AN INVESTIGATION IN THE FORMATION OF RIMSTONE DAMS

This study is an investigation in the origins of rimstone dams both on the surface and in caves. A literature review and laboratory experiments were conducted. Rimstone dams are common underwater mineral precipitate structures that generally form perpendicular to direction of water flow. A logarithmic relationship between rimstone dam size and flow gradient has been established. Rimstone dam morphology ranges from long sinuous single walls to stair-stepped horseshoe-shaped birdbaths with curve of horseshoe pointing down gradient. The current accepted hypothesis for the origins of rimstone dams is water flowing over an irregular surface in a stream experiences degassing of carbon dioxide as a result of turbulence in the water column. This degassing is expected to cause a precipitation reaction to occur that develops the dam structure. This hypothesis of rimstone dam formation does not address several critical issues. First, if rimstone dams originate at a site of a streambed irregularity, the logarithmic relationship between rimstone dam size and flow gradient seems unlikely. Secondly, degassing does not account for instances of non-carbonate rimstone dams such as ice, silica, and goethite dams which do not require carbon dioxide degassing. Finally, if degassing at a site of increased turbulence causes rimstone dam formation, precipitation would occur at the water-air interface. If so, the transport mechanism to bring the precipitate from the water surface to the streambed is lacking. A rapid-growth model of a cave stream environment was constructed to determine if a rimstone dam could be created in a laboratory setting. A highly soluble salt, sodium thiosulfate, was used in an attempt to grow rimstone dams. Only rippled flowstone was formed. More research will have to be conducted and experiments designed to reproduce rimstone dam formation in a laboratory setting.