Channel patterns

Channel patterns are types of sedimentary deposits formed by streams and rivers . Collectively, they are called fluvial deposits. Their shape and sediment characteristics are easily identified and enormously complex. Understanding of fluvial deposits is essential to economic geology because many of these ancient deposits are a good source of petroleum . Extremely old fluvial deposits are found extensively on land and often indicate much different environments. For example, a large river deposit is located outside of Flagstaff, Arizona where there now exists nothing but high desert . There is no indication of the source of vast channel cuts, sand bars, point bars, and cut banks seen captured in the sediments.

In order to understand and identify channel deposits, the types and natures of streams and rivers must first be learned. Each river has its own unique settings in which it flowed. No

two rivers have ever been the same. There are some general characters of rivers, however, that are easily observed. Rivers that originate and flow down steep slopes are usually straight and deep compared to a meandering river that flows across relatively flat ground. Meanders are wide curves in rivers that follow the path of least resistance along plains and valleys. In aerial view, the sinuous curves seem to wander over the topography . This wavy appearance gives rise to their name meandering rivers. Braided channels occur where steep rivers meet flat lying valleys. Their river's sediment load is rapidly dumped, building a flat surface along which the water , previously contained in a single channel, spreads out forming a series of interlocking shallow channels. The channels cut back and forth across the flat plain that gains them the name "braided" channels. The final large type of river group is the anastomosing river where different channels of the river are separated by permanent alluvial islands. From the air, the rivers have many channels that eventually coalesce to form one large possibly meandering river.

The effect of these variable types of waterways is a wide variety of sedimentary deposits. All the depositional types are a result of the increase and decrease of the force of flowing water. As velocities change, so does the sediment-carrying capacity of the water. The stronger the velocity of water, as in a steep channel, the greater the size and amount of sediment the river can transport. Large boulders are not often moved unless storm conditions exist where velocities can reach dangerous levels. Even cobble-sized stones need immense water velocities to be moved. However, sand and silt are much more easily moved along river bottoms. Many studies on the physics of water and its carrying capacity have produced information from which geologists can infer the amount of water in a channel at a particular time in history.

Channel bars are longitudinal deposits of mostly sand that accumulates in the centers of rivers. Their development is constant as new sands accumulate on top of old. The moving water is constantly rolling sand grains along the bottoms of rivers and on top of bars. Because the bars have elevation in the rivers, they act as a sort of brake for the water. When the water slows down as it moves around the bars, it deposits its load of sand, increasing the size of the bar and further slowing the water. To further complicate the picture, the channel bottom acts as a drag on the water column. The rough surface of the sand creates friction and slows the water immediately above it while the surface of the water is not slowed. The result is that lighter sediments not lying near the bottom are carried farther and longer in the water column and down the river.

Bars are rarely composed of any sediment larger than sand-sized particles. No type of sand bars is stable. They can migrate over the bottom surface of the channel with changing seasons and water flows. The bars can migrate from side to side in the channel and down the river as the water carries more sediment over their tops. When covered by additional sediment, these bars are buried in the channel and remain as a geological facies (specific identifiable geological pattern).

Meanders in rivers produce a variety of fluvial structures. As the water flows along in a meander, the point at which it first hits a turn in the river course receives the most force from the water. As the water continually strikes this bank, it erodes the sediments in front of it. A sharp and well-excavated corner is formed and appropriately called the cutbank. As the water turns the corner, most of its force is absorbed by the cutbank and it loses much of its velocity around the bend. At this point in the semi-circular water pattern, the water has the least velocity and unloads its sediment. The result is a build-up of sand called a point bar. Point bars will often have finer sediments than channel bars because the water loses so much velocity on the outer edges of the point bar that it cannot hold anything but the finest grained particles or clasts.

Because the water now has so little velocity, its course is easily turned sideways where it regains velocity and begins another meander. From the air, a meandering river is an extraordinary view since all the previous channels and turns can be seen as a result of this depositional pattern of cutbanks, channels bars, and point banks. Older and more massive rivers, such as the Mississippi, have an extensive history that is easily seen from an airplane.

As a meandering river ages, it develops interesting features called oxbow lakes . The cutting of the banks around the corners continues until one bank nearly touches the other. It looks something like an omega sign (Ω) from the Greek alphabet. Eventually the corners cut enough land away to meet. When this happens, the flow of the river is stronger where the channel is straighter. As a result, the river takes a new course. The abandoned meander does not receive any new sediment and the remaining water is sealed by new point bank bars along the new course. The unique lake that is formed is named an oxbow lake. The lake will eventually fill and die. Trees may overgrow it and a whole new cycle of river meandering leaves it stranded until the river cuts its way back.

These sedimentary structures are just a few of the interesting patterns formed by rivers and streams. They are found in a large range of size scales and are important to geologists worldwide.

See also Petroleum, detection; Stream valleys, channels, and floodplains