bream, pike, and perch die because of a lack of oxygen. There may be more than ten species of fish in some lakes. There are more than 30 artificial lakes with the area of one hectare and with the total area of 100 ha, 20 of which being used publicly. The biggest artificial lake is Puide Reservoir on Jõku River (11.5 ha). The best-known river of the area is Väike-Emajõgi, starting from Lake Pühajärv and falling into Lake Võrtsjärv, flowing fully on the territory of Valga County. Another important river is Pedeli, which flows through Valga and is the biggest tributary of the River Emajõgi. The River Õhne is flowing through Tõrva and the Jõku River is the tributary of the Õhne River. The River Mustjõgi flows through the south-eastern corner of the county towards Võru County. Nature protection The territory under nature protection composes around 20% of the whole territory of Valga County. The most important nature objects under protection are Otepää Nature Park, Karula National Park and Koiva-Mustjõe Landscape Conservation Area. There are 60 different protection areas such as national parks, nature protection areas, landscape protection areas and their sub-types such as parks, stands and arboretum. The biggest one in Valga County and also in Estonia is Otepää Nature Park with the area of 22,430 ha. The territory under protection, also including nature reserves and permanent habitats form the area of 43,431 ha. There are 28 indigenous trees and 5 glacial boulders under protection as a single object. The highest tree under protection is Tsuura spruce and it is also the largest spruce in Estonia – the height of 29 m and the circumference of 4.32 m. The thickest tree, Pühajärv War Oak, has the circumference of 6.98 m. The biggest glacial erratic of South Estonia is located in Valga County – Helgikivi with the circumference of 30.2 m and with the volume above the land of 61 m³. Substrate The substrate of the county is formed by Devonian sandstones which are quite porous, rich in micas and slightly cemented. Light-coloured sandstones from the Middle Devonian Burtniek stage, clay and marl layers are spread on the majority of the territory of the county. On the narrow stripe of the county's northern part, the sandstones of the Middle Devonian Aruküla Stage become visible. They are cross-bedded with the interlayers of siltstones, clays and dolomites. The substrate is covered by