Channel Migration Assessment

Appendix A - Glossary


Accretion: The gradual deposition of sediment along the edges of a channel by lateral migration. One of the bar formation processes that creates bars opposite of the meander bend.

Active channel: The portion of the channel or floodplain network that receives periodic scour and/or fill during sediment transport events.

Aggradation: The process of building up a surface by sediment deposition.

Alluvial channel: A channel formed in material (sand, gravel, cobbles, or small boulders) that moves during floods. Alluvial channels convey channel bed and bank materials under present flow conditions and adjust their dimensions, shape, and gradient under the present hydrologic regime. For the most part, streamflow, sediment supply, and woody debris control how alluvial channels change over time.

Alluvial fan: A low, outspread mass of loose materials (sand, cobbles, boulders), with variable slope, shaped like an open fan or a segment of a cone, deposited by a stream at the place where it issues from a narrow mountain or upland valley; or where a tributary stream is near or at its junction with the main stream. It is steepest near its apex which points upstream and slopes convexly outward (downstream) with a gradual decrease in gradient.

Alluvial terrace: An abandoned floodplain, produced by past vertical instability in the fluvial system. Alluvial terraces are inactive depositional surfaces within a current hydrologic, climatic, and tectonic setting. Alluvial terraces can result from a lowering of the river’s base level, from channel incision, or from changes in hydrology.

Alluvium: Material (sand, gravel, cobbles, or small boulders) that is deposited by flowing water. Two processes occur simultaneously in anastomosing channels: (1) avulsion, which creates a pattern of multiple channels; and (2) lateral migration of the individual channels that exist within the anastomosing pattern (i.e., individual meander belts).

Anabranching: One subcategory of island braided channel pattern that is characterized by low width–depth ratio, gentle gradient, variable peak discharge, frequent flooding and high sediment load. Anabranching rivers consist of multiple channels separated by vegetated semi-permanent alluvial floodplain islands excised from existing floodplain or formed by within-channel or deltaic accretion. The anabranching can be a response to the streams inability to transport high sediment load due to gentle channel slope and dominance of aggradation process. Anabranching provides greater efficiency for transporting sediment. The development of anabranches is related to rapid and frequent avulsions of the river channels and lateral migration.

Anastomosing: Anastomosing channels are a subcategory of the island-braided channel pattern with interconnected, coexisting channels separated by terraces or floodplain islands, with erosion-resistant cohesive banks, gentle gradient, and relatively low width-depth ratios of individual channels.. The distinguishing feature of anastomosing channels is that hydraulic and sediment transport dynamics of each channel are independent of the other channels. Anastomosing channels are generally stable in the short term with cohesive banks, low width to depth ratio channels, and gentle channel gradient that exhibit little or no lateral migration. The dominant channel migration process is avulsion.

Avulsion: The process in which a stream rapidly abandons a developed channel and creates a new one. Channels may avulse into an abandoned channel (second-order avulsion) or create a new channel (first-order avulsion) depending on the preexisting boundary conditions that initiate the avulsion.

Avulsion Hazard Zone (AHZ): The portion of the CMZ that delineates avulsion hazards not accounted for in the HMZ. Refer to Glossary of channel migration components.

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Bankfull stage: The stream level that corresponds to the discharge at which channel activity (sediment transport, the formation and/or reformation of bars, the formation and/or alteration of bends and meanders, etc.) results in the normally occurring morphologic characteristics of channel.

Bar: [streams] a general term for a ridge-like accumulation of sand, gravel, or other alluvial material formed in the channel, along the banks, or at the mouth of a stream where a decrease in velocity induces deposition; e.g. a channel bar or a meander bar. Examples include:

Point bars - Bars that are formed on the inside of meander channels.
Side bars - Bars that are formed along the edges of relatively straight sections of rivers.
Mid-channel bars - Bars found within the channel that become more noticeable during low flow periods.
Delta bars - Bars formed immediately downstream of the main confluences of a tributary and the main channel.

Braided stream: A channel or stream that has interconnecting multiple channels formed by flow that repeatedly divides and converges around mid-channel bars. In the plan view, the channel resembles strands of a complex braid. Braiding is generally confined to broad, shallow streams of low sinuosity, variable discharge, high bedload, non-cohesive bank material, and a steep gradient. At bank-full discharge, braided streams often have steeper slopes and shallower, broader, and less stable channel cross sections than meandering streams. During periods of high discharge, the entire stream channel may contain water and the islands are covered to become submerged bars. During such high discharge, some of the islands could erode, but the sediment would be re-deposited as the discharge decreases, forming new islands or submerged bars. Islands may become resistant to erosion if they become inhabited by vegetation.

Channel confinement: The width between the channel’s valley walls relative to the width of the active channel. Used to describe how much a channel can potentially shift within its valley.

Channel gradient: The angle between the channel flow and the horizontal length of the stream reach. Measured as the change in channel elevation divided by stream or reach length. .

