History of the Puula Lake Complex, Central Finland, and shifts in its outlet

Abstract

Events associated with shifts in the outlet of a complex of large lakes in Central Finland are studied by means of levellings of ancient shorelines, trend surface analysis, pollen analysis and radiocarbon dating.

The area emerged from beneath the ice of the last glaciation around 10 250 - 10 150 BP and was isolated from the Baltic during the Ancylus Lake stage, around 9200 BP. During the early history of the lake complex, 9200 - 5800 BP, referred to here as the Early Puula and Tammipuula stages, its waters discharged to the north-west via the River Tammijoki and for the last 2000 years of this time, in a bifurcated system, westwards via the River Vanjoki as well.  This latter direction of flow served as the only outlet in the interval 5800 - 4500 BP (6600 - 5100 cal BP), during which time the transgressive Vannipuula stage reached its maximum extent. This stage ended with the opening up of a third outlet channel, Tainionvirta, around 4500 BP.

The transgressive phases in the history of the lake complex and the drops in water level associated with the opening of new outlet channels have had a marked effect in regulating peat accumulation in the mires of the littoral zone. On the other hand, the rapidly growing mires have also served to retard the creation of new outlet channels and to prevent the passage of water towards the lowest points in the terrain at both Koskipää in the commune of Hartola and Kissakoski in Hirvensalmi. Finally the whole watercourse was divided in two by the artificial lowering of the water level in Lake Puulavesi in 1854. No evidence exists, however, for the change in outlet in the Pertunmaa area described previously as having taken place around 5000 BP.