Post-glacial history of lake Näsijärvi, Finland, and the origin of the Tammerkoski Rapids
Lake Näsijärvi is a body of water of 265 km2 in the northern part of the Kokemäenjoki watercourse, central Finland. The lake was isolated from the Baltic Sea during the Ancylus Lake. The effect of post‑glacial land uplift on the lake after its isolation and possible shifts in the outlet of the lake complex were studied by means of levellings of the old shore marks, by sedimentological analyses, and by radiocarbon datings.
The threshold of the present northern watershed at Sapsalampi lies at about 115 m a.s.l. Old shore marks above this altitude were discovered in the northern part of the lake complex. In the south the old shore lies at an altitude of 97 m a.s.l. and the gradient of this ancient shoreline is 0.21 m/km. Thus, the lake first drained over the northern threshold to the north. This is further indicated by the presence of a partly dry channel from the northern threshold to the River Lapuanjoki valley. As a result of land uplift the basin was tilted and a new outlet channel, the Tammerkoski Rapids, opened to the south, in the centre of the present city of Tampere, about 6,700 14C yr BP (ca. 7,500 cal yr BP). The opening of the new outlet led to a lowering of Lake Näsijärvi's water level by about two metres. Later the land tilting resulted in the isolation of the norther basins from Lake Näsijärvi proper.