Landform Analysis, Vol. 22: 75–87, 2013 doi: http://dx.doi.org/10.12657/landfana.022.006 Formation and development of a 1st-order valley network in postglacial areas (the Dębnica catchment) Małgorzata Mazurek, Renata Paluszkiewicz Institute of Geoecology and Geoinformation, Adam Mickiewicz University in Poznań, Poland, [email protected] Abstract: This article describes the formation and development of the 1st-order valley network in the postglacial catchment of the Dębnica River (NW Poland). Its valley network is an example of a system of polygenetic depressions, and its 1st-order sections are comprised of denudational hollows and valleys, dissections, and erosional-denudational valleys. In the article, both landforms with no permanent flow and those nourished by groundwater and included in the river runoff network are characterised. In the course of their development, dry valleys have continued to grow in width and length, while the gradient of their bottom has decreased due to slope wash processes. In groundwater outflow zones, in turn, the modelling of their slopes indicate re- treat as a result of seepage erosion and mass movements, which has led to the preservation of their convex parts and the development of a concave section at the foot, passing into the flat bottom of an alcove. In headwater alcoves, which mark the start of a valley or are inserted into older valley landforms, morphological contrasts are enhanced because of an increase in their width and depth, as well as changes in their cross-profile. Key words: valley network, erosional-denudational valleys, unchanneled colluvial valley, headwater alcoves, Dębnica catchment Introduction The process of the river network’s integration and development is still in progress, but the main stage took The postglacial relief zone in Poland offers an oppor- place at the close of the Vistulian (Kozarski et al. 1988, tunity for tracing the development patterns of a polyge- Florek 1991, Błaszkiewicz 1998). In that period, an im- netic valley network (Piasecki 1982), largely determined portant role in the organisation of the valley network was by the morphogenesis of postglacial depressions modi- played by meltwater coming from the melting of blocks fied by fluvial and denudational processes controlled by of dead ice and hydrogenic ice, as observed by Florek climate, neotectonics (Florek 1991, Błaszkiewicz 2005, (1991), Andrzejewski (1994) and Błaszkiewicz (2005). Chybiorz 2008) especially in Holocene (Zwoliński et al. The end of subaerial deglaciation also meant the con- 2008), and in the recent centuries, by a rise in multi-di- clusion of the most important stage in the development mensional human intervention (Florek 1991, Szpikowski of the present-day drainage system of Pomerania (Le- 2010). wandowski, Nita 2008), which now includes higher-order A uniform valley system on the northern slope of Po- streams. merania started to form during the recession of inland ice The further evolution of the valley network which in the Pomeranian Stage of the Vistulian Glaciation. The thus formed included the appearance and development of development of the valley network involved a gradual in- 1st-order landforms (using Horton’s hierarchical classifi- tegration of depressions produced by glacial, fluvioglacial cation of river networks), such as denudational hollows, and periglacial processes into a system of river runoff. As erosional dissections and valleys, as well as erosion- a result, within the present-day river valleys located on the al-denudational valleys. The pattern of this lower-order northern slope of Pomerania, including the Parsęta River colluvial valley network (Montgomery, Buffington 1997) Basin, one can find valley sections of various ages and is subsequent to the directions of the trunk valleys de- derivations (Sylwestrzak 1978, Piasecki 1982, Błaszkie- termined by the structure and relief of the older substra- wicz 1998, 2005). Predominant among them are marginal tum. Such landform complexes were identified by Marsz valleys (or pradolinas), running mostly latitudinally, as (1964, 1995), Churska (1965), Szupryczyński (1967), well as longitudinal trough valleys (subglacial troughs) Gołębiewski (1981), Kostrzewski et al. (1997), Florek and glaciofluvial valleys linked by erosional sections with et al. (1999), Smolska (2005), Majewski (2008) and a fluvial genesis (Zwoliński 1989). Paluszkiewicz (2008, 2011). 