Before the days of dams and levees, great rivers used to overflow from time to time and drop nutrients in their wake. Plants in the resulting flood plains would in turn help purify the water and limit erosion. Today, in Minnesota, there's a federal conservation program in which farmers are being encouraged to turn riverfront cropland into meadow lands. These green buffers take advantage of nature's own water purification and soil protection systems. Minnesota Public Radio's Mark Steil has our report.