| Abstrakt: |
Due to erosion and leaching the great rivers contain considerable quantities of silt (mud). It is estimated that the Rhine carries a mean value of 60–70 g.m-3; the variation of the mud content as a function of water discharge shows values up to several hundreds of grams per m3. The annual supply of mud from these sources is around 4⊗106 tons of dry matter. Near the river mouth, changes in water current velocity and other water movements result in deposition of the river silts. A further important factor is the change in salinity, which causes flocculation and deflocculation. Of course the geological history of the river delta is an important factor for the distribution of sediments over a given area. Chemical analysis of the river silts shows that SiO2 (or other eroded silicates) is a major constituent. Other inorganic compounds are iron and aluminium oxides and calcium (bi)carbonate. Organic matter may form an important part of the silts. Its chemical nature is not yet well described. Part of the silicon compounds are clays of the type of montmorillonite (Al2Si4O10(OH)2) and kaolinite (Al2Si2O5(OH)4). As may be expected from their chemical nature, these compounds are strong ion-exchangers, which may contain Mg, Ca and Fe ions and even anions such as phosphates. The large deposits of the Rhine silt is therefore rather fertile, which has a great influence on the growth of both the rooted and the suspended vegetation. The first has caused enormous amounts of peat, which can be found as layers formed in periods of a relatively low sea level, in which the sediments were comparatively dry. Due to a temporary elevation of the sea level, new layers of silt have been formed on top of the peat. Since the years 1000–1200, man has stimulated the formation of new lands by building dikes. Originally only lands that fell dry during low sea level were reclaimed; later on, lands a few metres below sea level were reclaimed. In these reclaimed areas groundwater level is always near the soil surface. Lakes have been formed mostly by human action; in the older days peat digging, and nowadays sand digging have created large but shallow lakes, of which the water is more or less ‘standing groundwater’. The water may still contain a large quantity of humic acids from the remaining peat. Due to the influence of the sea deposits, CaC03 is also present in large amounts. These factors together render the lake waters rich in humus and alkaline (pH 7–10). Most of the lakes serve as reservoirs for the surrounding agricultural areas. The water movement in spring and summer is therefore opposite to that in fall and wintertime. This fact and the short renewal times of these lakes cause large chemical and biological changes in which the influence of the sediments on the chemistry of the overlying water plays an important role. For instance in the spring, when water flows in from the IJselmeer, the pH and the concentrations of Cl-, Ca++, HCO3- and CO3 = increase. In autumn, when the rainy season begins, and water is admitted from the polder, the concentrations of phosphate, ammonia, silicate and iron increase. However, primary production becomes low due to the decreasing light intensities and the turbidity of the water. When the light increases again in spring a great part of the nutrients disappear from the lake, in which process the sediments play a not unimportant part. [ABSTRACT FROM AUTHOR] |
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