undergone much tectonic action, as all the visible layers of sedimentary rock are approximately horizontal. Karst topography is found throughout the Driftless area. This is characterized by caves and cave systems, disappearing streams, blind valleys, underground streams, sinkholes, springs, and cold streams. Disappearing streams occur where surface waters sink down into the earth through fractured bedrock or a sinkhole, either joining an aquifer, or becoming an underground stream. Blind valleys are formed by disappearing streams and lack an outlet to any other stream. Sinkholes result from the collapse of a cave's roof, and surface water can flow directly into them. Disappearing streams can re-emerge as large, cold springs. Cold streams with cold springs as their sources are superb trout habitat. Due to the rapid movement of underground water through regions with karst topography, groundwater contamination is a major concern in the Driftless area. Rivers The Mississippi River passes through the Driftless Area between and including Pool 2 and Pool 13. As rivers and streams approach their confluence with the Mississippi, their canyons grow progressively steeper and deeper, particularly in the last in their journey to their mouths. The change in elevation above sea level from ridgetops lining a stream to its confluence with the main-stem Mississippi can reach well past in only a few miles. The Waukon Municipal Airport is reliably established as being above sea level. The Army Corps of Engineers maintains a river level in Pool 9 of about above sea level, which covers Lansing. Maps and signs issued by the Iowa Department of Transportation indicate Waukon and Lansing are apart on Iowa Highway 9. This is a drop of more than in less than (and this along a very minor tributary of the Mississippi). "The role of isostatic rebound on the process of stream incision in the area is not clearly understood." There are many small towns in the Driftless Area, especially in river valleys, at or upstream from the Mississippi. Small towns in a deep steep valley going down to the Mississippi are at risk every 50 to 100 years or so of a major flood, as with the wreck of Gays Mills, Wisconsin, in August 2007, or the holding of the levee in Houston, Minnesota, (on the South Fork Root River) at the same time. Metropolitan areas have flood walls (See 2007 Midwest flooding). In August 2018, the region yet again experienced record-breaking flooding in