History

Siltasaari is part of the Heinolanharju ridge, extending north–south, formed by the Ice Age. Water masses from the ancient lake Päijänne fractured the embankment some 6,100 years ago, forcing a channel to form for the Jyrängönkoski rapids toward the south-east, where it remains today.

The upper reaches of the stream, along with its narrowest part, were used for crossing the river already in prehistoric...