Małgorzata Mazurek

Abstract The ways in which the valley and channel of a river are initiated are closely connected with paths water circulation within a slope system takes, with groundwater as an important morphogenetic factor in the development of those landforms. Being the factor that initiates processes of headward erosion, groundwater outflows have also been recognised since the 1980s as a factor forming relief elements (e.g. Laity, 1983; Laity and Malin, 1985; Howard and McLane, 1988; Baker, 1990; Dunne, 1990; Parker and Higgins, 1990; Nash, 1997; Lamb et al., 2006). The chief goal of the work detailed in this article has thus been to survey research conducted to date on seepage erosion and its role in the development of the headwater sections of river valleys. This has entailed the identification of areas in which seepage erosion has been studied, in the wider world and in Poland. The current state of knowledge on the contribution groundwater outflows make to the formation of a drainage system is also presented. Seepage erosion (Dunne, 1990; Lamb et al., 2006) is a process embracing mechanical and chemical action leading to the loosening, tearing off and carrying away of material from the zone of groundwater exfiltration. One result of groundwater sapping is the development of erosional undercuttings, which undermines the stability of slopes and causes their destruction via mass movement (Higgins, 1984; Laity and Malin, 1985; Baker et al., 1990). As a result of headward erosion, an area of groundwater outflow comes to be associated with a developing spring-head alcove, or an arcuate depression, often with steep slopes separated from the slopes of the initial depression by a distinct knickpoint. Together with slope and fluvial processes, seepage erosion contributes significantly to the development of valley forms in various morphoclimatic zones, including the temperate. The morphology of zones of groundwater outflows in Poland has been examined in the course of spring-hydrological and geomorphological studies. For example, the upper Parsęta basin features 88 river channels identified as having morphological features characteristic of an erosional effect of groundwater outflows (Mazurek, 2010). The spring-head alcoves predominating here are arcuate or paraboloid in shape and poorly branched (Photo 1; Mazurek, 2006, 2010; Mazurek and Paluszkiewicz, 2013). Reliefforming processes found to occur in these are: 1. seepage erosion, 2. mass movement, 3. wash, 4. geochemical processes, 5. biogenic processes, and 6. human impact (Mazurek, 2010, Plate 2, Fig. 1). The slopes of the alcoves develop by progressive headward retreat as a result of repeated episodes of sub-slope seepage erosion and gravity-induced mass movement. This sequence of processes keeps alcove slopes steep and leads to the formation of a concave section at the foot of the slope that passes into a flat erosional bottom. Water exfiltrating onto the alcove bottom under hydrostatic pressure washes out its sediments, thus deepening it uniformly, as is indicated by small differences in the bottom gradient. The share of seepage erosion in the formation of river valleys is still a topical research problem. There has been too little field research into relief-forming effects of groundwater outflows and their interaction with other morphogenetic processes that would corroborate the computer simulations and laboratory experiments conducted. There are also too few data about the intensity of the effects of seepage erosion and the rate of development of spring-head alcoves. Today great opportunities are opened up by the appearance of new research methods and techniques (like geotechnical studies, aerial and land-based laser scanning, and hydrogeological modelling), which allow for a quantitative assessment of headwater processes and the determination of their significance in the development of relief under conditions of advancing change in the climate and water cycle.

Małgorzata Mazurek

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