Origin of the NW European lowlands
The lowlands are in fact a large delta, this delta consists mainly out of river sediments.
Delta features:
- Naturally fertile (nutrient rich sediments)
- Sufficient water supply, by river and shallow groundwater
- Transport routes (river)
- Very limited relief.
These features made it a good spot for settlements, because it has a very rich and useful soil and area.
Formation of Lowlands
The lowlands lie on the foot of the Ardennes and the Renish Massif (rijns massief). The lowlands are
formed by tectonics, climate, sea level changes and human inducements over the last 14 million years.
The palaeography of the NW lowlands has a large
impact on the current status of the landscape.
During the Cretaceous the were calcareous deposition
in the shallow warm areas (145 -65 million years BP)
During the Tertiary (65-2.6 million BP) there was
tectonic uplifting of the mainland NW Europe
(ardennes, Eiffel since ~14 million BP). The cause that
the British Islands and the mid high Netherlands are
formed. Also the Northsea floor subducted. This tilting
is still going on the southeast is uplifted and the
northwest is subsiding.
Since 12 million years there are 4 rivers supplying sediments to the delta of NL. This are from the
southeast the Schelde, the Rhine, Meuse and from the east Eridanos.
The delta growth proceeded into a major influence by the river Rhine, the catchment area became
larger.
This Eridanos was removed from the delta during to the glacial period.
During the Late Pleistocene (Saalien ice period ~0.18 -0,13 Ma BP)
the flow of the Rhine and the Meuse was blocked by the icesheet,
this caused them to flow westward.
The current subsurface still consists of these river deposits. Currently
only in the flood regions of the rivers the depositions and the river
sediments are visible.
,Rivers processes, deposits and landforms
Delta: An accretion of clay, silt, sand that forms when the river debouches(uitmonden) into a sea/lake.
The formation of a delta is a mobile process, the coastal processes move the sediments along the coast.
These movements can form lagoons and beach ridges. The sediments always come from the
hinterlands.
Mechanism of sediment transport
Bedload: Rock, gravel and coarse sand. This will stay more close to the riverbed it will not be picked up
high in the stream
Suspension: find sand, silt, clay. These will be picked up by the eddies in water and be distributed over
the whole river depth.
Deposition: this depends on the waterflow velocity. Larger
particles need more energy to be moved so a higher flow
velocity is needed to make them move. For smaller particles the
flow velocity have to be reduced a lot before it settles. For clay
the water has to come to a standstill.
Erosion: For larger particles there is relatively less stream power
needed to erode them from the riverbed. For the smallest
particles like clay there is more stream power needed, because
the particles are more connected to each other. The clay
particles have a diffuse double layer which bind them together.
River types
The type of river is mainly dependent on the sediments,
slope and flow velocity.
There are 4 types of rivers: Braided, meandering,
straight and anastomosing.
The straight river move the least while the braided river
is the most unstable and moves the most.
,Braided river(vlechtende rivier):
Has a high stream power, the gradient is large and has a high discharge.
The transported sediments are mainly bedload, so larger sediments.
The braided river has, terrace edges, a main channel, mid-channel bar and
a inter channel
Meandering river:
Has an intermediate slope and it moves in the landscape, the sediments
are mixed.
A meandering river has scroll/point bars (rol/schuifbalken) these bars are
in the inside bend of the river. Swales are between the bars and the bars
are clearly visible in the landscape.
At the outer bends the river will erode the material and this is than
deposited in the inner bends.
Natural levee(overwal) these are formed when a river inundates. The
coarser materials are deposited first and the finer materials further from
the river. A fining upward sequence can be found.
Flood basin (overstromingsbekken) this is the area furthest
from the river, here the finest sediments will settle.
Oxbow lake(hoefijzer meer): This is meandering bend that has
been cut off from the original flow and a lake is formed.
Straight River:
River is the most stable, has the least slope and transports the
smallest sediments. Banks are erosion-resistant therefore no
scroll bars visible.
Anastomosing river:
The river is composed of 2 or more interconnected channels, in the middle flood basins are present.
The flood basins are hollow shaped, the flood basin of a braided river is more concave shaped.
River types in the Rhine delta:
Formerly the Rhine was a braiding river, however the river got a weaker stream and the slope is not
large enough. However in the oude IJssel there are still some traces of a braided system visible
The river in NL changes it type from a meandering river towards a straight river (Coastal river).
The central river has a high oscillating and lateral migration of the river.
This change is river type is a consequence:
- the decrease in slope,
- the tidal influence of the sea which the river is flowing to
- Less erodible river banks (clay & peat)
In the east the river transports coarser grain size than going to the west, also the eastern river incises
into the landscape.
Terrace Crossing
Terrace crossing(Terrassenkruising) is the border between the incision (upstream) and the deposition
(downstream)
The location of the crossing is determined by:
- Tectonics
- Climate
- Sea level
Sedimentation Wedge (sedimentatie wig) behind this wedge the sedimentation will fill up the river
downstream of the terrace crossing. The highest sedimentation takes place at the smallest gradient.
, During the Holocene the terrace crossing moved from the west towards the south east. This movement
was caused by an sea level increase. The tectonic uplift will cause that the crossing will move
downstream.
Sediments downstream crossing:
The older sediments are below the younger sedimentary layers, because the younger sedimentary
layers are overlayed by new ones due to inundations.
Sediments upstream crossing:
The older terraces are higher in the landscape the than the younger terraces.
Terrace formation
That terraces are formed is a result of the river ‘searching’ for an optimal dynamic equilibrium between;
discharge, sediment load and gradient.
Below the crossing the river will deposit the load because the discharge becomes less and the gradient
decreases.
Disrupted by:
- Sea level rise
- Climate change
- Tectonics (uplift of hinterland => downstream movement of crossing)
Climate change
Cold periods:
- No vegetation on the bank
- Melt water is transported on the permafrost, causing high peaks in discharge and sediments in the
water
- The river has a braided pattern and a wide plain.
The river transports high amounts of sediments and a braided river normally deposits a lot.
Warm periods:
- Vegetation present on the bank
- The river has high discharge due to melting of water. There is low amounts of sediments in the
river, so it is hungry for sediments. This caused an incision in the riverbed.
- Incision caused a formation of meandering river.
The cold and warmer periods followed each other forming the terrace landscape.
During these cold periods in the late Pleistocene glacial (Weichsaalien 15.000 Bp) these highest terraces
are formed by the braided river deposits and the river was incised during the interglacial periods.
In the Meuse valley 4 terraces are formed.
Several braided river channels formed an irregular shaped terrace landscape. The change between cold
and warmer periods incised several channels forming an variety of higher and lower terraces.
During the colder periods the incised riverbeds are again filled up with sediments.
During the last period (younger dryas) river dunes were formed with aeolian deposition, especially
visible at the east bank. The wind came from the west so parabolic dunes are also visible. The aeolian
sand was caught by the heath on the banks.
Levees, Crevasse and avulsion
Downstream of the terraces crossing the levees (overwal) are present. These have an upward fining
sequence and are formed during inundations of the river. It consists mainly of sandy clay or light clay.
Crevasse is a natural breach of a levee. Behind the breach a fan shaped deposition plain is present, it
consist of mainly out of coarse sediments.