Channel migration: The lateral or downstream shifting of a river channel within a river valley. The dynamic physical processes of rivers, including the movement of water, sediment and wood, cause the river channel in some areas to move, or "migrate," over time. This is a natural process in response to gravity and topography and allows the river to release energy and distribute its sediment load. Migration processes include bank erosion and avulsion. Aggradation causes vertical channel change which is another migration process. The area within which a river channel is likely to move over a period of time is referred to as the channel migration zone (CMZ)

Channel reach: A specific portion of the length of a channel that has similar physical features, such as gradient and confinement.

Channel pattern: A configuration of a stream reach as seen in planform. Generally recognized channel patterns include meandering, braided, wandering, island braided and straight.

Cut-off avulsion: A type of avulsion that bisects the neck of a meander and connects the apex of one meander with another downstream. Cut-off avulsions leave behind an abandoned oxbow channel or lake. See also meander bend cut-off.

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Degradation: Incision, or down-cutting of the channel bed.

Delta: A body of alluvium consisting mostly of stratified clay, silt, sand and gravel, nearly flat and fan-shaped, deposited at or near the mouth of a river or stream where it enters a body of relatively quiet water, usually a sea or lake.

Disconnected Migration Area (DMA): The area located landward of man-made structures that restrict channel migration. Not all man-made structures are exempted from the CMZ shoreline regulations. The specific exemptions are outlined in Step 5 and WAC 173-26-221(3)(b). Refer to Glossary of channel migration components.

Distributary channel: A divergent stream flowing away from the main stream and not returning to it, as in a delta or on a flood plain. It may be produced by stream deposition choking the original channel. These typically occur at the mouth or delta of a river where it empties in a lake or ocean or on an alluvial fan.

Dynamic equilibrium: The stream's energy (discharge and slope) and the sediment load it can transport are directly related. Any increase in energy will allow erosion to take place resulting in an increase in sediment load. A decrease in energy will result in deposition of some of the load. The feedback mechanisms resulting from these basic processes exhibit a dynamic form of stability, known as dynamic equilibrium. Dynamic equilibrium refers to the ability of system to persist within a range of conditions. Maintaining this balance requires the presence of a series of self-correcting mechanisms. A disturbance to the stream system triggers a response from these self-correcting mechanisms allowing maintenance of the dynamic equilibrium.

Entrenched channel: A channel that has cut vertically downward into streambed deposits or bedrock, resulting in a channel that is much deeper than it is wide.

Erosion Hazard Area (EHA): The area of the CMZ unaccounted for in the AHZ or the HMZ that delineates channel susceptibility to bank erosion from stream flow or mass wasting. The EHA is defined by the ES and the GS. Refer to Glossary of channel migration components.

Erosion Setback (ES): As part of the EHA, the ES encompasses the area outside the HMZ and AHZ that is susceptible to channel erosion; it includes those areas that are not at risk of avulsions, but are susceptible to stream or river erosion. Refer to Glossary of channel migration components.

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Failure: A mass wasting event where a bank hillslope’s face destabilizes and moves downslope.

Flood frequency: Number of times a flood above a given discharge or stage is likely to be exceeded or equaled in any given year. It is often calculated as a recurrence interval (e.g., 2-year flood) or a probability of occurrence (e.g., 50% probability of occurring). (Recurrence Interval: the average time interval in years in which a flow of a given magnitude will recur)

Fluvial: Of or pertaining to rivers or streams; produced by stream action, e.g. fluvial landform.

Geomorphology: The branch of geology and geography which deals with the form of the earth, the general configuration of its surface, and the changes that take place in the evolution of land forms.

Geotechnical Setback (GS): As part of the EHA, the GS extends from the outer boundary of the ES for the purpose of establishing a stable slope configuration following mass wasting. GS delineation accounts for the natural adjustment process that an embankment over-steepened by channel erosion will go through. Refer to Glossary of channel migration components below.

Glossary of channel migration components Definition
Historical Migration Zone (HMZ) The collective area the channel occupied in the historical record. Other terms such as historic occupation tract are used in some published analyses.
Avulsion Hazard Zone (AHZ) The area not included in the HMZ that is at risk of avulsion over the timeline of the CMZ.
Erosion Hazard Area (EHA) The area not included in the HMZ or the AHZ that is at risk of bank erosion from stream flow or mass wasting over the timeline of the CMZ.
EHA=ES+GS The EHA has two components: the Erosion Setback (ES) and the Geotechnical Setback (GS). The ES is the area at risk of future bank erosion by stream flow; the GS is defined by channel and terrace banks that are at risk of mass wasting (due to erosion of the toe). The GS projects from the ES at a side slope angle that forms a stable bank configuration, thereby accounting for mass wasting processes that will promote a stable angle of repose.
Disconnected migration zone (DMA) The area located in CMZ where man-made structures restrict channel migration. Not all man-made structures are exempted from the CMZ. The specific exemptions are outlined in Step 5 and WAC 173-26-221(3)(b).
CMZ=HMZ+AHZ+EHA-DMA CMZ is the sum of its components. DMZ only applies if areas meet legal exemptions.

Historical Migration Zone (HMZ): The portion of the CMZ study area that the channel occupied in the historical record. Refer to Glossary of channel migration components (above). Other analyses may use different terms like the Historic Channel Occupation Tract (HCOT). HMZ and HCOT are synonymous.

