Lahankoski is a briskly roaring rapid, that flows in the River Mustijoki in Pornainen. The rapid begins with a steep, three-metre high waterfall, whereas the latter part of the stream has been utilized for hydropower production. In spite of the power station, the first and highest waterfall in the rapids is still close to its natural state.
The waterfall is formed, when the slowly flowing river pours down a rocky cliff, which extends across the whole width of the stream bed. Before the rapids the river flow is calm. The steepness of the cliff varies along the width of the waterfall, being at best, completely vertical. After the fall, the water to continues its way under the nearby road bridge, and from there to the reservoir of the hydroelectric power plant.
Lahankoski has reportedly got its name from the fact that in the past the river was used often for transporting. As the water level in the rapids was low, the boats had to be portaged along the shore past the waterfalls (portage is laahata in Finnish). Lahankoski has also played an important role in the history of the nearby Laha estate (which is more than 200 years old). On the west side of the first waterfall there has been a sawmill, which was operational until 1912. Below the fall, on the other hand, there has been a watermill of the nearby Kupsenkylä village. The concrete foundations of the mill are still visible in the river. In addition, some remnants of an old log floating flume can still be seen at the river, as log floating was practiced there until the 1950s.
The Laha estate watermill was built in 1892, in the lower part of the rapid, where it served as a small power station in addition to the normal milling activity. Electricity was produced with two generators for the use of the estate itself and the neighbouring houses. Power generation ended in the 1960s, when the turbines broke due to freezing. Lahankoski was then able to flow free until the year 2010, when the hydroelectric plant was renewed, and most of the rapids lost its natural water flow. Happily, however, the highest and steepest waterfall in the rapid was left outside of the hydropower production, even though it is said that at the early stages of the project that was also considered.
Lahankoski can be easily reached by car along the Meijerintie road, and there are several parking places near the rapids. The main waterfall is easy to admire from the wooden road bridge, from where you can see the fall straight ahead. Excluding the driest periods of the year, there is enough water to cover most of the rocky cliff, and the roar of the rapids is an impressive sound. The cultural landscape of the Laha estate around the waterfall is open and idyllic countryside. During the summer, a variety of events are arranged in the area, including exhibitions and film screenings in the renewed mill building. In addition, it is possible to ride or play tennis, if you like to.