Cause of open holes below mouth of Manley Slough No flash player has been set up. Please select a player to play Flash videos. Video not playing? Update your browser or download the video clip Espen Jervsjo talks with Knut Kielland about what causes open holes in the river. Process Observed: Open Water Date Process Observed: Tuesday, March 1, 2011 Galleries: Below mouth of Manley Slough Gallery