Morphodynamic system of mountain headwater areas

Bibliogr. s. 45-49

W opracowaniu przedstawiono stan badań dotyczących lejów źródłowych. Zwrócono szczególną uwagę na odrębne podejście do lejów źródłowych w geomorfologii i hydrologii. Podkreślono ich znaczenie w rozwoju obszarów górskich. Przeprowadzone analizy wykazały, że leje źródłowe powinny być traktowane jak integralne części zlewni górskich, a kompleksowe podejście do lejów źródłowych jest możliwe dzięki stosowaniu metod badawczych z różnych dziedzin naukowych. Wyniki wielu badań pozwoliły wnioskować, że wykształcenie lejów źródłowych bezpośrednio wpływa na dostawę energii i materii do systemu fluwialnego, a tym samym na natężenie procesów hydrologicznych i geomorfologicznych poniżej tych lejów. Rola lejów źródłowych w rozwoju rzeźby jest więc istotna nie tylko z punktu widzenia morfometrii pasm górskich, ale również ze względu na częstość występowania i natężenie różnych procesów morfogenetycznych.

Headwater areas are complex systems affected by a number of morphological, hydrological and biological processes. This complexity is of interest to researchers. Many papers underscore the difference between headwater systems and mountain fluvial systems with respect to hydrology, geomorphology and biology. The differences are rooted in the close relationship between slope processes and river channel processes. In this paper, the headwater area is defined as a depressed landform with older Pliocene and Pleistocene setting, which forms a complex morphological subsystem of a mountain catchment and is located on the boundary between two subsystems – the slope subsystem and the fluvial subsystem. The headwater area develops via the joint action of surface processes and linear processes, which vary in intensity. Headwater areas develop differently, as the action of surface and linear processes is different in each case. The purpose of the paper is to describe the complexity of headwater areas in terms of relief and rates of morphological change as well as the role headwater areas play in mountain area development. The complexity of headwater areas demands an interdisciplinary approach based on a large variety of analytical methods including methods related to geomorphology, sedimentology, hydrology, hydrochemistry, dendrochronology, geodesy, statistics and geographic information systems. Headwater areas are Neogene landforms with sides and a floor – features typical of river valleys. Important parts of a headwater area include an upper edge and a transition zone. The presence of an upper edge suggests the action of mass movements shaping relief. The transition zone is the area between the headwater area and the fluvial system. This is the area where top-acting slope processes interact with bottom-acting fluvial processes. Headwater areas are classified as erosion-type and landslide-type based on the dominant morphological process present. Landslide action alters the longitudinal and transversal profiles of headwater areas. This leads to parts of slopes with unusually large gradients for a middle mountain region. Erosion-type headwater areas retain most of their older relief (Pleistocene, Pliocene). Erosion incisions and generally landforms shaped by the flow of water are very common. Steep slopes and steep sides of incisions are uncommon. Neotectonic activity also affects the formation of headwater areas. Neotectonic lift makes headwater areas deeper. A headwater area is a buffer zone between the slope system and the river channel system. Its relief directly affects the flow of energy and matter to the fluvial system. Slope processes and fluvial processes overlap in first-order and second-order stream channels depending on just how developed a given valley is. Research on buffer zone relief has shown that a gradient threshold value can be identified for areas homogeneous in terms of relief and geological structure. This value is important from the perspective of clastic material reaching the fluvial system. Clastic material is transported via slope processes such as sliding, torrential flow as well as debris and mud flow. The deposition of transported material usually occurs in the buffer zone. The development of a flood wave in higher-order streams, and consequently the energy in a river channel system, depends on the morphometry of the given headwater area. One of the better units of measurement of the energy in the transition zone between the headwater area and the fluvial system is unit stream power, which can be expressed as the relationship between headwater surface area, mean headwater area gradient, and stream channel width in the transition zone. Unit stream power was calculated for 108 headwater areas located in the flysch Outer Carpathians. Unit stream power increases with headwater surface area. The location of headwater areas affects the morphometry of the entire mountain range. Existing research has suggested that mountain passes are extensions of valleys and erosion incisions and are the result of back erosion acting over a very long period of time. This is especially true if erosion incisions on two sides of a ridge point to the same location on the ridge. The authors of this paper, on the other hand, believe that valleys and passes follow tectonic lines and do not result from intense backward erosion. Analysis of headwater areas in the Gorce, Beskid Sądecki and Bieszczady mountain ranges suggests that their relief is connected to geological structure.The presence of numerous and expansive headwater areas fragments mountain slopes and alters their longitudinal profile. Slopes become depressed in headwater areas and the top parts of slopes become steeper due to headwater areas becoming deeper. The development of headwater areas via sliding and creeping moves the tops of ridges further back. In effect, the headwater area assumes a convex-concave longitudinal profile. Headwater areas without a clear edge line tend to assume a concave longitudinal profile. Natural processes acting on headwater areas also alter the transversal profiles of slopes. The relief of the transversal profile is linked to older parts of headwater areas. These landforms tend to resemble wide and shallow valleys, which are often filled with colluvia at the bottom. At lower elevations, headwater areas become flat valleys filled with colluvia. In some cases, headwater areas become V-shaped valleys. The role played by headwater areas in relief development is relevant from the perspective of mountain range morphometry as well as in terms of the frequency and intensity of morphological processes. Headwater area relief affects natural processes further downstream. This is the direct result of mineral matter transport conditions in the headwater area.