Environmental and Human Impact on Gully Erosion Recorded in Fan Sediments: A Case Study of the Vilnius area, Lithuania

In temperate climates the activation of slope erosion is usually associated with the deforestation of an area and the beginning of agriculture (Bork 1989; Lang 2003; Starkel 2005; Dotterweich 2008; Dreibrodt et al. 2010, 2013; Fuchs et al. 2010; Dotterweich et al. 2012). In some places, gully erosion began as early as the Neolithic period, especially in loess areas (Śnieszko 1995; Bork 1989; Dotterweich 2008; Szwarczewski 2009; Dreibrodt et al. 2013), and continued with varying intensity over later phases; this would have been also related to the local history of settlements (e.g. Vanwalleghem et al. 2006; Dotterweich 2013; Superson at al. 2016). Research carried out on the Russian Plain indicates that the older phases were climatically determined, while younger ones were associated with deforestation and the occupation of land for cultivation (Panin et al. 2009; Belyaev et al. 2005, 2020; Piech 2021). A similar sequence occurs in the Suwałki lake district in NE Poland (Smolska 2007).

Gully fan deposits are also a valuable geoarchive—a source of information about the phases of human economic activity in the past (e.g. Reiß et al. 2009; Küster et al. 2011).

In post-glacial areas, the oldest gullies occur in the central part of northern Poland, i.e., in Kujawy (Sinkiewicz 1989; Jaworski 2018; Kittel 2015). In this region, during the Neolithic period, areas of cultivation occupied small areas, but, according to Sinkiewicz (1989), the cleared parts of forests, which was done mainly for grazing, contributed to increased surface runoff and the initiation of gully erosion from about 4500 BP. In many regions of Poland, this process started later—in the Bronze or Iron Age (Twardy 2008; 2011, Majewski 2014; Majewski & Paluszkiewicz 2019) or even in the Middle Ages (Smolska & Szwarczewski 2014; Szwarczewski et al. 2020).

In Lithuania, there are, locally, more gullies, including on the slopes of the Vilnia valley in Vilnius, South Lithuania (Fig. 1AB). So far, no studies of gully erosion have been carried out in this area. The aim of this research was the identification of the natural and anthropogenic conditions for the development of the gullies, and their age in the relation to the course of the processes taking place in the post-glacial areas in Central and Eastern Europe. This goal was achieved through the following research tasks: (1) morphological analysis of the cuts which occur on the slopes of the Vilnia valley near Vilnius and the fans at their outlets; (2) identification of the sediments which form the gully fans, and the lithological and sedimentological features of these sediments using the example of two selected forms; (3) determining the age of the selected forms based on radiocarbon dating, and (4) describing the stages of development of the selected forms based on the vertical diversity of the sediments which build the fans.

Figure 1.

Vilnius is located on the Neris river, at the mouth of the Vilnia tributary. The moraine plateau, mostly undulating, is located at an altitude of 160–190 m above sea level. Some individual moraine hills reach over 200 m above sea level. Along the Vilnia valley (east of Vilnius) there is a high fluvioglacial level (150–155 m above sea level) and river terraces: the highest terrace is 120–122.5 m above sea level and the flood plain is 115 m above sea level (Goubyte 2012; Petrošius et al. 2018) (Fig. 1C). The study area was located beyond the border of the last glaciation, in the Medeninkai (Saalian, Wartanian) glaciation zone (Goubyte 2012).

In the lower part of the Vilnia river, the valley is cut into the fluvioglacial level to a depth of some 30 m. Locally, at the junction of the valley and the surrounding moraine plateau, the elevations can reach over 50 m. The slopes of the valley have a significant inclination ranging from about 12 degrees to over 20 degrees. These are dissected by gullies and by dry erosional and denudation valleys of periglacial origin. There are 23 gullies on the northern slopes of the river valley between Vilnius and Naujoji Vilnia (Fig 1B). Five of these forms are large and cut through the fluvioglacial level and the moraine plateau, while the others only cut through the fluvioglacial level. Many of the existing gullies face towards the riverbed and no fans have been preserved at their outlets. Two gullies with clear fans at their outlets near Leoniškės were selected for detailed study. These fans are crossed by Belmonto street (Fig. 1C). All these forms occur in the Pavilniai Regional Park (Petrošius et al. 2018). The gullies selected for research are characteristic forms for the slopes of the lower Vilnia valley.

