it
Incised Meander
Entrenched Meander - Downcutting by a stream in which the rate of
vertical incision is greater than the rate of lateral migration
of the fluvial channel; in this case the meander loop is preserved
as the stream downcuts
Influent Stream
A stream that is contributing or losing water to the groundwater
system (a “losing stream”)
Insequent Stream
A drainage system that develops by random headward erosion across
a uniformly sloping surface underlain by horizontally stratified
sedimentary rock
Intermittent Stream
“
Ephemeral Stream” A stream segment that transports water intermittently,
usually after a rainfall or snowmelt event; it is not fed from spring
or groundwater flow and is dry when precipitation is lacking
Knickpoint
A point of significant or abrupt change stream gradient, often attributed
to headward erosion related to baselevel change or tectonic uplift
Landform
Individual features or morphological forms that comprise part of
the larger landscape and result for natural processes;
Landscape
The larger scale morphology of the Earth’s surface
Lacustrine
Of or related to lakes or the processes of sedimentation in lakes
Laminar Flow
The type of flow in which water molecules flow in parallel trajectories
downstream; There is little or no mixing between layers of water
Lateral Planation
Erosion of the land surface or floodplain by horizontal (lateral)
migration of a stream channel
Load
The grain size and volume of sediment transported by a stream
Mass Wasting
The downslope movement of unconsolidated materials (colluvium) or
rock debris under the influence of gravity; Rates of mass wasting
vary substantially from the rapid process of rock falls or debris
flows to the slow process of seasonal or continuous creep.
Meander
A floodplain stream channel feature consisting of a curved loop in
which channel bank erosion is concentrated along the outside of
the loop and deposition of sediment occurs on the inside (lateral
accretion deposits or point bar deposition); Through time the stream
channel migrates laterally across the floodplain or in the downstream
direction.
Meander Belt
The active portion of a floodplain containing the highest concentration
of meanders; Sometimes designated as that portion of the floodplain
lying between two lines tangent the apex of the outermost meander
bends on each side of the floodplain;
Morphometry
The numerical analysis of a drainage basin involving stream numbers,
length measrement (as in stream lengths of each stream order) and
areal measurement of drainage basins of each order
Outwash
Sediments released during melting along the margins of a glacier
or ice sheet and deposited by meltwater streams (glacial-fluvial
sediment)
Outwash Plain
The floodplain or braidplan of a glacial meltwater stream;
Paired Terraces
Terraces on opposite sides of the floodplain or valley whose surfaces
lie at the same elevation
Parallel Drainage Pattern
A general pattern in which most streams flow down a relatively uniform
slope parallel to each other.
Pediment
An inclined slope formed by fluvial erosion, generally along mountain
fronts; They may resemble broad alluvial fans in morphology but
consist of a thin veneer or cover of coarse alluvial sediment over
the erosional surface.
Peneplain
Conceptual model of a landscape that has undergone long periods of
erosion and is reduced to a low relief or nearly planer surface;
Piezometric Surface
The level to which water from an aquifer rises in a well or open
conduit; it is generally describes the variation in elevation of
the groundwater table with in a porous medium
Precipiton
A precipitation event in WILSIM that is sufficient to generate surface
runoff that leads to erosion of the land surface within individual
cells, downslope diffusion, or transport of eroded sediment in
a stream channel; Precipitons in WILSIM vary in intensity (total
volume or precipitation) and in duration.
Profile
See slope profile
Runoff
The overland flow of water (sheetwash) generated when there is an
excess of precipitation such that surface soil or porous rock become
saturated and can not accept additional water; Runoff occurs when
there is sufficient precipitation over an extended period of time
to saturate the surface materials, or the rate of precipitation
is too fast to allow for all the water to infiltrate into the soil
or porous rock;
Slope Profile
A measure and graph of the elevation along a specific line indicated
on a map or the land surface; Longitudinal profiles are constructed
down slope or down a stream channel, Latitudinal profiles are constructed
across or perpendicular to the direction in which the surface slopes;
Stream Competence
The ability of a stream to erode the landscape or to transport the
sediment available to it; often defined by the largest particle
size that can be transported as bedload under mean discharge or
alternatively, flood discharge
Stream Order
The numerical designation for individual stream segments from lowest
order (smallest tributaries) to higher order, increasing incrementally
at stream junctions
Stream Piracy
The capture of one stream segment or portion of a drainage by the
headward erosion of an adjacent stream
Subsequent Stream
A stream or drainage network whose form develops by selective or
differential erosion of weaker rock
Superposed Stream
A Stream or drainage network that has eroded into a landscape exposing
a pre-existing landsurface or surfaces that had been covered by
sedimentary rock; The drainage pattern may inherit some aspects
of the older surface morphology.
Suspended Load
Sediment (generally finer grains) kept in suspension by turbulence
and may be deposited when discharge decreases or turbulence diminishes;
In order for a grain to be suspended, the turbulent flow velocity
must equal or exceed the fall velocity for that particle.
Tectonics
Deformation of the Earth’s crust which gives rise to diverse
landforms including folds, faults and fault escarpments, large scale
plateau or mountain ranges;
Tectonic Basin
A sedimentary basin or enclosed depression formed by tectonic deformation
that causes subsidence of the crustal rock
Terrace
Flat elevated surface features within or adjacent to the active floodplain
that consist of remnants of former floodplains or planer surfaces
of fluvial erosion; These features are left stranded above the
floodplain during periods of active incision or downcutting by
the stream. They are most prominently preserved when the rate of
downcutting exceeds the rate of lateral migration and erosion of
the stream channel across its floodplain.
Through Flow
Water moving through permeable soil horizons generally along pathways
that are a combination of downward and downslope vectors;
Trellis Drainage Pattern
A drainage characterized by mostly parallel stream segments which
join each other at right angles;
Turbulent Flow
The type flow in which individual water molecules have nearly random
directions of motion superimposed upon the down stream flow trajectory;
Unpaired Terraces
Terraces on opposite sides of the floodplain or valley whose surfaces
lie at differing and unmatched elevations; Usually result from
stream downcutting in which rate of lateral migration of the stream
is significant compared to the rate of vertical incision;
Valley Train
Large or thick accumulations of glacial outwash deposited between
the restrictions of valley side walls;
Viscosity
A measure of the resistance to flow within a fluid; “thicker” fluids
have higher viscosity and flow more slowly because of great internal
friction;