Paleochannels and recent changes in the Pulmankijoki valley, northern Lapland

Abstract

The age of peat on the bottom of palaeochannels (oxbow-lakes) was determined with C14-dating for getting a general view of the former development of the valley. The sea ratreated from the valley as a result of land uplift. During Tapes I phase, about 7000 B.P., the mouth of the river was slightly north of the Äyskäriniemi area.

The lowest boggy oxbow-lakes are situated at a level of 17-22 m and the age of deepest peat horizons varies there from modern to 600 years. Several dating results at a level of 28-34 m are a little more than 2000 years. It is evident that in Lapland before our Christian era there prevailed a rainy and cool climatological phase which favored bog formation. The oldest dated peat stratum was 4410(80 years at a level of 36 m. The height of the palaeochannel explains the age of the peat very significantly (r=0.854).

It was found out that the cut-off of the meander-loop at Äyskäriniemi happened 1980 as a result of "normal" river erosion. Since the cut-off a channel fill extending 130 cm above the summer time river surface has developed, mostly as a result of a back current. The channel fill has four separate strata deposited in different years. Sediments 0.5 cm thick on average are annually deposited in the middle parts of the new 1.0-1.3 m deep oxbow-lake. On the channel fill four species of plants had become established by 1988 and 12 species in 1989. Over the years 1961-1985 the channel in the river delta moved over a distance of 300 m.