Channel Structure

Nearly all channels are formed, maintained, and altered by flows and sediment loads. Channel equilibrium involves the relation among four basic factors: sediment discharge, Q, sediment particle size, D, streamflow, Q^, and stream slope, S. Using median particle size, D50, this relationship is often expressed as

This relationship states that a measure of sediment load (sediment discharge Q times median particle size D50) is proportional to a measure of the streamflow power (streamflow QW times slope S).

Channel equilibrium occurs when the streamflow power is constant over the length of the stream. If this occurs, no net changes in the channel shape will occur. If a change occurs in either the left or right-hand side of the above equation, the balance and hence equilibrium will be temporarily lost. Ifone variable changes, one or more of the other variables must change appropriately if equilibrium is to be maintained. Reaching equilibrium typically involves erosion and/or deposition. Assuming increasing flows from runoff in the downstream direction, the channel slope has to be decreasing in the downstream direction. If the slope is too steep, sediment is deposited to reduce that steepness. This is why stream channels that experience increasing downstream flows have decreasing slopes in the downstream direction.

If streams in equilibrium have constant streamflow power, Q^S, over distance, the sediment load, QjD50, must also be constant. Hence, if sediment deposition is occurring in the downstream direction to decrease stream slopes, the

Figure 3 Two types of floodplains, the hydrological and topographic. Modified from Loucks DP and van Beek E (2005) Water Resources Systems Planning and Management. Paris: UNESCO.

median particle size, D50, will be decreasing and the sediment discharge, Q, along with streamflow, Q, will be increasing. This is typically observed in channels with increasing downstream streamflows. A stream seeking a new equilibrium tends to erode more sediment and larger particle sizes. Alluvial streams that are free to adjust to changes in these four variables generally do so and reestablish new equilibrium conditions. Nonalluvial streams such as those flowing over bedrock or in artificial, concrete channels are unable to maintain this equilibrium relationship because of their inability to pick up additional sediment.

Stream and river valleys are created over time by the stream or river depositing sediment as it moves back and forth across the valley floor. These processes of lateral migration and sediment deposition, usually occurring during flood pulse flows, continually modify the flood-plain. Through time, as the channel migrates, it will maintain the same average size and shape as long as the channel stays in equilibrium.

For streams and rivers whose flows increase with distance downstream, channel width and depth also increase downstream due to increasing drainage area and discharge. Even among different types of streams, a common sequence of structural changes, as shown in Figure 4, is observable from headwaters to mouth.

The longitudinal profile of many streams can be divided into three zones. The changes in the three zones are characterized in Figure 5. Zone 1, the headwater zone, has the steepest slopes. In this zone, sediments erode from slopes of the watershed and move downstream. The rivers in hilly regions are characterized by the swiftness of the flow in restricted and/or steep channels, the occurrence of landslides, and the formation of rapids along their courses. The control of rivers in the upper reaches is known as 'torrent control'.

Zone 2, the transfer zone, receives some of these sediments and hence is usually characterized by wider floodplains and more meandering channel patterns. The flatter slopes in Zone 3 receive most of the coarser sediments.

Though Figure 5 displays headwaters as mountain streams, these general patterns and changes apply to watersheds with relatively small topographic relief from the headwaters to the mouth. Erosion and deposition occur in all zones, but the zone concept focuses on the most dominant process.

Pools and Riffles

Most streams share a similar attribute of alternating, regularly spaced, deep and shallow areas called pools and riffles (Figure 6). Pools and riffles are associated with the deepest path along the channel (thalweg). This deepest path 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 pool-to-pool or riffle-to-riffle spacing, where they exist, is normally c. 5-7 times the channel width at bank-full discharge.

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