11 November 2012

Traces of a mighty paleo-Danube in the Altmühl Valley

The morphology of the river valleys depends on the amount of the water delivered. By morphology we do not only understand the width of the valley: the term also includes the development of the bends. The more abundant the water discharge of a river is, the larger bends it forms. If we consider that the water discharge of a river varies widely even within a year, the more so it was in the various geological ages. The bend development of long abandoned riverbeds indicate the water discharge conditions of old ages. Therefore, a geographer always becomes suspicious if he sees a small brooklet – that cannot be even properly called a stream – running in a large and wide valley. The Wellheim valley in Bavaria is such a place.

The ancient riverbed forms of the Wellheim dry valley. Source: urdonautal.info

It’s like a green river meandering between the limestone ranges of the Frankish Alb. However, there is no trace of water here. The light green of the fields and the dark green of the forests, which form a sharp line, draws the half to one kilometer wide valley of a huge ancient river. Between Rennertshofen and Dollnstein it is called Wellheim’s (dry) valley, while between Dollnstein and Kelheim at the Danube the Altmühl stream runs in it. This paleo-river valley meanders at a total length of 120 kilometers between the Jura limestone rocks covered with forest. In the last 35 kilometers, under the town of Dietfurt it is filled by the bed of the Danube-Main-Rhine canal.

Once a mighty river, now the dry valley of Wellheim
Even the Altmühl stream does not thave enough water discharge to develop a valley system of this size. It only runs for 220 kilometers fom its source in Middle Franconia to the Danube. Its average water discharge is 17 m³/s, and even the largest ever measured value was only 159 m³/s. It gets lost in this large valley as our foot if we put it in a giant’s shoe. Once a much mightier river had to run here. Which river might have it been?

The Altmühl stream at the town of Eichstätt

The only nearby river which has enough water to develop such a valley is the Danube. However, if we go back a bit in time, to the Pliocene age (from 5.332 to 2.528 million years), it turns out that this is by no means so clear. The current valley of the Upper Danube was long covered by sea after the Alpine orogeny. Both the source and the estuary of the Danube was in a completely different place than it is today. At that time it came from the Alps, and gradually filled up the shallow, brackish sea lagoon which reached up from somewhere the Black Sea to today’s Southern Germany. When the present Altmühl valley was developed, the delta of the Danube was somewhere around Vienna.

The estuary of the paleo-Danube and paleo-Main at Dollstein. Source: urdonautal.info

The paleo-Danube, which developed this valley system, had a much greater water discharge at this section than it is today. At that time it was the paleo-Main running in the upper valley of the Altmühl stream, and the Fulda and Neckar also brought here their water. But what happened to these tributaries? For the serious loss of water suffered by the Danube since the Pliocene period, another river is responsible: the Rhine, which has been gradually pushing the continental divide to the east. With its valley sinking down by way of a trench, it has taken over one by one the tributaries of the Danube. However, the Rhine cannot be blamed for the drying out of the Altmühl valley.
Once the valley of the Danube, today of a stream – the Schutter. Source: wiki

Near the city of Ingolstadt, a small stream flows into the Danube. In the Middle Ages the local mills stood over the water of the Schutter. Until 80 thousand years ago, this stream was also a bed of the Danube, after the main branch disappeared from the Altmühl valley. But in vain you would look for the Danube here, you would only find this thin brooklet in the wide valley, to the west of Ingolstadt. The river spent much less time here, as it soon found a new bed for itself in the valley of a southern tributary. The cutting back of the Neuburg stream again shortened the Danube by a few kilometers. The shortening has led to the increase of falling, since the river makes the same height difference on a shorter stretch. The Neuburg section has not yet been fully adopted by the river. The landscape here is completely different from the broad and flat valley of the Altmühl.
The Weltenburg monastery to the west of Kelheim, the oldest abbey brewery of the world. Source here

The Danube meandering next to the Weltenburg monastery, in a width of 60-100 meters between the limestone walls, is only a pale replica of that mighty paleo-river which, at the turn of the Pliocene and Pleistocene, developed the Altmühl valley. The Danube, once deprived from its complete Alpine catchment area, formed a much smaller valley and smaller bends in its new, seemingly final section.

In Hungary the Érmellék is comparable to the Altmühl valley. Here the old bed of the Tisza was “inherited” by the Ér stream.

Suggested literature:

    Franz X. Bogner: Im Urdonautal der Altmühl, 2001

Translation by: Tamás Sajó

No comments:

Post a Comment