Naturschutzgebiet Aare bei Altreu: A Protected Alluvial Landscape and Riverine Habitat in Solothurn

The reserve was formally established in 1992 under the federal ordinance on the protection of alluvial areas of national importance, with the Bern cantonal government setting out the detailed protection objectives in a decision of December 1995. A separate protection and management concept for the Widi area was adopted in 1993 and revised in 1994.

Its protection responds to centuries of transformation of the Aare landscape. Earlier flood events once regularly inundated the broad Witi plain between the river and the Jura, sustaining a rich mosaic of wet meadows, willow woods, and shifting channels. The second Jura water correction, which aimed to drain farmland and protect settlements from flooding, straightened and embanked much of the Aare and armored the outer banks with blockwork, leaving the river functioning like an artificial canal for most of its course. The areas inside the great meanders near Arch and Altreu were among the few places where enough natural floodplain character survived to justify federal designation, and the reserve now safeguards those remnants against further loss.

The landscape of the reserve is defined by the wide, slow meanders of the Aare across the flat plain of the lower Aare valley. At this point the river sits on average around 425 meters above sea level, with surface water often lying several meters below the surrounding farmland of the Witi plain. The northern meander near Arch describes an arc roughly 800 meters across, encircling the wooded Widi island. The next loop, about a kilometer in diameter, swings around the Eichacker before turning again near Altreu.

Inside these curves, gently sloping banks and seasonal shallows have created a layered mosaic of habitats: reed-fringed shorelines, marshy flats, and low willow woods rooted in soft alluvial soils. Outside the meanders the same river is heavily armored; stone blockwork on the outer bends marks a sharp boundary between the engineered channel and the cultivated fields, making the protected floodplain islands appear as narrow ribbons of green within an intensively farmed plain.

Ecologically, Aare bei Altreu stands out as one of the few remaining examples of softwood floodplain forest along the Swiss Aare. Silver willow, a species that once formed extensive gallery forests along naturally meandering river courses, still occurs here in remnant stands within the Weichholzaue zone, where younger soils along active channels support rapid tree turnover and dense shrubby undergrowth.

Beyond the willow stands, reed beds, sedges, and other water and marsh plants colonize the shallow banks and periodically flooded flats. Seasonal hydrological variation is central to the site's ecological character: small shifts in water level open up fresh sediment for pioneer species, maintain saturated marshy ground, and shape the boundaries between open water, reed, willow thicket, and drier grassy areas. The mosaic of microhabitats along the meanders sustains a rich community of wetland-dependent invertebrates, amphibians, and birds in a landscape where such variety has largely been lost.

While the reserve is small, its concentration of wetland and alluvial habitats gives it significant value for amphibians and waterbirds. The shallow banks, ponds, and seasonally flooded margins provide breeding and refuge habitat for frog, toad, and newt species adapted to dynamic floodplain conditions, and the site is integrated into the federal inventory of amphibian spawning areas of national importance through the adjacent Widi reserve.

The wider corridor along which Aare bei Altreu lies supports diverse birdlife, as documented by long-term ornithological studies of the Aare plain west of Solothurn. The reed beds shelter typical marsh birds, while the open water and gravel margins are used by waterfowl and migratory species that move between the Aare and the large Grenchner Witi waterbird reserve downstream. Small mammals, fish in the Aare itself, and a wide range of wetland invertebrates contribute to the food web that the floodplain mosaic sustains.

The reserve's central conservation role is to protect one of the last near-natural alluvial sections of the Aare in a region where most of the river has been straightened, deepened, and armored. By preserving the inner meander curves, the silver willow stands, and the associated reed and marsh habitats, the site maintains ecological processes and species that depend on natural flooding dynamics, even at a very small spatial scale.

The protection objectives set by the Bern cantonal government in 1995 emphasize the maintenance of a near-natural floodplain landscape of national importance, the promotion of characteristic floodplain flora and fauna, and the safeguarding of the area as a key amphibian habitat. The site's incorporation into the broader Witi landscape protection zone on the Solothurn side strengthens these goals by linking Aare bei Altreu to neighboring refuges such as Eichacker-Wannengraben, the Grenchner Witi, and Pro Natura's Wannengraben reserve, forming an interconnected network of aquatic and wetland habitats along the river corridor.

The landscape of Aare bei Altreu is intimately tied to the long history of human effort to control and harness the Aare. The plain west of Solothurn, including the Witi area, was once regularly shaped by river floods and was gradually converted to stable farmland through drainage, embankments, and the large-scale river corrections of the nineteenth and twentieth centuries. Small villages such as Altreu, Arch, Leuzigen, Bettlach, and Selzach grew up in this tension between settlement and floodplain.

Today the narrow wooded strips along the river serve dual roles as protected ecological corridors and as part of recreational infrastructure: the national Aare cycling route and various footpaths run along the bank, accompanied by information boards explaining the meanders and their ecological significance. Conservation organizations such as Pro Natura play an active role in managing adjacent refuges like Wannengraben, reflecting a regional culture of cooperative land stewardship between cantonal authorities, conservation NGOs, and local landholders.

Aare bei Altreu is a textbook example of how a very small protected area can preserve processes and habitats that have disappeared elsewhere along a major European river. Its combination of intact meander geometry, surviving silver willow floodplain forest, and integration into a broader corridor of amphibian and bird reserves gives it an outsized ecological importance relative to its modest size.

Spring and early summer are typically the most rewarding periods for visiting the floodplain, when the water level is moderate, amphibian activity peaks in the surrounding spawning ponds, and the willow and herbaceous layers are at their freshest. Late spring and early summer also bring active birdlife along the reeds and water margins.

In warmer months the reed beds and riparian vegetation are fully developed and the meander landscape reads clearly from the riverside paths, though summer heat can reduce amphibian visibility. Autumn brings a quieter atmosphere and changing colors in the willow stands, while winter visits reveal the structure of the meanders and the contrast between the protected wooded banks and the surrounding farmland under bare conditions. Visitors generally experience the reserve on foot or by bicycle along the riverside paths, including sections of the national Aare Route.