Rewilding Abramsån

Healing the silent wounds of a northern Swedish river

Rewilding Abramsån

Healing the silent wounds of a northern Swedish river

Background and project outline

Forestry has for a long time been one of Sweden’s largest industries. In the early 20th century, when there was no network of forestry roads, streams and rivers served as timber highways. For the purpose of transporting logs to coastal sawmills, boulders and blocks were removed from nearly all rivers in northern Sweden. It was laborious and dangerous work that also caused devastating damage to aquatic ecosystems. Before the 1950s, rocks were manually removed, sometimes with dynamite, and dumped on riverbanks, severing natural land-water connections. But this was little compared to what was to follow. After World War II, bulldozers took over from manual labour, accelerating river degradation and rendering once-diverse habitats even more homogenized.

Abramsån, 45 kilometres south of the Arctic Circle and a tributary of the Råne River, shares the same fate. Channelized before 1880, not only was it stripped of its natural course but, unlike most other rivers, floored with wooden boards over many kilometres to further ease log transportation. In the summer of 2023, Rewilding Sweden initiated a multi-year rewilding project to restore this remarkable river, reviving its natural processes and reversing over a century of human impact.

Funding: LOVA (Swedish Agency for Marine and Water Management), County Administrative Board of Norrbotten, EKOEnergy

Prerequisites and rewilding approach

The challenge

A straightened river has reduced capacity to trap organic matter, like leaves, due to rapid currents and few obstructing elements. This lowers carbon (energy) input to the system and thereby food for leaf-shredding insect larvae – the initiators of the boreal aquatic food web. Leaves still fall, but without boulders, backwaters, or woody debris to trap them, they wash away. The lack of large stones creates a homogenous ecosystem with few resting and foraging spots for fish, while less varied turbulence affects oxygen levels. Stone-lined banks disrupt land–water interaction and nutrient exchange. Without leaf litter, the stream depends almost entirely on algae for energy. In northern latitudes, low temperatures and little sunlight reduce algal growth, making food scarce for scrapers – aquatic grazers feeding on periphyton in rapids. Some flow-adapted species, like filter-feeding blackfly larvae (Simuliidae), may still thrive, but they rely on fine organic particles, now limited due shredder shortage. With fewer insects, fish decline. Without natural flooding, much native riparian vegetation does poorly, including deciduous trees – leading to bird decrease. In the end, the whole ecosystem suffers.

The solution

Phase 1: Restoring structural complexity

River shape is controlled by slope, discharge of water, and soil type. Channels naturally meander in fine-grained soils of low slope, but follow a straighter path in coarse, erosion-resistant stony terrain. Machine-restoration, as in Abramsån, seeks to counteract structural simplification in rigid sections the river itself cannot reshape.

During the initial restoration phase of Abramsån, Rewilding Sweden reinstated boulders taken out during the timber-floating era and expanded channel width, structure, and depth to reduce flow velocity and trap fine sediments (habitat) and organic matter (food source). We further increased structural complexity by adding dead wood – slow-degrading conifers in most cases – with long-term ecological gain without decreasing yearly leaf litter input. The wood floor was removed, and logs were recycled as log jams to the benefit of salmonids or to mimic natural dead wood.

Phase 2: Adding fine sediment

Even in turbulent moraine rivers, gravel and sand are a natural part of the sediment mix. They are essential microhabitats for the larvae of aquatic insects, important spawning areas for river fish, and substrates for freshwater pearl mussels. Timber floating and almost a century of channelization removed these fine materials.

Although Abramsån was restored structurally in the first phase, it was still lacking in fine sediments, resulting in loss of habitat and artificially shallow water as water flowed into sand-filled cavities that had once naturally been filled with sand. To alleviate this, Rewilding Sweden used a helicopter to add 180 tons of sand and gravel to a 100-meter test reach. The effect of the measure on aquatic life is followed by periodic electrofishing, and a researcher from the Norwegian Institute for Nature Research (NINA) follows the response of aquatic insects’ larvae. The helicopter operation was funded by EKOenergy Environmental Fund.

Expected impacts

We expect restoration responses to fall into three distinct stages:

◆ Level 1: Direct responses
Increased structural complexity; reduced overall velocity, more varied oxygenation levels; more large woody debris and increased leaf trapping.

◆ Level 2: Consequential effects
Enhanced macroinvertebrate functional β-diversity driven by greater habitat heterogeneity; increased shredder abundance linked to higher organic matter retention; more sand/gravel dwellers in re-sanded reaches; improved lateral connectivity between water and land from increased water retention.

◆ Level 3: End goals (depending also on other landscape factors)
Increased riverine fish presence; enhanced riverside bird community responding to natural flooding and more swamp forests habitats.