In August 1941, the Municipal Waterworks of Amsterdam started an investigation on the biological an hydrographical conditions of the water in some ”wielen”. ”Wielen” are exceptionally deep pools. They originate from old dikebursts caused by high floods of the river. The new dike has been constructed around the ”wiel” which had been formed behind the breach by the whirling water (see figure A). The drinking water supply from the dunes of the Westcoast of Holland, which, owing to a rise of the salty groundwater level is running short, is a serious problem. New sources of fresh water must be found and a plan has been made for the construction of some deep basins for the storage of river water.