1.3. The Prespa Lakes: hydro-geological and climatic context

In order to answer the question of how current wetland management in Greek Prespa will be influenced by future climate change, the first step is to look at past reactions of the lake-wetland system to climate change. 

The geomorphology of Prespa

The Prespa Lakes occupy a tectonic basin, which is subsiding, surrounded by mountains. The lakes are bordered by impermeable granitic rocks to the north and east. Rainfall on the northern and eastern mountains triggers a lot of surface run-off, erosion and sediment transport. Where the numerous small rivers exit the mountain front, they have created widespread alluvial plains with wetlands, which border the northern and eastern sides of the Prespa Lakes.

To the west of the basin, there are limestone mountains containing extensive cave systems created by the dissolution of the rock by water. Most rainwater falling on these mountains disappears into these underground systems. The few rivers on the western side of the lake don’t carry much sediment at all, as the limestone rock dissolves and only contains limited  amounts of clay, silt, and sand that can subsequently be transported and deposited. Therefore, this side of the lake is characterised by only small alluvial plains and wetlands in-between the limestone cliffs. On the western side of Great Prespa Lake, the water is in direct contact with the limestone, and lake water from Great Prespa flows through cave systems to Lake Ohrid, which is 160m lower.

All the discharge into the lake from groundwater and rivers is created by precipitation in the catchment area. Annual lake level fluctuations are strongly related to total precipitation over the wet season (October to April). The dominant northerly and southerly winds create substantial wave action in Great Prespa Lake, and clockwise water currents re-distribute the incoming river sediments around the lake shorelines. Wave action is much more limited in Lesser Prespa Lake and river sediments remain near the river mouths there.

The alluvial isthmus separating Great and Lesser Prespa lakes has been formed by sediments brought down by the Agios Germanos River. These sediments were re-distributed by wave action along the coastline of the isthmus. Over the past 10,000 years, wave action, sediment supply and water level fluctuations interacted to form the isthmus. It is underlain by horizontally and vertically accumulated beach deposits; the surface displays ridges and depressions that are parallel to the shoreline. Some of these depressions are very deep and wide, and filled by water, such as the Vromolimni lakes, which are an important bird-nesting site.

Lake water levels in Prespa

The water level of the Prespa lakes has fluctuated by up to 10m in the past. Up to the 1950s, the hills surrounding the catchment were, for the greater part, bare due to intensive land use, largely grazing by sheep and goat flocks, leading to high rates of erosion and sediment transport by rivers, as well as much surface run-off. Decreasing land-use pressure on the higher mountains since the 1950s has resulted in re-forestation and/or re-vegetation and a decrease in erosion, sediment transport and surface run-off. 

Water level fluctuations used to be the same in both lakes, as they were connected by a natural channel through the isthmus. A sluice was constructed in this channel during the 1980s, preventing the free outflow of lake water. The water level of Great Prespa Lake fell abruptly by 5-6 m between 1987-1995, during a regional drought episode, and has declined ever since. 

The long-term falling water level trend is a function of climate change and water abstraction. There has been a progressive increase in water abstraction from the lake catchment area since the 1950s, mainly for irrigated agriculture in the form of intensive bean and apple cultivation in Greece and North Macedonia respectively, but also for urban use. Without such water abstraction, the water level of Great Prespa Lake would likely be at least 2m higher at present. Lesser Prespa Lake is now around 6-8m higher than Great Prespa Lake. This height difference causes a considerable underground water flow through the permeable isthmus, from the smaller and higher lake into the lower and larger Great Prespa Lake.

Concluding remarks

In summary, changes in river sediment transport and water flow/input are strongly affected by land use and climate variability. In turn, these changes and lake water management impact upon lake level variability, water flow between the lakes, and shoreline erosion and deposition patterns. 

Further reading 

van der Schriek, T. and Giannakopoulos, C. (2017), Reconstructing absolute water level variability of Lake Prespa (SW Balkans) during the Medieval Climate Anomaly and Little Ice Age from a prograding beach ridge complex. The Holocene 27 (11): 1720-1730. DOI 10.1177/0959683617708449