A permanent feature of the Dutch river system in it’s original, natural state is siltation of river mouths causing flooding of upstream areas.
This started already by the time the Romans arrived in the Low Countries, about 2000 years ago. Then the river Rhine had its most important outlet near Katwijk, on the North Sea coast, passing the locations of present-day Utrecht and Leiden on a course some 30 km to the north of the present northernmost branch of the river system, the Lek/Nieuwe Maas. The Romans took this main branch of the Rhine river system as the northern boundary of their empire.
Flooding of the Betuwe area and construction of the Rhine-IJssel bypass
At that time floods threatened human settlements in the Betuwe area, at the time a relatively densely occupied and economically important region. These settlements were not yet protected by embankments, but were located on deposits of sand and gravel along the rivers that remained dry most of the time, but suffered when flood levels were higher than normal, which occurred more and more often due to the silting up of the river mouth at Katwijk.
The Romans tried to improve the situation by connecting the Rhine river to the IJssel river. Until then the IJssel river was just a collector of brooks draining the sandy hills of the Veluwe and Salland, transporting the water north to the Almere Lake. By making the connection the IJssel could deflect Rhine water to the north relieving pressure on the Betuwe area. This connection is still there as the Pannerdens Kanaal, making the IJssel part of the major river system of The Netherlands.
Shift of the Rhine’s main branch to the Lek river – cutting off the old branch to Katwijk
For the moment this solution was helpful but it didn’t take away the structural problem. In time this caused the main flow of river to seek a different course altogether, breaking through to the river Lek and reaching the North Sea more to the south. In the same way as the IJssel, the Lek wasn’t an original part of the major river system, but just a collector of local rain water (in this case draining an extensive peat area).
Being a peat river, the Lek had never deposited sand and gravel along its course, so in a later stage, when constructing embankments started (in the 11th century), for the Lek these couldn’t have a stable foundation. Nowadays this is still a problem for which no satisfactory solution has been found. Also in the 11th century the silting up of the old Rhine mouth at Katwijk had progressed so far that the flow through this outlet was almost completely blocked backing up the water that still flowed into this old branch. Again flooding was the result until finally this whole branch was cut off by damming it at its beginning. Now the Kromme Rijn, Leidse Rijn and Oude Rijn are still to be found as relicts in the landscape, but are no longer part of the active river system.
Further shift to the south – the Waal river and its joint outlet with the Maas river
In the meantime an even more southernly branch – which we now know as the Waal – started to gradually over as the most important branch. By now two-thirds of the Rhine water takes this southern (Waal) branch, leaving one-third for the northern (Nederrijn-Lek-Nieuwe Waterweg) branch and the IJssel branch together.
Over most of its length the Waal runs parallel to the river Maas that originates in the north of France and flows through Belgium to reach The Netherlands. The Maas has no separate outlet to the sea, but used to flow into the Waal just east from the town of Gorcum, while there was an open connection between the two rivers at a point where they flow directly next to each other, some 30 km east of the conjunction.
New siltation problems – three bypasses as a solution
During the 18th and 19th century the history of a river mouth silting up and causing flooding upstream repeated itself for the joint Maas and Waal rivers, to the west of Rotterdam. Initially this problem was solved by deflecting flood waters to inundation polders in order to protect more vulnerable areas, and by building a new port at Hellevoetsluis (on the Haringvliet estuary) joined by a canal to the port of Rotterdam. During the last half of the 19th century however, it became clear that more radical interventions were needed. These were the digging of three bypasses that could serve a outlets for the combined Rhine and Maas watersheds, and would not silt up any more. These were (1) the Bergse Maas, extending the Maas to the west and moving the conjunction with the Waal to the west of the Biesbosch area; (2) digging a new channel for the Waal (under the name of Nieuwe Merwede) through this same Biesbosch area – meeting the Bergse Maas in a joint outlet into the Hollands Diep and from there the Haringvliet estuary; and (3) digging a shipping canal from the port of Rotterdam directly west to the coast at Hoek van Holland. From these three the numbers one and two do not have any siltation problems anymore; number three needs regular dredging to keep the depth sufficient for big sea-going ships. All in all flooding problems resulting from siltation of river outlets are now something of the past.
Closing the opening between Waal and Maas and discussion to reopen it
At the same time as the bypasses were constructed, around 1900, the old course of the Maas was cut off at the starting point of the bypass, while the open connection to the Waal more to the east, near the village of Kerkdriel, was closed.
Recently an idea has come up to reopen this connection, not for water management reasons but from ecological motives. The State Forestry Agency together with a nature protection NGO (Natuurmonumenten) is strongly advocating this idea, because to enhance aquatic biodiversity it would be good to keep a point of exchange between the two rivers with their different characteristics and different watersheds.
The opening needs to be narrow in order to avoid undesirable water management impacts. The level of the Waal at this point is higher than the level of the Maas, so making a large opening would generate a considerable water flow from the Waal to the Maas. The Waal carrying about three times as much water as the Maas, this would lead to serious flooding of the downriver part of the Maas every time the water discharge of the Waal would be high. And when the water level in the Waal would be low, it would mean that too much water would be lost to the Maas, leaving the downstream part of the Waal with insufficient depth to accommodate inland shipping from the port of Rotterdam to the German hinterland. So for these reasons, a decision to make a narrow opening for ecological purposes may result from the present public debate, but a larger opening is out of the question.
Author & drawings : ir. Steef Buijs