The first traces of human presence in the studied area come from the Mesolithic period. However, these are isolated stone artefacts found in the area of today's city. In the early Middle Ages (6th century AD) there was a settlement here, indicated by a cultural layer within the Vilnius Lower Castle (Stančikaitė et al. 2008). Since then, the human impact on the natural environment has been systematically increasing due to the clearing of forests and continued grain cultivation. Intensive and organized settlement began at the turn of the thirteenth and fourteenth centuries (Stančikaitė et al. 2008). In the past economic and territorial development of Vilnius, several phases can be distinguished, including an initial phase related to the establishment of the city, and the late nineteenth century and the interwar period when the city significantly expanded territorially. The 1950s–1980s period was also important, as numerous industrial plants were built in the city, the population increased, and new, large housing estates were built (http://www.truelithuania.com/history-of-vilnius-634).

On the basis of the digital elevation model (DEM), longitudinal and transverse profiles for two selected gullies were made. The volume of the gullies and the volume of the alluvial-deluvial fans at their outlets were estimated using a geometric method according to previous work (e.g., Govers & Poesen 1988; Klimaczak 1988). Based on the DEM, characteristic sections were determined (Fig. 2) in terms of their cross-section, and the depth and the width of the gullies were measured. In the case of the larger gully, the depth below the slope bend was taken into account, which indicated the difference in inclination between the gully and the former slope of the older erosional-denudation valley. The cross-sectional area (triangular area) was related to the length of the gully section represented by this area. The sum of the volumes of individual fragments constituted the entire volume of the gully. A similar procedure was used for the fans. Their area was determined based on the DEM, and the average depth was determined based on drillings.

Figure 2.

The deposits which built the fans were identified on the basis of manual drilling with an Eijkelkamp auger. Drillings were performed in the proximal, middle, and distal parts of both selected fans (Fig. 1D). The samples were taken from each macroscopically visible series for grain size analysis. Sediment grain size was determined using the sieve method (Mycielska-Dowgiałło 1995) and the settling tube method, i.e., the Casagrande method, modified by Prószyński (Myślińska 2001). The organic matter content was determined by loss on ignition in a muffle furnace at a temperature of 550 °C for four hours (Myślińska 2001). The sedimentological indices: mean grain diameter (Mz), standard deviation as sorting (σ₁), skewness (Sk₁), and kurtosis (K_G) were calculated according to Folk & Ward (1957).

The relationship between the grain size indices (Mz, σ₁) allows conclusions to be made about the processes which were responsible for the deposition of sediments (Folk & Ward 1957, Mycielska-Dowgiałło 1995; Mycielska-Dowgiałło & Ludwikowska-Kędzia 2011; Szmańda 2011).

Selected sediments containing greater amounts of organic matter (peat, fossil soils) were radiocarbon dated in the Radiocarbon Laboratory in the Nature Research Centre in Vilnius.

The prevailing opinion is that gully erosion begins with deforestation and the use of these areas for agriculture (e.g., Bork 1989; Starkel 2005). In Central Europe, gully erosion has so far been correlated with settlement phases, which confirms the anthropogenic determinants of this process.

The research carried out in the vicinity of Vilnius (SE Lithuania) also suggests that the environmental changes caused by human impact, namely the development of settlements, are of significant importance. There is a record of relatively minor changes in Lithuania, predominantly changes in forest species, which are mainly found in lake or peat sediments (e.g. Stančikaite et al. 2013). Primitive economies such as hunting and gathering dominated for a long time. It was only changes in land use which resulted in more intensive local soil erosion. A permanent settlement on the site of the Lower Castle in Vilnius existed from around the sixth century AD (Stančikaitė et al. 2008). Evidence of the continuous cultivation of cereals and a clearly marked increase in the landscape being cleared of woodland indicates an increase in human activity in this area during the later thirteenth and early fourteenth centuries AD. The age of the studied sediments (Table 1) corresponds to archaeological and palynological data (Stančikaitė et al. 2008).

