The gradual and imperceptible withdrawal of water from land which it covers “by the lowering of its surface level from any cause.” 91 N.W. 2d 57, 60. If the retreat of the waters is permanent—i.e., not merely seasonal—the owner of the contiguous property acquires ownership of the dry land thus created. See 152 N.W. 796, 798. See dereliction, see also accretion, avulsion.