(9) The origins of the Tainionvirta
The river Tainionvirta is not the first link of lake Jääsjärvi to the ocean. During the Ice Age, about 20,000 years ago, Hartola, like other parts of Finland, was covered by a layer of ice about two kilometers thick. When the ice left the area, melt water covered the land (the Baltic Ice Lake and the Yoldian Sea). About 11,000 years ago Hartola started to emerge from under the water on the shore of the Yoldian Sea. As the land, which had been pressed down by the ice, began rising, the link of the sea to the ocean was broken and the Ancylus Lake was formed (about 10,600 years ago). The large lake of the ancient Päijänne was formed, with present-day Jääsjärvi a part of it.
It discharged to the north, approximately along the route of the present-day Kalajoki river to the present Gulf of Bothnia. As the land rose further, this link was broken again, and about 7,000 years ago the waters of Päijänne broke a route to the south, through Kymijoki to the Gulf of Finland. Jääsjärvi became a separate lake. First it discharged to Päijänne via Tammijärvi lake in Luhanka, then via Rautavesi and Vanjärvi lakes to the middle of the present-day Tainionvirta. About 6,500–6,000 years ago the waters of Jääsjärvi broke the Koskipää isthmus and the present-day Tainionvirta came into being. It is estimated that the present-day Ekonkoski rapid was earlier a sound of the lake and the isthmus and the breaking point was just below the rapid.
The ancient history of the shores of Jääsjärvi and the Tainionvirta have been studied by the poet and geologist Aaro Hellaakoski.
upper part of the Tainionvirta, then called the Koskipää River, in 1818
In the middle of the 19th century the river was cleared in order to win more land for clayey fields and to enable timber drafting. At that time the surface of Jääsjärvi was lowered by 1.5 meters. Before that, the present-day headland where the Koskenniska bridge is located, was an island (See map below.). The modern brook leading from the neck of the river to Lokkilammikko pond and to the Korentosilta bridge actually follows the old route of the river. The brook has been cleared and does not maintain its natural form.
On the shore of Jääsjärvi, at the end of the Aurinkoranta road and along Kiiskentie road you can see old banks dating from the period before the Tainionvirta broke the Koskipää isthmus. Before the present houses were built archeological excavations found ancient camping sites with the bases of fire pits and arrowheads.