The colluvial deposits originating from soil erosion were described in the vicinity of the Rudamina stronghold (SW Lithuania), and the beginning of this accumulation was dated to 1410±80 BP (Szwarczewski et al. 2008). The age of the colluvial sediments from the Sejny lake district in NE Poland is similar (Smolska & Szwarczewski 2008).

Gully sediments in Lithuania have not yet been studied. The absolute datings obtained from fossil soils under the distal deposits of fan I turned out to be imprecise (Table 1). The significant range and error (i.e., standard deviation) in the obtained dating can be interpreted as being from a period when the fossil soil was gradually covered by sediments from gully erosion. The supply of top soil particles to the fan from the gully catchment area, where soil erosion processes began earlier than the gully erosion, should also be taken into account. In such situations previously formed soil could have been subject to redeposition (Zolitschka et al. 2003; Lang 2003). At the bottom of fan I, the humic horizon of both in situ and redeposited soil was dated. A similar situation was recorded in NE Poland (Smolska & Szwarczewski 2016). Therefore, it can be assumed that during the period of the sixth to the thirteenth century AD, the erosion processes began along the bottom of the older, dry valley, but the supply of sediments to the foot of the slope was negligible and soil continued to develop. With the development of settlements at the turn of the thirteenth and fourteenth century, and also due to climatic fluctuations, the valley floor was intensively ‘cut up’ and the gully developed. Such larger gullies are characteristic for the northern slopes of the Vilnia valley. In the lower and middle parts, the older dry valley was completely transformed (Fig. 1C and 2) and a vast fan was formed. The older, dry valleys located in its hinterland undoubtedly played an important role in the development of this large form. Analogous morphological situations have been described from sites in Poland (e.g., Sinkiewicz 1989; Smolska 2007; Twardy 2008, Jaworski 2018). The smaller gully began to develop at a similar time, i.e., during the years 1271–1435 (Table 1) and its fan quickly covered the palaeochannel which was located adjacent to the slope of the Vilnia valley.

Previous studies of sediments from gully erosion have pointed out their similarity to channel alluvium and, in the case of soil redeposition from the surface of deforested catchments, to colluvium (Twardy 2003, 2008; Smolska 2007, 2011; Kittel 2014; Majewski 2014; Piech 2021). The studied fans are dominated by alluvial type deposits (called proluvia in Poland), only the youngest unit, deposited since the seventeenth century, is characterized by increased recharge from soil erosion (deluvial deposits).

The gullies around Vilnius which were studied are diversified in terms of their morphology. Mostly short forms predominate and occur on the slopes of the Vilnia valley; while larger forms developed only along older, dry valleys.

The creation of the gullies was the result of land use changes related to the intensive settlement of the Vilnius area in the Middle Ages. Their further development was due to climate fluctuations and the intensity of human economic activity in the past.

The dating of the fossil soil level gave an inaccurate date (±200 years), which indicates the problems associated with dating buried soils, especially humus accumulation levels containing in situ soil and redeposited older soils from gully catchments.

The climatic conditions (Little Ice Age) caused the rapid development of the gullies and smaller deposition within the fans. An analysis of sedimentological indices (Mz, σ₁, Sk₁, and K_G), the proportion of admixtures of the coarsest and finest fractions (gravel and clay), and organic matter content allowed three lithological units to be distinguished, which corresponded to the stages of gully development relating, accordingly, to medieval and early modern periods, and one from the seventeenth century. Until the seventeenth century, sediments from gullying predominated in the construction of the fans, and later, colluvial (deluvial) deposits from soil erosion from gully catchments.