VII.2.3. Work of rivers; river sections
The kinetic energy of water is proportionately increasing with its volume and fall. Its speed is reduced by the inner friction between its own particles (the viscosity of water at 0 oC is twice as much as at 25 oC), as well as its external friction, that is the friction between its own particles and the wall of the riverbed. (e.g. detrital or shallow riverbed). The power of the water in the river may dislocate some looser parts of the riverbed and the particles drift on in the water. The river has a deepening force (linear) and a lateral force, directed at the walls. This erosion depends on the speed, the volume of the water and the quality of the rocks of the riverbed.
The alluvium is carried by the river either in a dissolved form, or, its particles are rolling (saltation) or floating (flotation) in the water. If the speed of the river decreases, it will deposit its larger-size alluvium. At places where the fall of the river is more significant, or at places where the riverbed is narrowing, energy increases and erosion becomes more significant. When the river is flooding it carries more alluvium. (Moholi 1988). During transport larger size rocks break up into smaller rocks and pebbles, eventually sand. Alluvium is the ’chisel’ of the river. It performs corrosive tasks with the help of rough rocks and pebbles.
Fast rivers with much water in them work more. The triple rule of destruction-transport- accumulation is valid in this case, too. Depending on these features different sections of rivers can be identified.
The Upper course is the steepest part of land in the river. It is usually small, but flows very fast. The energies are high and the river cuts away at its
bed. As it does, it quickly deepens its valley through down cutting. This creates a steep sided V- shaped valley. Downward erosion is the dominant process. Waterfalls are typical in this area.
If the work of the river is as much as the amount of alluvium it carries, it becomes Middle course. In the middle course the rivers starts to flow slowly, because it transports a lot of sediment. The river moves down its wide asymmetrical valley in a meandering way. The meanders can be very attractive in water tourism. The river gets wider, the river begins to erode sideways, the valley side becomes less steep, a flood plain starts to develop. Erosion and deposition occur.
When arriving at a flat area the river slows down and its energy is not enough to transport the sediments. Deposition occurs. The V-shape is wide. The river approaches its mouth and it winds its way slowly across a large floodplain. Deposition is now the dominant process. And the river builds sand beds, sand banks, small islands. This section of the river is called Lower course. This is how large plain areas came into being in the geological past When rivers leave mountaineous areas they build large alluvial cones. (e.g. Szigetköz, Csallóköz). When the courses of rivers change in time and space river terraces come into being.
„although all river sections have their own values from the point of view of water tourism, the most ideal of the three is the Middle course: The speed of the water is in the ideal zone, travelling is safe, its water warms up quickly thus it provides an appropriate environment for holiday makers and sportspeople alike. (Donka 2012)”.