ASGEOGRAPHYREVISIONHYDROLOGYANDFLUVIALGEOMORPHOLOGY

1.3 RIVERCHANNELPROCESSES

KEYTERMSANDDEFINITIONSAbrasion or corrasion is the wearing away of the bed and bank by the load carried by a river.Attrition is the wearing away of the load carried by a river. It creates smaller, rounder particles.Capacity refers to the total load that a stream can carry.Competence refers to the size of the largest particle that a stream can carry.A delta is a depositional feature formed where a river flows into the sea or a lake such that the rate of deposition of sediment exceeds the rate of removal of sediment by marine processes.Discharge is the volume of water in a river channel passing a certain point per unit of time. It is usually expressed in cubic metres per second (cumecs).

KEYTERMSANDDEFINITIONSA gorge is a narrow and deep section of a river valley with vertical or near-vertical valley sides.Helicoidal flow is a ‘corkscrewing’ motion responsible for the formation of meanders.Hydraulic action is the force of air and water on the sides of rivers and in cracks.Laminar flow is where water flows in sheets or laminae parallel to the channel bed.Levees are raised banks of sediment along a river rising above the level of the flood plain and created by fluvial deposition. In many cases these natural levees are enhanced by artificial levees built to prevent flooding.Load is the particles of sediment and dissolved matter that are carried along by a river.

KEYTERMSANDDEFINITIONSPoint bars are depositional features of rivers formed of well sorted sediment with a gentle slope on the inside of meanders.Pools are deep hollows scoured in the bed of a river, typically located on the outside edge of meanders.Riffles are small-scale ridges formed of coarse pebbles and cobbles and found approximately half-way between meanders in the straight section of a river.River cliffs are erosional features on the outside of a meander. They are formed by the undercutting of the river bank by the river current, which is at its fastest on the outside of a meander. They vary in height but may be high and have a vertical gradient.

KEYTERMSANDDEFINITIONSSolution is the removal of chemical ions, especially calcium.Turbulent flow is a chaotic flow that involves eddying and ertical movement.A waterfall is a point on a river where the gradient increases to vertical or near vertical such that the water plunges over the edge. This is usually a result of the change in resistance of the underlying rocks. Where the change in gradient is less severe rapids will be formed. These may also be formed where a river narrows.

TOPICSUMMARYThe load of a river varies with discharge and velocity.The capacity of a stream refers to the largest amount of debris that a stream can carry.The competence refers to the diameter of the largest particle that can be carried.The critical erosion velocity is the lowest velocity at which grains of a given size can be moved.Deposition occurs due to a shallowing of gradient which decreases velocity and energy; a decrease in the volume of water in the channel; and/or an increase in the friction between water and channel.The main types of erosion are hydraulic action, abrasion, attrition and solution.

TOPICSUMMARYThere are a number of factors affecting rates of erosion, including load, velocity, gradient, geology, pH and human impact.Global sediment yields vary widely.There are three main types of flow: laminar, turbulent and helicoidal.Channel types include straight, meandering and braided.Braiding occurs when the channel is divided by islands or bars.Straight channels are rare.Meanders are complex.Meanders have an asymmetric cross-section. They are deeper on the outside bank and shallower on the inside bank. In between meanders channels are more symmetrical.

TOPICSUMMARYIncised meanders are those that are especially well developed.Intrenched meanders are symmetrical, and occur when downcutting is fast enough to offset the lateral migration of meanders.Ingrown meanders are the result of lateral meander migration, and are asymmetric in cross-section.Features of erosion include waterfalls, gorges and potholes.Deposition includes floodplains, deltas, levees, alluvial fans and cones.Some features are the result of both erosion and deposition, such as ox-bow lakes.

ADDITIONALWORK1. Describe and explain how river channel characteristics vary downstream.2. Study Figure 1.12. Describe and suggest reasons for variations in global sediment yield.3. Explain how meanders are formed.4. With the use of examples, explain how waterfalls are formed.5. Explain how deltas are formed. Suggest why living on deltas can be both beneficial and hazardous for people.

SUGGESTEDWEBSITESFor a slide show on general river processes visit:www.slideshare.net/maliadamit/river-channel-processeslandforms-1026801

For videos and animations of stream processes visit:http://serc.carleton.edu/NAGTWorkshops/geomorph/visualizations/erosion_deposition.html

For animations of deposition visit: http://highered.mcgraw-hill.com/sites/0072402466/student_view0/chapter10/animations_and_movies.html#