Wpływ wydin i procesów eolicznych na kształtowanie się dolin rzek nizinnych u schyłku plejstocenu i w holocenie

Abstract

Przedstawiono pogląd na rolę wydm i procesów eolicznych w kształtowaniu się dolin Wisły, Bugu, Narwi. Wieprza, Liwca i , Świdra od schyłku plejstocenu do chwili obecnej. Z udziałem wydm i procesów eolicznych autor wiąże lokalne modyfikacje sposobu rozwinięcia koryta rzecznego .oraz aluwiów. Są one tym bardziej wyraźne. im mniejszy jest ciek. Przedstawiono zespół czynników wskazujących. że wydmy i procesy eoliczne generalnie opóźniły proces meandrowania rzek, który w holocenie mógł nastąpić lokalnie ze znacznym opóźnieniem w stosunku do zmian klimatycznych.ON THE INFLUENCE OF DUNES AND EOLIAN PROCESSES ON DEVELOPMENT OF LOWLAND RIVER VALLEYS IN THE LATEST PLEISTOCENE AND HOLOCENEThe studies showed that eolian processes were acting more intensely and longer in lowland river valleys than in highlands. This phenomenon may be explained by a delay in invasion of forests in river valleys in relation to highlands. In valleys, development of forest cover was primarily impeded by unfavourable conditions prevailing in sandy Pleistocene terraces and water regime. The water regime was there disadvantageous because of close water connections between terrace and river, large oscillaiions in water level in annual cycles and resulting numerous rises.The influence of dunes on development of a valley was varying, depending on size of river and amount of eolian material present in the valley. The mode of occurrence of eolian deposits in valley may cast some light on the course of processes responsible for its development (Fig. 5). The mechanism of influence of dunes and eolian processes on the course of sedimentation and erosion of alluvia was a follows :1. Direct supply of eolian material to river channel was resulting in increase of load transported by river. The amount of material nowadays brought to river valleys by winds equals 15,000 t per 1 km2 at the average, rising to about 50,000 t per 1 km2 under particularly advantageous conditions. Quantitative estimations of morphological effects attributable to winds (Figs. 1-4) gave further support to the above .estimations based on measurements. of amounts of present-day material deposited by winds.2. The extent of lateral river channel changes was limited by supply of eolian material coming from erosion of dune covered terraces. This suggests that the width of modelled valley was reversely proportional to. the number of dunes in its area.3. The entrace of dunes into river was resulting in local rise of water level and, therefore, change in fluviodynamic processes.4. Dunes occurring within the range of water rises were bearing some influence on flow as well as the course of sedimentation and erosion in time of a rise (Fig. 7). Vast areas of sedimentation of organic soils, i.e. intra-dune swamps, were originating in places where distribution of dunes impeded outflow of rise water. Dunes were also rising groundwater table and under specific setting, i.e. when dune lines were forming natural groundwater-table-rising steps, they resulted in swampy character of river valley on its ·whole width.5. A specific type of flood-facies deposits, characterized by alternation of sands and silts (Fig. 6) is connected with dunes. The presence of such deposits in geological record indicates contribution of eolian processes to development of a valley.The above discussed set of agents is responsible for delay in development of river in relation to climatic changes. The delay in transition of rivers from braided-channel phase to the meandering was varying, depending on size of a river. Generally, the smaller the river, the large! the delay. Modification of geological structure of terraces by dunes and eolian processes could be so lalrge that it becomes possible to differentiate subtype of river controlled by eolian processes with reference to morphological features and the nature of sedimentation.