Origins and development of the ancient outflow channel of the river Vantaanjoki, southern Finland, as indicated by fluvial sediments

Abstract

The city of Helsinki was founded in 1550 at the mouth of the River Vantaanjoki on the Gulf of Finland. There is a clearly distinguishable overgrown river bed located to the west of the present channel which would seem far too large for the minor stream flowing through it nowadays. The aim here was to determine whether this stream and its valley do in fact represent an ancient channel of the larger river, for how long this channel functioned and how and when it ceased to do so. The problem was approached by means of morphological surveys, corings, stratigraphic analyses and radiocarbon datings.

It may be concluded from the shoreline displacement data and the topography of the area that the threshold of the ancient channel emerged from beneath the sea around 4500 BP (5100 cal. BP). Subsequently two shallow lakes developed at approximately the same altitude around 4000–3500 BP (4500–3800 cal. BP), later drying up as the thresholds that retained them were eroded away. A channel some 50–70 m broad, 5–8 m deep and 8 km in length was created in the valley of the River. The channel began to fill rapidly with fluvial sediment around 2200 BP (2200 cal. BP), however, once the present course of the river was opened up over a low glaciofluvial ridge which had previously restrained its waters in the east. As the new channel gained in depth, the flow in the old one came to an end around 2000 BP (1900 cal. BP), by which time substantial quantities of fluvial sediment had accumulated in it.