75 Małgorzata Mazurek, Renata Paluszkiewicz The network of small valleys in Pomerania was initiat- 2002, Błaszkiewicz 2005). The increased depth of rain- ed in the Late Vistulian by wash and solifluction processes water infiltration contributed to the gradual development when there was permafrost in the substratum. The ame- of the local, transitional and regional systems of ground- lioration of climatic conditions and gradual degradation water flow (Petelski, Sadurski 1987) while limiting over- of the permafrost, accompanied by an expansion of forest land flow (Rotnicki 1991). Lewandowski and Nita (2008) vegetation, limited the significance of overland flow in fa- found that the groundwater level had stabilised at the start vour of groundwater flow. The melting of blocks of dead of the Atlantic, while the development of a regional sys- ice and ground ice had morphological consequences that tem of groundwater flow probably brought about spatial were accompanied by changes in the hydrogeological con- stabilisation of the river network, as was the case in NE ditions (see e.g. Nowaczyk 1994, Bajkiewicz-Grabowska Poland (Bajkiewicz-Grabowska 2002). A consequence of A B Ostre Bardo 2 Rudno Ostre Bardo 1 Baltic Sea Rudno 1 Rudno 3 V-22 V-23 Ostre Bardo V-20 V-21 Parsęta V-19 V-18 V-17 Ostre Bardo 4 V-25 V-16 Rudno 2 Ostre Bardo 5 V-24 V-24 Ostre Bardo 6 V-15 V-23 V-25 Ostre Bardo 7 V-14 Kołacz 7 V-17 V-21 V-22 V-26 V-1 1 5 V-20 V-27 V-19 Kołacz 6 2 6 Buślary 2 V-18 3 a 7 V-13 Kołacz 5 4 8 V-2 V 3-16 V-12 V-28 Buślary 1 V-11 Kołacz 4 V-10 Kołacz 3 V-1 Buślarki V-9 V-8 V-7 Lake V-6 V-29 Kołacz Skowrończe Kołacz 2 c V-5 Kołacz V-4 V-30 V-3 Kołacz 1 m a.s.l. V-2 170 V-1 Hills 45 C 0 1 km ogra W Buślarki Hills Dębnica V-1 POŁCZYN ZDRÓJ b V-2 Luboradza 1 V-3 Lake Koprzywno V-5 V-12 V-4 a V-11 V-6 V-7 Lake Dębno Koprzywno 2 V-8 V-9 Lake Koprzywno 1 m a.s.l. Koprzywno 223 41 05 km Lake Kołbackie V-10 Fig. 1. A. Study area in the Dębnica catchment (NW Poland). River network in the Dębnica catchment against its hypsometry, B. Loca- tion of study site: the lower Dębnica River valley, C. Location of study site: the upper Dębnica River valley Headwater alcoves, erosional-denudational hollows and valleys in which topographic measurements and morphological mapping were carried out have been numbered or named 1 – erosional-denudational hollows and valleys, 2 – headwater alcoves, 3 – river network, lake, 4 – groundwater outflows, 5 – town, 6 – watershed of the Dębnica catchment, 7 – limits of sections of river course: a - upper, b – middle, c – lower, 8 – study areas 76 Formation and development of a 1st-order valley network in postglacial areas (the Dębnica catchment) these events was a decline in the number and length of the Parsęta River Basin (Fig. 1), comprising the postgla- streams flowing in small, 1st-order valleys. Some valleys cial geoecosystem of the Drawsko Lakeland situated in located within the aeration zone dropped out of the drain- a wide and morphogenetically diversified marginal zone age network and functioned as intermittent streams only of the Pomeranian Stage of the Vistulian Glaciation, with during high groundwater stages. a Late Glacial and Holocene retouch. The valley network Slope processes intensified in the neo-Holocene (the in the Dębnica catchment is an example of a system of sub-Atlantic) as a result of human activity, and then again sections of various ages and glacial, fluvioglacial, fluvial, in the early Middle Ages and the Little Ice Age (Smol- and erosional-denudational in origin. ska 2005, Majewski 2008). In the scarp zones of uplands and on valley slopes, small, poorly branched erosional valleys developed, while in the upper sections of older Study methods denudational and erosional-denudational valleys there ap- peared badlands and gullies, which grew deeper and more In the research, use was made of the following source elongated. Not all of these valleys are part of the pres- materials: topographic maps at a scale of 1:10,000, geo- ent-day drainage network; only some are used by 1st-or- logical maps at a scale of 1:50,000, hydrographic maps der streams as elements of river runoff. at a scale of 1:50,000, and a Computer Map of Poland’s Some dissections are very young, having developed as Hydrographic Division (KMPH 2004). In the Dębnica a result of deforestation over the last 100–200 years. To- catchment, a detailed geomorphological and hydrolog- day, the formation of new forms of rill erosion, mostly ep- ical mapping of selected 1st-order valley sections was isodic or ephemeral in nature, can be observed on slopes conducted. Because of the size of the valleys and head- used in agricultural. Their development is facilitated water alcoves, as well as the insufficient accuracy of the by morphological conditions, the physical properties of topographic maps, it was necessary to perform direct the soils, sparse vegetation cover, and farming practices topographic measurements of these landforms. The meas- (Kos trzewski et al. 1989). The afforestation of zones with urements were carried out using a GPS receiver and sup- a great amplitude of relief has curbed denudation process- plemented with topographic measurements.