The river clay landscape
Just like the sea clay scenery and the dune scenery, the river clay scenery originates from the Holocene period. After the last ice age (the Weichselene) the temperature rose and the rate of water drainage gradually increased. This influenced both the eastern (east of the line Leerdam-Culemborg-Vianen) and the western (west of this line) river clay.
The eastern river clay scenery
The riverbeds became more narrow and more twisty as a result of the water drainage in the Holocene. Consequently, the capacity of the rivers was reduced, which led to regular flooding. Because of the floods, the water flowed out of the rivers and deposited sand and clay next to the bed. The sand was heavier and had larger grains than the clay. For that reason, the sand sank earlier and stopped closer to the riverbed. The sand created so-called river banks in this way: a sandy strip situated somewhat higher than the river bed itself. Further away from the river, the clay sunk down to the soil in basins. A basin is a lower area between rivers, which exists of heavy clay. Today, the basins lie between one and two metres below the river banks. The basin soils are mostly used as grass land, because they do not drain water very effectively.
The rivers changed direction regularly through time. The water level of the rivers rose because of both the rising sea level and the silted up beds. Eventually, rivers would flood and change direction. The old riverbeds with the banks are called 'flow ridges'.
Further characteristics of the river clay scenery are the so-called swamps. Swamps are river dunes created at the end of the last ice age. Most of these dunes have been covered over with clay, though some still remain today. The swamps consist of sand blown in by the wind. A deposit supplied by the wind, is called an 'Aeolian deposit'.
The western river clay scenery
The western river clay scenery is different from the eastern river clay scenery, because the sea had a greater influence on the area. The drainage of these rivers depends on the tides, as a result of which the western river clay scenery has different deposits than the eastern river clay scenery. During flood tide the river water cannot drain away easily: the water is obstructed by the rising sea water. This leads to very easy flooding. Both river banks and flow ridges are made of clay. The reason for this is that no sand is deposited due to the low rate of flow, which makes the sand stay on the bottom of the river. The basins in the western river clay scenery exist of silty peat. The swamps were raised by both clay and peat.
The first habitation in the river clay scenery started around 2000 years before Christ. People chose to live in places situated high up, in order to be safe from the water. These higher places were for instance, river banks, flow ridges and swamps. Later, people artificially raised the land themselves, creating what we call terps. The first dikes were built around the 11th century. In contrast with the dikes we see in the Netherlands today, which are built parallel to the water, these dikes were built at right angles to rivers.
It may not seem very useful to build a dike at right angles to a river, but back then there was a very good reason for this method. The dikes did not have to 'turn' the water, but rather keep the water away from the lower areas of the village.
The basins used to be very wet. Not only was rain water gathered there, but seepage water and flood water too. There was too much water to return to the river, so a large proportion flowed from east to west, via a slope. To stop this water, so-called 'behind quays' or 'behind dikes' were built. Eventually, dikes were built alongside the rivers, resulting in closed dike rings.
Watercourses were dug to drain water out of the basins. Ditches were also dug at right angles to these watercourses. During high water levels, the dikes were sometimes too low and often burst. The water would then patter behind the dikes, through a large hole. The hole would be so deep that the dike could not be closed at the same place. The dike was therefore built around the hole, resulting in a twist. Nowadays, dikes that broke through often, can easily be recognized through their twists.
To increase the water storage and overcome the bursting of the dikes, a winter dike was built, at some distance from the river. When the water of the river would flow over the summer dike (the dike that was built directly alongside the river), the water could flow into the area in between the two dikes. In that way the water would not reach the villages. The area between the two dikes is called river foreland. During high water levels, the water would leave a layer of sandy clay in the river forelands. Consequently, the river forelands are situated much higher than the land on the other side of the dike. The river forelands are, besides for water storage, also used for extraction of sand, clay and gravel. In the summer, they are also used as meadow land pasture.
The river clay scenery faces many different problems:
Firstly, there are some problems regarding the dikes. The dikes need a lot of maintenance. The water level in the rivers rose as a result of the Deltaworks, and the drainage rate decreased. Because of this, large amounts of sand are deposited into the rivers, meaning the rivers are are situated higher, thus raising the water level. The water level also becomes higher because of the rising sea level. Consequently, it is increasingly important to raise and / or strengthen the existing dikes. The Dutch will always have to fight the water!
The river clay scenery is also threatened by some environmental problems. Firstly, the extraction of clay, sand and gravel has a great influence on the flora and fauna. The depth and the surface of the extraction causes a shortage of oxygen on the bottom of these pools. Because of this, the water is not survivable for many types of plants and animals, so the ecological diversity is low. The river forelands are also damaged, primarily because of pollution of the river water.