Summary

Natural science is the main characteristic feature of the present civilization. Science and technology have modified our homes, places of work and enjoyment, means of communications. Science expands man’s intellectual horizons. The range of human knowledge doubles every twelve years. The second half of the twentieth century brought a number of technical innovations – transistor, ICs. satellites, etc. Recent developments of nuclear weapons make wars of annihilation possible and pose the question of the survival of the human race.

2. WHAT HAPPENS TO A RIVER AS IT APPROACHES ITS MOUTH?

The flat area of land over which a river meanders is called a flood plain. During times of flood, a river will overflow its banks and cover any surrounding flat land. As the speed at which the water flows across the flood plain is less than in the main channel, then the fine material transported in suspension by the river will be deposited. Each time a river floods a thin layer of silt, or alluvium, is spread over the flood plain. The Egyptians used to rely upon the annual flooding by the River Nile to water their crops and to add silt to their fields until the opening of the Aswan Dam in 1970.

When a river floods, it is the coarsest material which will be deposited first. This coarse material can form small embankments alongside a river which the Americans call levees. Large rivers, like the Mississippi, carry tremendous quantities of material in suspension, especially in times of flood.

However, during times when the river level falls and its speed is reduced, large amounts of silt will fall out of suspension onto the bed of the river. In time the bed of the river will build up so that, when water levels are high again, the river is more likely to overflow its banks. To try to prevent this happening, large artificial levees are built. The Mississippi now flows at a much higher level than the surrounding flood plain, and cities like New Orleans and St Louis are protected by levees that are up to 16 metres high. The problem is, what happens should these levees break?

Large rivers transport great amounts of fine material down to their mouths. If a river flows into a relatively calm sea, or lake, then its speed will reduce and the fine material will be deposited. The deposited material will slowly build upwards and outwards to form a delta. River deltas provide some of the best soils in the world for farming (e.g. River Nile) but they are also prone to serious flooding as the land is so flat (e.g. Bangladesh). The Mississippi delta is extending rapidly into the Gulf of Mexico. As in all deltas, deposition blocks the main channel of the river so the Mississippi has to divide into a series of smaller channels called distributaries. These channels need constant dredging if they are to be used by ships.