The intention is a long-term monitoring of different sites at the inoperative gravel lake "Tenderingssee". This lake is formed by digging gravel in the district of Wesel on the northwestern edge of the so called Ruhrgebiet. The lake has an area of 45 ha and a maximum depth of 15m. Since 1996, the lake has been considered as an European Community (EC) bathing water. The lake is a well known dive location within the mentioned area. Furthermore, fishing and windsurfing are common leisure activities. Up to now the years averaging visibility is very good. The local gravel plant owner intent to restart the rinsing of gravel from other places close to the lake. The operator of the gravel plant promise to the authorities, that only a light impact to the visibility will happen. The intention is to dumping the rinse water underneath the lake-ground to prevent strong influence to the quality of water. The measured values will be the visibility as well as the in water temperature. The temperature will be measured by a mini temperature logger "tempmate-B2”.