Late-glacial and post-glacial development of the valleys Oulanka river basin, north-eastern Finland

Abstract

The valleys of the Oulanka river and its tributaries, located in north­ eastern Finland, gained deep deposits of glaciofluvial drift during the last glacial retreat. The principal aim of this study is to date the deposi­tion of these sediments, the prevailing conditions and the landforms created, and to trace post‑glacial changes in the relief of the area. The methods involved mapping of the valley floor landforms, grain‑size and stratigraphical analysis of these deposits, construction of profiles of the valleys and certain landforms by levelling, and sampling of minerogenic and organogenic horizons in the valley floor for pollen and diatom analysis and radiocarbon dating. Recent changes in relief were surveyed over the period 1975‑78. The majority of the glaciofluvial material was deposited proglacially around 9300‑9500 B.P. Initially the valleys themselves were flooded by water most probably having a direct outlet into the White Sea. The subsequent infilling of the valleys up to the prevailing water level then led to the accumulation of supra‑aquatic valley‑train deposits on top of the subaquatic delta formation. The valley‑train delta assumed more or less its present form immediately upon deposition in the lower reaches of the river valleys, but further upstream the final relief only emerged some centuries later, with the melting of the buried dead ice. The major role in the postglacial changes in relief is attributed to fluvial processes, regulated by isostatic land uplift. Downcutting was most rapid in the early post‑glacial period, declining later with the decreasing rate of land uplift. The dominant modern process is lateral erosion, although this is now restricted to only the lower reaches of the Oulanka river, where the meanders of the river are migrating downstream at varying speeds.