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Incision: The process of downcutting into a stream channel leading to a decrease in the channel bed elevation. Incision is often caused by a decrease in sediment supply and/or increase in sediment transport capacity. A decrease in base level can cause headcutting that migrates upstream and produces incision upstream and initiating aggradation downstream.

Island braided pattern: A multi-thread channel pattern that has multiple, interconnected, coexisting channels. This channel patterns has vegetated islands between channels, whereas braided channels have bare bars. The vegetated islands generally are not inundated during high to moderate frequency floods whereas braided channels become inundated during small floods. Anabranching and anastomosing channel patterns are two subcategories of this pattern.

Lateral migration: The bank erosion process where the side to side movement of meander migration undercuts the bank. Lateral growth increases the meander bend amplitude.

Main channel: The main stream channel is the dominant channel with the deepest or lowest thalweg, the widest width within defined banks, and the most water during low flow periods. Main channel locations can be transient over time. Braided channels may not have a defined main channel.

Mass wasting: The down slope movement of material due to gravity (rather than water, wind, or ice, for example).

Meander: One of a series of freely developing sinuous curves or loops produced as the stream moves from side to side of its floodplain. Meander bend is the convex side of a meander. Meander bend migration is the lateral or downstream movement of a sinuous curve in a stream within a river valley

Meander-bend cutoffs: The shortened channel resulting when a stream cuts through a meander neck or narrow strip of land between adjacent meander bends. Bend cutoffs leave behind an oxbow lake. See also avulsion cut-off.

Meander scar: A crescent-shaped, concave or linear mark on the face of a bluff or valley wall, produced by the lateral erosion of a meandering stream which impinged upon and undercut the bluff; if it's no longer adjacent to the modern stream channel it indicates an abandoned route of the stream.

Meander scroll: One of a series of long, parallel, close fitting, crescent-shaped ridges and troughs formed along the inner bank of a stream meander as the channel migrated laterally down-valley and toward the outer bank. (Figure 5).

Multi-threaded channels: A section of a stream that is characterized by several discernable channels separated by bare sediment bars or vegetated islands.

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Oxbow: A closely looping stream meander having an extreme curvature such that only a neck of land is left between the two parts of the stream.

Oxbow lake: A crescent-shaped, body of standing water along a stream created by a meander-bend cutoff or avulsion. Once isolated, oxbow lakes will slowly fill up with sediment, as point bar sands and gravels are buried by silts, clays, and organic material carried in by floods and by sediment slumping in from sides as rain fills up lake.

Planform: The shape and size of channel and overbank features as viewed from above.

Relic channel: An abandoned channel that is not presently active.

Rectification: The operation of matching a scanned photograph or map with projected electronic data (I.e. parcel lines or an orthophoto).

River [streams]: A general term for a natural, freshwater surface stream of considerable volume and generally with a permanent base flow, moving in a defined channel toward a larger river, lake, or sea. Rivers are a subset of streams.

Secondary channel: Any channel on or in a floodplain that carries water (intermittently or perennially in time; continuously or interrupted in space) away from, away from and back into, or along the main channel. Secondary channels include: side channels, wall-based channels, distributary channels, anabranch channels, abandoned channels, overflow channels, chutes, and swales.

Stream: (a) Any body of running water that moves under gravity to progressively lower levels, in a relatively narrow but clearly defined channel on the ground surface, in a subterranean cavern, or beneath or in a glacier and transports sediment and dissolved particles. (b) A term used in quantitative geomorphology interchangeably with channel; (c) (under the shoreline management act). A naturally occurring body of periodic or continuously flowing water where: (1) The mean annual flow is greater than twenty cubic feet per second; and (2) The water is contained within a channel. A channel is an open conduit either naturally or artificially created. This definition does not include artificially created irrigation, return flow, or stockwatering channels [WAC 173-22-030]. Rivers, creeks, brooks and runs are all streams.

Stream power: The amount of work (material transportation) a channel reach can does. It is measured by flow per unit of time times channel slope. Work and energy have the same units. Stream power has a number of definitions depending on the time rate at which either work is done or energy is expended. It is a useful index for describing the erosive capacity of streams, and relates to channel pattern, development of bed forms, sediment transport, and the shape of the longitudinal profile.

Stream reach: A length of a stream channel that is uniform with respect to discharge, gradient, channel shape, bank composition, valley shape, sediment supply and other factors.

Study reach: The portion of the channel that is within the study area.

Swale: A vegetated ephemeral channel that may or may not correspond to a relic channel; Small secondary channel or linear digressional features on point bar deposits. Associated with the point bar are a series of arcuate ridges and swales. The ridges are formed by lateral channel movement and are relic lateral bars separated by low-lying swales. Swales are locations where fine-grained sediments accumulate following original creation.

Thalweg: The line that defines the deepest part of a channel.

Translational migration: The down valley movement of meander bends caused by bank erosion concentrated at the outer bank between the bend apex and the downstream crossing.

Valley: An elongate, relatively large, externally drained depression that is primarily developed by stream erosion or glacial activity.

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