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Rivers in northern Sweden do not always become wider or richer in species further downstream. Natural barriers shape the flow and stop plants from spreading, from Umeå University shows. The study is published in the journal Communications Earth & Environment.

"Our results suggest that many foundational assumptions in river science may not apply here," says Lina Polvi Sjöberg, Associate Professor at the Department of Ecology, Environment and Geoscience at Umeå University.

The rivers in northern Sweden flow through terrain shaped by the last ice age. The landscape is dotted with lakes and covered in sediment brought by the ice: sand, gravel and plenty of boulders.

The study shows that this creates natural barriers that disrupt the flow of water. It also hinders the transport of sediment and the dispersal of plant seeds. All of this affects both the shape of the streams and the mix of plant types found along the shores.

"We found that these landscapes are naturally fragmented, and that local conditions—such as sediment type and proximity to lakes—play a much larger role than previously thought," says Lina Polvi Sjöberg.

Together with researcher Lovisa Lind, she studied two catchments in northern Sweden, Bjurbäcken and Hjuksån, located above and below the highest coastline after the last glaciation. Using maps and field studies, they analyzed the shape of the streams and the plant life along tens of kilometers of the shores.

Their findings challenge widely accepted theories that streams become wider and more biologically diverse downstream. The researchers found no clear relationships between the drainage area and channel width, and no consistent increase in plant species diversity downstream within these medium-sized catchments.

"In the catchment below the former highest coastline, we did see slightly stronger patterns, likely due to finer sediments from the sea. But overall, the presence of lakes and coarse glacial deposits breaks up the expected downstream trends," says Polvi Sjöberg.

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

The researchers also saw an unexpected pattern: that species density (the number of species in a given area) was constant—or even decreased—downstream.

These results are important for river restoration in areas shaped by the Ice Age. In these fragmented systems, passive recovery—where plants recolonize naturally—will probably not succeed. Instead, active interventions such as planting and physically reshaping the stream channels might be necessary.

"Restoration strategies need to be adapted to these local realities," says Polvi Sjöberg.