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Lakes are a crucial part of the Earth’s ecosystems - they provide habitats for many different animal and plant species, provide fresh water for continental areas, and also shape local climates. Lakes are invaluable for humans, providing fresh water, food, hydropower, and numerous services from tourism to fishing. Hydrology has an important role in understanding the effects of waters and water ecosystems to the Earth’s biosphere. The chemical compositions of waters give us information about the concentration of nutrients, acidity and purity of the waters. This information can be used to evaluate the overall state of the waters, which, in turn, can be a baseline for improving the quality of water and ensure sustainability for ecosystems and human activity that depend on those waters. The aforementioned is the reason why an expedition and research by Lake Viljandi in Estonia was carried out. The purpose of the expedition was to evaluate and to compare different water ecosystems around and by Lake Viljandi. It was accomplished by measuring hydrological parameters at five different sites around Lake Viljandi. The four hypotheses stated in the paper aimed to find out the impact of vegetation to waters around Lake Viljandi, impact of the City of Viljandi to the quality of lake water and the differences of water characteristics between stream and lake waters. It was discovered that vegetation has a clear effect on the water characteristics, the environmental policies around Viljandi lake were found to be effective enough to avoid contamination of waters by nitrates. Soil was discovered to have a high oxygen-removing effect on the waters.