Channel Classification Summary
(Information compiled by A. Bryden)
Montgomery and Buffington (1997) have classified channel reach morphologies of mountain drainage basins into seven distinct types. This classification differentiates stream types by sediment supply, transport capacity, and interaction with hillslope processes. The seven distinct channel types demonstrate the tendency of alluvial channels to spatially organize sediment into distinct morphologies. Colluvial channels comprise the transport-limited end member morphology where hillslope processes dominate. Five channel types ranging from transport- limited to supply-limited are dune-ripple, pool-riffle, plane-bed, step-pool, and cascade channel morphologies. The final channel morphology is a bedrock channel where the transport capacity dominates the channel, leaving little to no deposition of sediment. Although the distinction is made between channel types, this does not preclude the understanding that morphology along a channel is constantly changing in response to local controls. A continuum of form exists between the channel types, giving rise to intermediate morphologies that may reflect characteristics of more than one type.
Schematic shows the general relation between valley bottom confinement, transport capacity and sediment supply, valley segments, and channel reach morphology. Schematic modified from Montgomery and Buffington (1997).
Please click a link below to learn more about channel classfiication and each channel type:
Channel Classification
Montgomery and Buffington (1997) noted a trend in the spatial arrangement of channel types to become increasingly transport-limited as drainage area increases. As valley-bottom confinement increases, so do the channel slope and transport capability. Although exceptions exist, increased confinement typically translates to an increase in transport capacity, resulting in a supply-limited channel. Increased confinement can be directly correlated with hillslope connectivity and sediment loading from hillslope processes. Nonetheless, the transport capabilities of a confined channel are relatively large and typically move bedload to downstream storage reaches during frequent flows. Unconfined reaches of channel are characterized by a decreased transport capacity, and by increased sediment storage, floodplain development, and flood attenuation. The storage of excess sediment in partially and unconfined valley bottom reaches suggests that these reaches are primarily transport-limited, although supply-limited conditions exist during less frequent flows.
The relations between the transport regime and valley-bottom confinement drive reach-scale tendencies in channel morphology. Local disturbances can force the formation of most channel types regardless of confinement, as discussed below. Confined channels can range in morphology from bedrock, to cascade, step-pool and plane-bed. The partially confined valley bottom channel morphologies are transitional between confined and unconfined valley bottoms. Morphologies of the partially confined channels commonly exhibit some level of sediment storage and floodplain development, allowing for the formation of pool-riffle channels. The unconfined channel morphologies are characteristically supply-limited segments which form channel morphologies dominated by alluvial sediment.
The dominance of transport capacity over sediment supply can lead to development of bedrock channels, which form in steep confined valley bottoms in both glaciated and unglaciated terrain. The channel morphology is characterized by a lack of alluvial bedforms. This does not mean that alluvial material is absent; some may be found deposited in scour holes or behind flow obstructions. Inclusion of woody debris can force alluvial deposition and bedforms to occur in reaches of channel that would otherwise be bedrock. The high transport capacity associated with the steep channel gradient flushes bedload to downstream storage reaches and indicates a supply- limited channel.
At the other end of the spectrum, sediment supply from hillslope processes can exceed the transport capacity and lead to development of a colluvial channel. More can be found regarding colluvial channels in the channel morphology of Colluvial Hollows.
References
Channel Types
| Cascade |
Planform |
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Schematic shows the general planform and profile morphology of cascade channel. Modified from (Montgomery and Buffington, 1997). |
Cascade:
The cascade channel type occurs on steep confined and partially confined reaches in both glaciated and unglaciated valley bottoms. The cascade channel is characterized by energy dissipation through tumbling, jet-and-wake flows over and around cobble- and boulder-size clasts. Longitudinal profiles show that bedform patterns are largely random, with small pool development at a frequency of less than a channel width. The sediment sources include fluvial, but are dominated by hillslope processes and debris flows. Two thresholds for sediment transport are suggested; when large bed-forming clasts are mobilized during infrequent floods, and when bedload is competently transported during typical flows. The competency to readily transport the bedload and lack of in-channel sediment storage define the cascade channel as supply-limited. “Because of the high transport capacity relative to sediment supply, cascade channels function primarily as sediment transport zones that rapidly deliver sediment to lower channels.” (Montgomery and Buffington 1997)
Two photographs of a confined cascading channel
(Photo taken by E. Wohl)
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References
| Step-Pool |
Plan |
Profile |
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Schematic shows the general planform and profile morphology of a step-pool channel. Modified from (Montgomery and Buffington, 1997).
