Abstract: A flexible sheet of fluid impervious material of sufficient area to substantially cover the sidewalls and bottom of the reservoir when substantially empty and to cover and form laterally fixed positioned and well defined inter-connected surface water canals when the reservoir is filled at various levels. The surface water canals are adapted to receive, store and channel surface water which normally falls on the reservoir as rain or snow to the edge of the reservoir. The means for forming the fixed horizontally positioned canals also function to form taut, substantially horizontal portions of the cover. The depth of the canals vary as the level of the reservoir changes. The cover is supported by displacement of water rather than by separate float means.