172 Living Energies
warmer and less able to transport sediment and nutrients. In the lower reaches of such rivers the banks tend to become barren for lack of them and the river has a greater tendency to flood and to form deltas at its confluence with the sea. The flow conditions in north->south and south->north flowing rivers is again different to the above. Their flow-patterns are governed more by their lateral inertia relative to the Earth's rotation than by the passage of the Sun across the heavens. In fig. 13.17 the section drawn through such a river exhibits an asymmetrical profile. Owing to the water's fluid inertia, the main body of water has a tendency to bank up against the riverbank on the western side, i.e. the side opposite to the direction of the Earth's rotation. Being the densest and heaviest, the coldest water is the most affected and therefore the main flow occurs along the western bank, where the channel is also generally deeper. Such rivers tend to be barren on the eastern
Fig. 13.22
side, because, being shallower, the water on that side is hotter and the deposition of sediment therefore takes place sooner. When a positive temperature gradient is operative its effect tends to be more marked on the western bank than on the eastern and consequently the nutrient flow is greater towards the west than towards the east with a commensurate difference in relative fertility, or, as was shown in fig. 9.8, the river acts to extract nutrients from the warmer bank and deposit them on the cooler one. If these rivers are at fairly high latitudes, however, and flow into cold or arctic seas, then as they move polewards the angle of incidence of the Sun's rays decreases, the water cools and such rivers carry their sediment far out into the seas, creating tongues and peninsulas in what is known as 'haff' formation. In the opposite case, such as the Nile, whose confluence with the sea is at much lower latitudes, in lieu of haff development, deltas form as the flow has become overheated and the water correspondingly
The rationale of trapezoid channels
Relative to the base unit volume, the volume of water carried by the channel increases exponentially as the waterlevel rises