THEMES ARISING FROM PALEOHYDROLOGICAL RESEARCH IN HIGH LATITUDES AND THE TEMPERATE ZONE

The most extensive continental areas of the northern hemisphere drained by large river basins are located in the temperate zone and in high latitudes. These areas experienced substantial change in water circulation at the turn from the last cold stage to the Holocene. During the cold stage water storage occurred in large ice sheets as well as in the foreland areas together with permafrost and seasonal storage in snow in areas of continental climate. Depending on the pattern of drainage systems large ice dammed lakes were formed.

During the Lateglacial and the early Holocene major transformations in water storage and drainage pattern occurred. Rapidly retreating ice sheets and permafrost left new topography over which lake basins with surficial storage were developed, then being subsequently modified with the gradual formation of a new drainage pattern, expansion of bogs and overgrowing of lakes. In continental climate areas permafrost still dominates. With extension of forest communities rivers changed their regime and their channels shifted from a braided to a meandering pattern.

The cooler and wetter phases, clearly evident during the Holocene coinciding with decreases in solar activity and with high frequencies of volcanic eruptions, are characterised by higher frequencies of extreme floods and their clustering. Rivers responded to these conditions by straightening and braiding of their channels, especially in mountain headwater catchments, where lowering of vertical climatic vegetation belts is also observed.

Special attention should be concentrated on human intervention, which initially caused an acceleration of runoff, increase of sediment load and aggradation, but later in the last century various human activities directly provoked opposite tendencies.