The composition of surface waters retained in lakes and reservoirs is directly related to the inflows of their surrounding watersheds, which are subject to underground and atmospheric exchanges of materials and energy. Therein lies an dependence on an multitude of factors. Natural waters, soils and the atmosphere are being increasingly polluted. Strategies must be developed for the use of surface waters which protect the high quality standards of drinking water, starting with the watershed via the lake to the end consumer.
In order to ensure drinking water quality, various measures must be considered in relation to each other. As a rule, priority is given to the avoidance of contaminants directly at their point of origin.
Total prevention of water pollution in congested areas is often not feasible, or only possible at great expense, so even after the refurbishing of watersheds, further improvement of water quality is required.
Complementary to the generally practiced technical methods of raw water treatment with all their associated problems of energy input requirements, costs, and waste products, there are an increasing number of environmentally sound treatment technologies which use ecological principles as a basis to support the self-cleaning properties of flowing and dammed waters.