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Step-Pool
This type of channel morphology occurs in steep confined and partially confined reaches in both glaciated and unglaciated valley bottoms. The step-pool channel is characterized by a vertical bedform pattern of channel-wide accumulations of large clasts or steps separated by pools spaced at a frequency of 1-4 channel widths. The vertical oscillatory flow forced by vertical bedforms increases energy dissipation beyond that of grain and bank roughness. Bedload and most step-forming clasts are mobilized during very infrequent flows and the step-pool morphology develops during the recession of the flood. Armoring of the bed surface and transport of bedload and pool-filling sediment over step-forming clasts during less frequent flows suggest that transport of non-step-forming clasts is supply-limited. Although similar in many ways to a cascade channel, the step-pool channel exhibits a fluvially organized morphology compared to the random arrangement of large clasts in a cascade channel.
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| Partially confined step-pool channel |
Partially confined step-pool channel |
Partially confined step-pool channel |
Confined Step-pool Channel |
Confined Step-pool Channel |
| (Photos by E. Wohl) |
(Photo by A. Bryden) |
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References
| Plane-Bed |
Plan |
Profile |
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Schematic shows the general planform and profile morphology of a plane-bed channel. Modified from (Montgomery and Buffington, 1997). |
Plane-Bed
The plane-bed channel occurs in moderate to high slopes at any level of confinement in both glaciated and unglaciated valley bottoms. The channel morphology is characterized by a planar bed, largely devoid of any organized or rhythmic bedforms. Local pool development is associated with forced flow convergence from channel obstructions and disturbances such as woody debris and debris flows. Energy dissipation is dominated by grain and bank roughness. The sediment transport capacity of a plane-bed channel is considered transitional between supply- and transport-limited morphologies. Larger clasts that armor the bed during frequent flows become mobilized near bankfull discharge. Thus, the channel exhibits transport-limited characteristics during infrequent flows and supply-limited characteristics during more frequent flows.
Photo of plane-bed reach
(Photo by E. Wohl)
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References
| Pool-Riffle |
Planform |
Profile |
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Schematic shows the general planform and profile morphology of a pool-riffle channel. Modified from (Montgomery and Buffington, 1997). |
Pool-Riffle
The pool-riffle channel can form in low gradient partially confined and unconfined valley bottoms in both glaciated and unglaciated terrain. The channel morphology is characterized by vertical pool-riffle bedforms and oscillating lateral bedforms or sinuosity in a well-developed floodplain with bar development on the inside of bends. Pools are topographic depressions that typically form at a spacing of 5-7 channel widths apart along bends. Pool development can be free-form scour or can be associated with flow obstructions such as woody debris. Riffles are sections of channel located along the inflection between bends and pools where energy is dissipated through tumbling flow and grain roughness. This channel form demonstrates a mix of transport- and supply- limited characteristics where the supply-limited tendencies are exhibited during frequent flows and the channel becomes transport-limited when bedload is mobilized during less frequent flows.
Pictures of a Partially Confined and Unconfined Reach
(Photos by E. Wohl)
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References
| Dune-Ripple |
Planform |
Profile |
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Schematic shows the general planform and profile morphology of a dune-ripple channel. Modified from (Montgomery and Buffington, 1997). |
Dune-Ripple
The dune-ripple channel forms in sand-bed, low gradient landscapes that are rare in the Colorado Front Range. The channel morphology is characterized by a series of regime- and scale-dependent mobile bedforms with significant sediment transport. Bedforms in a dune-ripple channel provide the primary hydraulic resistance to flow. The fined grained sediment and low gradient environment can be found at local scales but not at reach scales in the Colorado Front Range.
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References
Bedrock
Bedrock channel segments form in steep confined valley bottoms in both glaciated and unglaciated terrain. Channel morphology is characterized by a lack of alluvial bedforms. This does not that mean alluvial material is absent; some may be found deposited in scour holes or behind flow obstructions. Inclusion of enough woody debris can force alluvial deposition and bedforms to occur in reaches of channel that would otherwise be bedrock. The high transport capacity associated with the steep channel gradient flushes bedload to downstream storage reaches, which indicates a supply-limited channel.
