Formation Patterns of Mediterranean High-Mountain Water-Bodies in Sierra-Nevada, SE Spain

At present, there is a lack of detailed understanding on how the factors converging on water variables from mountain areas modify the quantity and quality of their watercourses, which are features determining these areas’ hydrological contribution to downstream regions. In order to remedy this situation to some extent, we studied the water-bodies of the western sector of the Sierra Nevada massif (Spain). Since thaw is a necessary but not sufficient contributor to the formation of these fragile water-bodies, we carried out field visits to identify their number, size and spatial distribution as well as their different modelling processes. The best-defined water-bodies were the result of glacial processes, such as overdeepening and moraine dams. These water-bodies are the highest in the massif (2918 m mean altitude), the largest and the deepest, making up 72% of the total. Another group is formed by hillside instability phenomena, which are very dynamic and are related to a variety of processes. The resulting water-bodies are irregular and located at lower altitudes (2842 m mean altitude), representing 25% of the total. The third group is the smallest (3%), with one subgroup formed by anthropic causes and another formed from unknown origin. It has recently been found that the Mediterranean and Atlantic watersheds of this massif are somewhat paradoxical in behaviour, since, despite its higher xericity, the Mediterranean watershed generally has higher water contents than the Atlantic. The overall cause of these discrepancies between watersheds is not connected to their formation processes. However, we found that the classification of water volumes by the manners of formation of their water-bodies is not coherent with the associated green fringes because of the anomalous behaviour of the water-bodies formed by moraine dams. This discrepancy is largely due to the passive role of the water retained in this type of water-body as it depends on the characteristics of its hollows. The water-bodies of Sierra Nevada close to the peak line (2918 m mean altitude) are therefore highly dependent on the glacial processes that created the hollows in which they are located. Slope instability created water-bodies mainly located at lower altitudes (2842 m mean altitude), representing tectonic weak zones or accumulation of debris, which are influenced by intense slope dynamics. These water-bodies are therefore more fragile, and their existence is probably more short-lived than that of bodies created under glacial conditions.

Hydrological Characteristics and Paradoxes of Mediterranean High-Mountain Water-Bodies of the Sierra-Nevada, SE Spain

Each lake complex must be understood before attempting any regional synthesis leading us to view these water-bodies as indicators of regional climate change. Therefore, in order to improve knowledge of these Mediterranean biotopes, we examined the dependence of the fringes of hygrophilous communities surrounding the water-bodies (green fringes) on their hydrological and geomorphological features. The climate of the western sector of this massif is cryo–oromediterranean, where thawing produces 53 hm3 of run-off and 11 hm3 of sub-surface flow. Part of this water is stored in 123 water-bodies located from 2480 to 3200 m a.s.l., 72 of which (58%) are located on the south-facing Mediterranean watershed. The total surface of the water sheet is approximately 170,000 m2, and volume is approximately 215,000 m3, of which 140,000 m3 (65%) are stored in the south-facing water-bodies. Green fringes surrounding 84 water-bodies have a total surface area of approximately 186,000 m2. Surprisingly, the more xeric Mediterranean watershed holds 58 such fringes (149,000 m2, 80%) while 26 are found on the Atlantic watershed (38,000 m2, 20%). Green fringes are mainly associated with small water-bodies (<5000 m3), which occupy 148,000 m2 on the Mediterranean watershed, while on the Atlantic side, green fringes occupy 31,000 m2. Sierra Nevada also has 46 times higher water-efficiency in the smaller water-bodies than in the large ones; 16.4 and 335.8 times higher on the Atlantic and Mediterranean watersheds, respectively. The differences in gradient of the massif hillsides must largely explain this uneven behaviour.