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.

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;

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;

A rock formation of low permeability and transmissivity that absorbs or transmits water slowly

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

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

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

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

A topographically enclosed depression or basin that has no stream outlet

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

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

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

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

A general term describing physical or mechanical erosion by diverse processes (wind, glacial, coastal, or fluvial)

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

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)

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

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

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

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

The volume measurement per unit time of the water passing through a specific cross-sectional area of a stream channel

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

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

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

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;

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

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

Pertaining to streams, rivers or their processes of water and sediment transport

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

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

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

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;

A measure of the elevation distribution with in a drainage basin

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

A point of significant or abrupt change stream gradient, often attributed to headward erosion related to baselevel change or tectonic uplift

Individual features or morphological forms that comprise part of the larger landscape and result for natural processes;

The larger scale morphology of the Earth’s surface

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

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.

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;

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

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.

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.

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

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.

See slope profile

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.

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

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;

A measure of the resistance to flow within a fluid; “thicker” fluids have higher viscosity and flow more slowly because of great internal friction;

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