Most rivers are considered as reaches with different geomorphological characteristics. The most simple divison generally made is to divide the river into Upper and Lower River reaches
The uppermost portion of a river system includes the river headwaters and low-order streams at higher elevation. The upper river basin is usually characterised by steep gradients and by erosion that carries sediment downstream. Streams in this upper region are usually steep and torrential, and often include rapids and waterfalls. These streams generally have little floodplain, although part of the bank and surrounding land may be wetted during periods of high flow.
The lower section of a river system (extending to the mouth), usually exhibits a larger river channel and lower slope – the landscape is usually flat. In the middle portion of the river there is a balance between erosion and deposition of sediment. Further downstream, the lower river sees mainly deposition, though localised erosion and reworking of sediments may also occur. The main channel of the river often forms a sinuous (meandering) path across the landscape unless artificially channelled.
This generalisation however does not hold for the Kunene River. The Kunene has a rather a typical Longitudinal Profile. The Upper Kunene is steep, however the river flattens out in the middle reaches before once again entering a steep descent towards the Atlantic Ocean in the Lower Kunene. This ensures that the water drains the Kunene River basin relatively quickly.
Seen from above, river systems display a tree-like pattern, with many small streams feeding into fewer larger rivers and eventually into one very large river. Many systems have been developed to classify the different “levels” of streams in this pattern. In the system devised by Strahler, each level of stream is assigned an order. Order 1 streams are the very smallest, uppermost streams (i.e., with no upstream tributaries). Two order 1 streams combine to form a stream of order 2. Order 3 streams are formed from the confluence of two order 2 streams. Each higher order of stream is formed from the confluence of two lower order streams, and the watersheds of lower-order streams are included within the watersheds of higher order streams. Orders 10–12 correspond to the main channels of large world rivers. In general, individual streams get wider and longer with increasing stream order.
Characteristics of a river system depend on the landscape, climate, other geographical features and natural processes. Most river systems can be divided into different sections from the headwaters to the river mouth.
The diagram below illustrates the concept of stream order using a schematic diagram of a hypothetical stream network.
Schematic diagram of Strahler stream order.
Source: State University of New York College of Environmental Science and Forestry
( click to enlarge )
River Channel Geomorphology
Characteristics of river channels can vary laterally (horizontally across the river channel), longitudinally (along the river channel from the headwaters to the mouth), vertically (from the water surface to the river bed), and over time. Various channel features and structures are formed as the flow of the river water interacts with the landscape over these four dimensions. The different features within a river provide complex habitats for numerous aquatic species, and also affect the way that humans interact with the river system.
River channels exist as single channels or, less commonly, as multiple channels, found where the river splits into numerous smaller channels separated by islands. Braided channels occur most commonly where the river system includes erodible banks, an abundance of coarse sediment, and rapid and frequent changes in river flow (FISRWG 1998). Anastomosed channels occur where sediments are relatively resistant to erosion, and the main river channel is divided by a rise in the downstream base level and rapid accumulation of sediment.
The islands found in multiple channel systems are susceptible to flooding during periods of high water, while erosional and depositional processes can continuously alter the location and shape of these islands. Erosion can occur from the upstream end of the islands, from meander formation, and from bank undercutting within the stream channel. Deposition can occur at the downstream end of the islands as well as on the inside of meander bends. Vegetation, such as forests, reeds, and grasses, can anchor sediments and protect against erosion. Vegetation may also lead to decreased flows and increased rates of sediment deposition. The complexity of braided systems provides a variety of habitat types for fish; aquatic organisms in these systems are usually well adapted to sudden changes in their environment.
Sinuosity refers to the amount of curving in a river channel, and can be calculated by dividing the length of the channel between two points by the length of the valley between those points. A river channel is considered to be meandering if the sinuosity is greater than 1.3.
Generally, sinuosity is related to river discharge and gradient; streams and rivers in the upper and middle portion of river systems exhibit low to moderate levels of sinuosity, while sinuosity is often higher in the lower portion of river systems.
Individual meander bends are constantly changing shape due to erosion and deposition. Erosion and undercutting of the bank occurs in the deep, outer portion of each bend, while sediment deposition occurs on the inside of the bend to form a shallow beach known as a point bar. Point bars can be rapidly colonised by vegetation such as reeds. This vegetation can protect against erosion, as the roots of the plants bind the soil and protect the soil from the current.
As erosion continues, the meander bend can increase in size until it doubles back on itself, eventually cutting through bank sediments to form an oxbow. On the inside of the bend, successive sediment deposition and formation of point bars, and the isolation of small dead arms, lead to the formation of scroll lakes.
Over time, the erosion of material from the outside of a bend and deposition on the inside leads to the downstream and lateral movement of meanders across the floodplain. As rivers sometimes form boundaries of land ownership or national borders, the lateral movement of the river over a scale of decades or even centuries can sometimes affect land tenure or sovereignty. Pressure to stabilize the river and eliminate natural shifts in river form over time can be attributed to these interests.
Schematic representation of a meander.
Source: Nile River Awareness Kit 2006
( click to enlarge )