Channel form along the corridor
The form of the channel can change as it moves through the threelongitudinal zones. Channel form is typically described by two maincharacteristics--thread (single or multiple) and sinuosity.
Single- and Multiple-Thread Streams
Single-thread (one-channel) streams are most common, butmultiple-thread streams occur in some landscapes. Multiple-threadstreams are further categorized as either braided or anastomosedstreams.
Three conditions tend to promote the formation of braided streams:
- Erodible banks.
- An abundance of coarse sediment.
- Rapid and frequent variations in discharge.
Braided streams typically get their start when a central sediment barbegins to form in a channel due to reduced streamflow or an increasein sediment load. The central bar causes water to flow into the twosmaller cross sections on either side. The smaller cross sectionresults in a higher velocity flow. Given erodible banks, this causes thechannels to widen. As they do this, flow velocity decreases, whichallows another central bar to form. The process is then repeated andmore channels are created.
In landscapes where braided streams occur naturally, the plant andanimal communities have adapted to frequent and rapid changes inthe channel and riparian area. In cases where disturbances triggerthe braiding process, however, physical conditions might be toodynamic for many species.
The second, less common category of multiple-thread channels iscalled anastomosed streams. They occur on much flatter gradientsthan braided streams and have channels that are narrow and deep(as opposed to the wide, shallow channels found in braided streams).Their banks are typically made up of fine, cohesive sediments,making them relatively erosion-resistant.
Anastomosed streams form when the downstream base level rises,causing a rapid buildup of sediment. Since bank materials are noteasily erodible, the original single-thread stream breaks up intomultiple channels. Streams entering deltas in a lake or bay are oftenanastomosed. Streams on alluvial fans, in contrast, can be braided oranastomosed.
Sinuosity
Natural channels are rarely straight. Sinuosity is a term indicating theamount of curvature in the channel. The sinuosity of a reach iscomputed by dividing the channel centerline length by the length ofthe valley centerline. If the channel length/valley length ratio is morethan about 1.3, the stream can be considered meandering in form.
Sinuosity is generally related to the product of discharge and gradient.Low to moderate levels of sinuosity are typically found in Zones 1 and2 of the longitudinal profile. Extremely sinuous streams often occur inthe broad, flat valleys of Zone 3.
Pools and Riffles
No matter the channel form, most streams share a similar attribute ofalternating, regularly spaced, deep and shallow areas called poolsand riffles. The pools and riffles are associated with the thalweg,which meanders within the channel. Pools typically form in thethalweg near the outside bank of bends. Riffle areas usually formbetween two bends at the point where the thalweg crosses over fromone side of the channel to the other.
The makeup of the streambed plays a role in determining pool andriffle characteristics. Gravel and cobble-bed streams typically haveregularly spaced pools and riffles that help maintain channel stabilityin a high-energy environment. Coarser sediment particles are found inriffle areas while smaller particles occur in pools. The pool-to-pool orriffle-to-riffle spacing is normally about 5 to 7 times the channel widthat bankfull discharge (Leopold et al. 1964).
Sand-bed streams, on the other hand, do not form true riffles sincethe grain size distribution in the riffle area is similar to that in thepools. However, sand-bed streams do have evenly spaced pools.High-gradient streams also usually have pools but not riffles, but for adifferent reason. In this case, water moves from pool to pool in astairstep fashion.
