Abstract: Disclosed is a fluid reservoir having a bottom and side and end walls which is lined with a flexible material secured about its marginal edges to a footing surrounding the reservoir. A cover is disposed over the reservoir with its marginal edges secured to the footing. A fan supplies air under pressure into the space between the cover and the fluid level to maintain the cover in a raised state above the fluid surface. Webs are provided along the outside of the cover to shape the cover under pressure and to relieve the stresses along the fastening elements used to connect the cover and the footing. The fan operates to supply air under pressure upon sensing a predetermined low pressure in the system, whereas a pressure relief valve opens to vent the air under pressure upon sensing a pressure higher than a predetermined pressure.