Channel form along the corridor

The form of the channel can change as it moves through the three longitudinal zones. Channel form is typically described by two main characteristics--thread (single or multiple) and sinuosity.

Single- and Multiple-Thread Streams
Single-thread (one-channel) streams are most common, but multiple-thread streams occur in some landscapes. Multiple-thread streams are further categorized as either braided or anastomosed streams.

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 bar begins to form in a channel due to reduced streamflow or an increase in sediment load. The central bar causes water to flow into the two smaller cross sections on either side. The smaller cross section results in a higher velocity flow. Given erodible banks, this causes the channels to widen. As they do this, flow velocity decreases, which allows another central bar to form. The process is then repeated and more channels are created.

In landscapes where braided streams occur naturally, the plant and animal communities have adapted to frequent and rapid changes in the channel and riparian area. In cases where disturbances trigger the braiding process, however, physical conditions might be too dynamic for many species.

The second, less common category of multiple-thread channels is called anastomosed streams. They occur on much flatter gradients than 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 not easily erodible, the original single-thread stream breaks up into multiple channels. Streams entering deltas in a lake or bay are often anastomosed. Streams on alluvial fans, in contrast, can be braided or anastomosed.

Sinuosity
Natural channels are rarely straight. Sinuosity is a term indicating the amount of curvature in the channel. The sinuosity of a reach is computed by dividing the channel centerline length by the length of the valley centerline. If the channel length/valley length ratio is more than 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 and 2 of the longitudinal profile. Extremely sinuous streams often occur in the broad, flat valleys of Zone 3.

Pools and Riffles
No matter the channel form, most streams share a similar attribute of alternating, regularly spaced, deep and shallow areas called pools and riffles. The pools and riffles are associated with the thalweg, which meanders within the channel. Pools typically form in the thalweg near the outside bank of bends. Riffle areas usually form between two bends at the point where the thalweg crosses over from one side of the channel to the other.

The makeup of the streambed plays a role in determining pool and riffle characteristics. Gravel and cobble-bed streams typically have regularly spaced pools and riffles that help maintain channel stability in a high-energy environment. Coarser sediment particles are found in riffle areas while smaller particles occur in pools. The pool-to-pool or riffle-to-riffle spacing is normally about 5 to 7 times the channel width at bankfull discharge (Leopold et al. 1964).

Sand-bed streams, on the other hand, do not form true riffles since the grain size distribution in the riffle area is similar to that in the pools. However, sand-bed streams do have evenly spaced pools. High-gradient streams also usually have pools but not riffles, but for a different reason. In this case, water moves from pool to pool in a stairstep fashion.