Mölndal Open Water

Organized by the local club MASS (Mölndal Allmänna Simsällskap) alongside Mölndal Open Water, the event was established as the open water swimming leg for the revival of the Göteborgsklassikern multi-sport circuit. Swimmers assemble along the waterfront at Stensjöns badplats, stepping off the lakeshore into smooth, clean freshwater protected from coastal winds and ocean chop. Held in late August, the competition features course options of 500, 1,000, and 2,500 meters, allowing both sprint enthusiasts and endurance swimmers to test their form in a safe, enclosed basin. In past editions, swimmers have experienced an intimate and welcoming atmosphere, marking the inaugural event's place on the regional sporting calendar.

Start and finish at Stensjöns badplats: the 1 km leg

Mölndal Open Water is an open water fitness race in Stensjön, with its start and finish at Stensjöns badplats. The distance of Mölndal Open Water that counts toward the Göteborgsklassikern series is 1 km. Most of the swimmers in the fitness class and in the Göteborgsklassikern swim breaststroke. A swim cap is compulsory at Mölndal Open Water, and wetsuit requirements follow the rules of the Swedish Swimming Federation. Completing the Göteborgsklassikern means swimming Mölndal Open Water in succession with the Hisingen Runt and Finalloppet races.

Stensjön and Rådasjön: interconnected lake waters and nature reserve

The swim course across Lake Stensjön covers a nutrient-rich freshwater basin of roughly 45 hectares, situated at an altitude of 49.5 meters above sea level and descending to a maximum depth of 11 meters. Swimmers encounter conditions where sighting buoys and holding a steady rhythm feel remarkably effortless. Water continually replenishes the lake through a channel known as Ståloppet, which draws directly from the pristine reservoir of Rådasjön. This neighboring lake is an essential natural asset for the community, functioning as the primary drinking water source for the municipality of Mölndal. Covering a sprawling surface area of 1.92 square kilometers at an elevation of 49.4 meters above sea level, Rådasjön drops to maximum depths of 23 meters.

The Mölndalsån watercourse drains Rådasjön, flowing through it from east to west before carrying its clean outflow downstream toward Stensjön. To safeguard the purity of this vital drinking supply, authorities established the entire waterbody and its perimeter as the Rådasjön Nature Reserve in January 2006.

From Södra Långevattnet to Mölndalsån: the forested watershed

Stensjön's purity is further bolstered by upland sources that cascade down from higher woodland elevations. The basin receives clear water from Södra Långevattnet, an elevated highland lake nestled within rugged mountains and forested ridges at 94.6 meters above sea level. Covering an area of 0.0983 square kilometers with depths reaching 13.5 meters, this pristine tarn collects woodland precipitation before draining northwards toward Stensjön and the Mölndalsån. This highland tarn forms part of Långevatten in Mölndal municipality, lying within the traditional province of Västergötland. The entire interconnected drainage network belongs to the primary catchment area of the Göta älv river system. Water leaving these rocky ridges travels through three distinct watercourses across a journey of 17 kilometers before finally reaching the open sea.

Downstream from the swim venue, Stensjön is drained by the Gullbergsån and the Mölndalsån river. The Mölndalsån flows steadily northward through the municipality, eventually emptying its fresh discharge directly into the urban canals of Gothenburg and the waters of Gullbergsån. This surrounding watershed in Västergötland also comprises a network of 13 interconnected lakes centered around Stora and Lilla Delsjön.

The Mölndalsån rapids and the historic Gunnebo bridge

Beginning in the 17th century, local builders harnessed the rushing rapids of the Mölndalsån to drive mechanical watermills. The constant availability of flowing water gave industries such as paper manufacturing, oil milling, and textile weaving an invaluable natural advantage. At the height of this industrial surge, more than sixty historical mills were recorded by name along the banks of the roaring rapids. The relentless turning of wooden wheels defined daily life along the waterway for generations until milling operations ceased altogether in 1930. Today, the roar of heavy machinery has given way to quiet river paths, where visitors can trace centuries of regional engineering alongside the water. Remnants of this heritage remain preserved along the perimeter of the lake, including an old crossing over the Gunnebo stream outlet that was charted on historical maps as far back as 1725.

Stensjöparken promenade: wintering swans and noble crayfish

Thanks to the continuous water flow passing through the channel at Gunnebo bridge, the stream outlet remains open and ice-free even in the coldest months of winter. This microclimate creates an oasis for Scandinavian wildlife, attracting hundreds of mallards, Canada geese, and various waterbird species throughout the winter season. Remarkably, since the 1930s, approximately one hundred migrating whooper swans have gathered each year to winter in these sheltered, open waters. Beneath the lake surface, Stensjön sustains a healthy aquatic fauna that thrives in its nutrient-rich environment. The waters support a population of native noble crayfish, known locally as flodkräfta. Looking down through the lake shallows, open water swimmers can appreciate the vibrant freshwater ecosystem flourishing beneath their strokes.

The event center at Stensjön Strandpromenaden sits within the green parkland of Stensjöparken, featuring wooden bathing piers and a small sandy beach described by visitors as a scenic natural retreat. Along the shore, well-kept walking paths loop around the water, while calm bays provide opportunities for recreational boating. Standing on the piers after completing your swim, with views stretching across the peaceful tree-lined waters, you take in the quiet harmony of Lake Stensjön.