Rivers are high-speed corridors for the spread of invasive exotic plants. Increasingly, these plants are pushing out native species and making floods more likely. A study conducted by Deltares, Utrecht University, Radboud University and the German Institute for Flood Plain Ecology has shown that exotic varieties like the Japanese knotweed and the Himalayan balsam grow faster and form denser vegetation in European flood plains than the native vegetation. The phenomenon is also seen Dutch river areas.
Particularly where a river deposits new sand, exotic species spread like a plague. The dense plant growth slows down the flow of the river and increases the flood risk in the summer and autumn.
River development calculated for a century and a half
Mijke van Oorschot (Utrecht University/Deltares) and fellow-researchers combined an existing computer model for water flows and the deposition of sand with a new model for the spread, growth and death of plants. In this case, willows, poplars and the Japanese knotweed. 'Model results showed that, in the worst case, the invasive species became dominant within a few years and that water levels rose by about 35%. The patterns were similar for rivers in Northwest Europe,' explains Mijke.