New Rhine

What the Karlsruhe engineer Johann Gottfried Tulla began in 1817 was completed this year: The river loops of the Upper Rhine have been pierced, the Rhine straightened and shortened. With the correction, the bed of the river was also deepened. The Rhine is now navigable as far as Basel.

For decades, construction workers had been working on the river with shovels, wheelbarrows and horse-drawn carts. The bed of the Rhine north of Karlsruhe had to be partially re-excavated: A total of 18 cuttings now shorten the original path of the water through its numerous meanders. The old river loops were filled in at their inlets. The result: the Rhine no longer flows in many branching tributaries, but is concentrated in a dead-straight main bed. In addition, dams were built along the river. These are intended to protect the residents from flooding. It is also hoped that draining the swamps will reduce the dreaded swamp fever, malaria.

The straightening of the Rhine initially met with fierce resistance from farmers and fishermen, some of whom used force of arms to break it up. However, when the straightened areas were spared from the floods after a flood, the project found more supporters and the work was accelerated.

Since the Rhine has been straightened, its course between Basel and Mannheim is 90 kilometres shorter than before. Because the river bed has also been lowered, heavy cargo ships can now reach as far as Basel.

On risks and side effects

Tulla had only wanted the best: “…In cultivated lands, streams, rivers and creeks should be canals and the management of the waters should be in the power of the inhabitants.” The advantages of river correction were clear: one wanted to facilitate navigation, to de-sump the land for agriculture and animal husbandry, and at the same time to curb the danger of epidemics and floods.

But Tulla’s project was controversial from the start. Farmers and fishermen feared economic disadvantages. Moreover, the further development of the Upper Rhine showed the unforeseen consequences of this intervention in nature: the Rhine no longer flowed leisurely through the loops, but flowed much faster on a straight path. As a result, it dug deeper into the ground, the groundwater level sank and species-rich wetlands dried up.

In addition, during floods, the excess water masses could no longer spread out in the Rhine floodplains. Instead, they flowed rapidly downstream and threatened the towns and countryside there. So Tulla did not really eliminate the flood danger, but only shifted it downstream.

Why do rivers flow in loops?

Like a snake, the Moselle winds its way through the country. Its bends and loops have made it famous. Yet it is not at all unusual for a river to meander along its way. In their middle and lower reaches, rivers often form loops, so-called meanders. Incidentally, the term comes from the Greek name “Maiandros” for the river Menderes in what is now western Turkey.

In these loops, the water flows at different speeds: On the outside, the water has a longer path, therefore it flows faster and transports more material. Therefore, the outside of the curve is more abraded. On this side, a steep slope develops over time, the impact slope. On the inside of the bend, on the other hand, the water flows more slowly, so that the material it carries, for example silt and gravel, settles. A shallow sliding slope forms here. Through erosion on the outside and deposition on the inside, the loops of the meander grow further and further outwards.

This process is self-reinforcing: the more the river course is bent, the more the flow velocities on the outside and inside differ, the more material is removed and deposited, and the more the loop grows outwards. Thus, even small, random deviations from the straight riverbed can grow into strong meanders over time.

As the meanders grow, the individual loops come closer and closer together. At some point, a meander breakthrough may occur. The water then flows on the direct path again. What remains of the old river course is a crescent-shaped old water arm that “loops around” a circulating mountain. Well-known circulating mountains can be found, for example, on the Saar or the Moselle. Oxbow lakes can also be found in the Rhine valley, in the Upper Rhine Graben. However, some of the meanders of the Rhine were artificially pierced in order to straighten the Rhine and develop it as a waterway.

From trickle to stream – flowing waters

Bubbling groundwater emerges from a spring and flows down the slope as a thin trickle or a small stream: a watercourse has been created. All flowing waters start out small. On their way towards their mouths, they merge with other watercourses and continue to grow until they have become a river or even a broad stream. At its lower end, the watercourse flows into another river, a lake or the sea.

Streams, rivers or creeks – terms that come fluidly to our lips are precisely distinguished from one another by scientists (geographers). They can be classified by their water volume, their length or their width: If the watercourse is less than half a metre wide, it is called a trickle; if it is more than 2 metres wide, it is called a stream. If the watercourse swells to a width of 10 metres, it is a river. And if it gets even wider, the river can be called a stream. The Amazon or the Nile, for example, are called rivers, but the Rhine and the Danube are also streams.

The amount of water in the flowing water increases from the source to the mouth. Nevertheless, it flows slower and slower downhill. This is because the slope down which it flows is steeper at the top than at the bottom. And because the water flows faster at the top and slower downstream, it can carry more sand and debris along the upper course than the lower. Thus, more sand and debris is removed from the upper course of a river, and more is deposited in the lower course.

Upper Rhine Graben

Doors rattle without the slightest wind blowing. Cups clatter in the cupboard as if moved by a ghostly hand. This is not a scary movie, but reality in the Upper Rhine Graben. In this region in southwest Germany, there are smaller earthquakes every few months. This is surprising, because there is no plate boundary to be seen here, far and wide.

The Upper Rhine Graben is a depression about 300 kilometres long and up to 40 kilometres wide between Basel and Frankfurt. At first it looks like an ordinary river valley, but it owes its formation to a weak point in the earth’s crust.

Along this weak spot, a trench collapsed in the course of the last 45 million years. In the process, the sinking rock disintegrated into fragments of different sizes and partially slid down into the trench. At the same time as the trench sank, the rock was lifted at its edges. This is how the “trench shoulders” developed, which can still be recognised today as the Black Forest and the Vosges. However, the erosion constantly compensated for the difference in height between the depression and the mountains: Debris and rock slabs slid in from the sides and filled up the trench again and again. Thick sediment layers accumulated at its bottom, through which the Rhine only found its way much later.

To this day, the earth’s crust is expanding and moving in the area of the Upper Rhine Graben. The trench sinks almost one millimetre every year. Thus, tensions are constantly building up in the rock, which are repeatedly discharged in smaller earthquakes. One earthquake, however, was unusually violent and reduced the city of Basel to rubble in 1356.

Whether such a strong quake could happen again soon is difficult to answer, even for experts. The processes in the earth’s crust are still far from being completely understood. There are several possible explanations for the formation of the rift: one cause could be that the African plate is pressing against the European plate from the south. In the process, the Alps were folded up and presumably also the Black Forest and the Vosges were lifted. The enormous pressures and tensions could have caused the rock of these older mountains to break apart, thus creating the rift valley. Another assumption is that magma from the Earth’s mantle pushed upwards, stretched the Earth’s crust and thus ripped open the trench. To find out exactly what happened, the Upper Rhine Graben is still monitored and studied by geologists today.