Holocene development and present hydrology of Lake Pyhäjärvi in Satakunta, southwestern Finland

Abstract

Lake Pyhäjärvi is located on Precambrian sandstone bedrock which has been preserved from erosion on account of its situation in a graben forma­tion. Its waters are retained by a diabase threshold in the northwest and an esker chain in the northeast. The Holocene development of the lake was studied by means of stratigraphical and morphological investigations supported by pollen and diatom analyses and radiocarbon datings.

The whole of the Pyhäjärvi area lay beneath the waters of the Baltic upon deglaciation around 9800–9700 B.P., the depth of water at the site of the lake itself being over 100 m. The rapid rate of land uplift compared with that found nowadays reduced this depth considerably in the course of the Yoldia and Ancylus Lake stages, but isolation of the basin to form an independent lake took place only during the Litorina Sea stage, approx. 5600 radiocarbon years B.P.

Since the outflow from the lake is at its northwestern end, where land uplift is most pronounced, the lake has been transgressive throughout its post‑isolation history. This caused a rise of some 3 m in the water level at the southeastern end in the space of about 5500 (radiocarbon) years, and some peat deposits which formed on the shore were later inundated by the waters of the lake. This natural development was interrupted in 1852–53, when the water level was lower6d artificially by almost two metres.

The now regulated lake of Pyhäjärvi accounts for as much as 25 % of the area of its own drainage basin, and partly for this reason evaporation exceeds the combined figure for inflow and direct precipitation in June and July. If some of the water were to be taken for domestic water supplies, as is projected, this would quite obviously lead to repeated infringements of the agreed limits for water level fluctuation, especially during dry periods.