How are rivers traced back to their source?
Where does the gravel come from?
Gravel is a fascinating material. Who hasn't admired the wealth of shapes and colors of these rounded stones on a river bank or in a clear mountain stream? Some pebbles are plain white, gray or black, there are green, brown, brick-red or salmon-colored ones. Quite a few consist of two or more colored components. Some are dotted or marbled in waves, others have glittering crystalline inclusions. No wonder that gravel is often used as a decorative covering in gardens and parks. But why do pebbles appear to be industrially manufactured, sorted by size, polished and rounded? Gravel never arises where it is found. All the colorful stones on the banks of our rivers have already come a long way. The color and composition of the pebbles point the way to their place of origin - and that is mostly in the mountains. Although the erosion of the surrounding rock ensures a constant replenishment of sediments from the Alps or low mountain ranges, only a small part of the sands and gravel finds its way into the water of our streams and rivers through the current weathering. For the Danube and the Rhine and their southern tributaries, the mobilization of Ice Age deposits from the Alps is by far the most important source of sediments. The rivers of the north German lowlands draw their sediments to a large extent from the remains of the Scandinavian ice sheet, which with its huge glacier rivers has deposited a layer of sediment that is often several hundred meters thick. This material therefore mostly comes from Northern Europe.
The Danube and Rhine draw their sediments from the Alps
Elbe, Oder and even the Thames mobilize the sediments of the Ice Ages that came to us from the Scandinavian mountains. The Svartisen at the Arctic Circle is only a modest remnant of the former ice sheet, which reached as far as England and northern Germany.
Just like in Greenland, the Alps looked 12,000 years ago ...
... and with the plane of the ice and the transport power of the water, sand and gravel were created ...
... which filled up the ice age river valleys as sediments. And the valleys of the Rhine and Danube once looked like the Watson River in Greenland ...
... what can no longer be recognized ...
... and the gravel has also found new interested parties.
As long as stones are transported through glaciers, they have an irregular, angular shape with a rough surface. Deposits from glaciers (moraines) consist of sand and rocks of very different sizes. Running water loosens the sands and smaller rock fractions from the moraines and transports them downstream. The mutual friction in the water polishes the stones until they gradually round into pebbles. This process began as early as the Ice Ages. What remained were the coarse gravel and the often tons of heavy blocks in the upper reaches of the Alpine rivers, which withstand even the strongest floods. But due to the constant erosion, they too will have to start their way down the river at some point, chopped up in portions. Due to its mineral composition, the origin of a pebble can often be traced back to a specific rock layer in a certain mountain range. Each pebble tells the story of how it was created and how long it went to where it was found.
A higher water pressure is required to move large stones than to move finer sediments. Coarser stones are only mobilized when there is a strong current and are deposited again much faster than finer materials, which are more easily picked up and transported much further. Stone blocks, gravel and coarse gravel therefore dominate the upper reaches of the alpine tributaries of the Danube and Rhine. Of course, sandbanks also form in the quiet zones of these flow stretches, but these are easily washed away again during floods. What remains are the larger rocks, which are only moved a little downstream in the event of exceptional floods. Finer gravel fractions and sands are preferentially deposited along the middle and lower reaches of the rivers. In this way, in the course of time, the rock fractions along the course of a river have literally segregated in terms of size and weight. Whether and at which point gravel or sand banks form depends not only on the sediment supply but also on the shape of the river. A curved course or river islands promote deposits on the banks facing away from the current. In shallow river valleys, large-scale relocations of the gravel banks are common, with steep valley profiles the sedimentation is rather limited to a narrow strip of banks. If the valleys are wide and flat, like they used to be on the Danube and its alpine tributaries, sand and gravel fields that are kilometers long and hundreds of meters wide can arise.
On the upper Danube, regulatory measures, dams and the regular dredging of the course of the river have brought the formation of new sediment banks to a complete standstill. The fate of the once extensive gravel banks along the southern tributaries of the Danube is similarly deplorable. The Isar is a pleasant exception here. Especially between Krün and Wolfratshausen and from Freising to the estuary, the complete variety of forms of natural bank areas has been preserved in sections. Hydraulic engineering measures are not limited to cutting high water peaks, but also to avoiding low water. Dams and reservoirs are tools with which the runoff of our rivers is adapted to the economic needs. A steady flow of water optimizes energy generation and water transport - but not the living conditions of the organisms in and around the river. The formation and maintenance of natural gravel and sand banks require, in addition to intact bed load transport, hydrological dynamics that are unaffected by human intervention. The Danube and its alpine tributaries were once characterized by spacious gravel and sand banks that could only arise in this form in the foothills of the Alps. The remnants of this natural heritage are among the most endangered ecosystems in our homeland today.
On the Argen in the Allgäu or on the upper Isar, the world is still okay.
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