Natural barriers in northern Sweden’s rivers challenge downstream assumptions

Rivers in northern Sweden do not always become wider or richer in downstream cash. Natural barriers shape the flow and prevent plants from spreading, new research from Umeå University show. The study is published in the journal Earth & Environment Communications.

“Our results suggest that many fundamental hypotheses in river sciences may not apply here,” explains Lina Polvi Sjöberg, associate professor in the Department of Ecology, the Environment and Geosciences at Umeå University.

The rivers of northern Sweden flow through a terrain shaped by the last glacial period. The landscape is dotted with lakes and covered with sediment brought by ice: sand, gravel and many rocks.

The study shows that this creates natural barriers that disrupt the flow of water. It also hinders the transport of sediment and the dispersion of vegetable seeds. All this affects both the shape of the rivers and the mixture of plant types found along the banks.

“We have found that these landscapes are naturally fragmented and that local conditions – such as the type of sediment and proximity to the lakes – play a much more important role than we thought,” said Lina Polvi Sjöberg.

With researcher Lovisa Lind, she studied two watersheds in northern Sweden, Bjurbäcken and Hjuksån, located above and below the highest coast after the last glaciation. Using maps and studies in the field, they analyzed the shape of streams and vegetable life along the tens of kilometers from the banks.

Their results question the theories widely accepted according to which flows become larger and more diversified downstream. The researchers found no clear relationship between the drainage area and the width of the channel, and no constant increase in the diversity of plant species downstream in these medium -sized watersheds.

“In the watershed under the old highest coast, we have seen slightly stronger models, probably due to finer sediments of the sea. But overall, the presence of coarse lakes and glacial deposits breaks the trends expected downstream,” said Polvi Sjöberg.

The study shows that the lakes stop the transport of vegetable seeds by water. This leads to differences in the composition of vegetable life between the neighboring parts of the rivers.

The researchers also saw an unexpected scheme: this density of species (the number of species in a given area) was constant – or even decreased – in the event of flow.

These results are important for the restoration of the river in the areas shaped by the ice age. In these fragmented systems, passive recovery – where plants naturally overcome – will probably not succeed. Instead, active interventions such as planting and physical remodeling of flow channels may be necessary.

“Catering strategies must be adapted to these local realities,” said Polvi Sjöberg.