In Minnesota Public Radio's Mississippi River series, we've reported on Twin Cities' efforts to re-integrate the riverfront into city planning, and we've looked at efforts to balance commercial and recreational uses of the river. We've also looked at the environmental impact of using the river as an industrial thoroughfare. In this final segment, we'll focus on the geology of the river, and in particular, its waterfalls. The only waterfalls along the 23-hundred miles of the MIssissippi are all clustered in the Twin Cities. Author Richard Arey recently published a limited edition book called Waterfalls of the Mississippi that tells their story. Arey says the biggest falls, River Warren Falls, in what is now downtown St. Paul were once the largest in North America: