RIVERS THE MIDDLE COURSE

What’s happening here? In the middle course, rivers aren’t as high above sea level as they are in the upper course. There is more energy because the discharge is greater. Discharge has increased because there is more water from tributaries which have joined, and because more water has arrived from throughflow and surface run-off. Less energy is lost to friction since less water is in contact with the bed and banks and the load size is smaller. So they erode sideways (laterally) as well as vertically. As this happens it twists from side to side (meanders) This means that the channels are wider and deeper than upstream.

Meanders A meander is a large bend found along a river’s course. Meanders tend to occur in series. Thus, meanders are generally found in groups. Normally found in the middle and lower course of a river. They are more pronounced in the latter and may be associated with ox-bow lakes. Generally created by a combination of erosion and deposition.

Formation of Meanders Water in a river channel does not flow uniformly. There is always an area within the water body where the flow is the fastest. This area of fastest flow tends to occur because of variations in the channel caused by: Different types of sediment Changes in channel slope/gradient Changes in bed roughness Variations in depth As a result water in the channel will twist and turn around stones and other obstructions resulting in areas of slower and faster water movement and creating deeper areas along the bed.

Variations in Velocity across a Meander

Formation of Meanders The result of this is turbulent flow and the development of a thalweg . The Thalweg is a line connecting the lowest points along the stream bed. It will therefore have the least resistance and the fastest flow. The thalweg accentuates the variations in velocity across the river channel and bends from side to side naturally and due to variations in the channel itself.

Thalweg

Formation of Meanders The end result is that water in river channels does not flow in a straight line, even in an apparently straight river channel since the thalweg meanders across the channel, from one side to the next. Such meandering is actually the normal behaviour of fluids and gases in motion. The river starts to flow from side to side in a winding course but still in a relatively straight channel.

Formation of Meanders As said before, turbulent flow within the channel causes the development of a thalweg. It also causes riffles and pools. Pools are areas of calmer and apparently slower moving water found in deeper parts of the river filled with fine sediments. Riffles are areas of faster and more turbulent flow are found in shallower parts of the river over larger stones and gravel . The creation of pools and riffles causes more erosion to occur on one side and more deposition on the other, which eventually causes the river channel to bend from side to side and meanders to develop.

Formation of Meanders Over time, the pool tends to move to opposite side of the channel and this is where a meander will develop. Water moving faster has energy to erode. This occurs on the outside of the pool. Water moving slowly tends to deposit material as it has little energy left to erode or transport material. This occurs on the inside of the pool.

How do meanders form? Erosion, transportation and deposition are all processes that create the characteristic features of meanders. There are several stages involved in the creation of meanders.

How do meanders form? Stage 1 In low flow conditions straight river channels have bars of sediment on their beds. Flowing water weaves around these bars of sediment. This creates deeper pathways where most of the water flows (thalweg) called pools and shallow areas where less water flows called riffles. This causes the river flow to swing from side to side.

Pools and Riffles

How do meanders form? Stage 2 Where the river swings towards the bank erosion causes undercutting. On the opposite side of the channel where the velocity is lower material is deposited. Therefore the river does not get any wider. Instead the shape or form shifts or migrates across the landscape and the sinuosity increases

Sinuosity

How do meanders form? Stage 3 Continued erosion along the outer bank, as the result of hydraulic action and abrasion, creates a river cliff or bluff. A slip-off slope and point bar forms on the inner bank. This is a gently sloping deposit of sand, gravel and pebbles. A point bar is a depositional feature made of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope. Point bars are found in abundance in mature or meandering streams. They are crescent-shaped and located on the inside of a stream bend The following image shows a the top of a slip-off slope.

How are meanders formed? Stage 4 Meanders are perpetuated through a process called helicoidal / helical flow. As the surface flow of water hits the outer bank it corkscrews, flows along the river bed then deposits eroded material on the inner bank.

How are meanders formed? Step 5 Erosion is greatest beyond the middle of the bend in the meander. This causes the meander to migrate downstream over time.

Meanders On the side where the fast water is, there is more erosion. This makes the river deeper and cuts into the bank, to make a river cliff. On the other side, where the slower water is, there is deposition. This makes a shallower area of sediment called a point bar. The gentle slope down to the point bar is called the slip-off slope. Y X

Cross-section at the meander