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Summary
In order to enrich the deeper layers of water in a lake with oxygen, it is proposed to create an artificial circulation. An impeller, installed at the bottom of the lake, with a diameter of about a tenth of the maximum depth, creates a rising current which entrains in a relatively short time (30 days) a water quantity equivalent to the volume of the lake. The power input required is comparatively small. Very slow currents at the bottom bring the water to the impeller which conveys it to the surface, whilst a backward current is formed between the upper warm water layer and the cold water layer of the bottom. In summer these two layers are only mixed in the immediate neighbourhood of the impeller; at some distance of the latter, the natural thermal superposition is not disturbed. In winter, however, the mixing zone is more extensive.