Stone, steel, and concrete are often poured into efforts to stabilize river channels, but the massive flood that scoured the valleys of southeastern Minnesota in August 2007 exposed the merits of a more natural approach to maintaining healthy rivers.
Much of the damage caused by raging floodwaters occurred where accumulated sediment confined a river or where rivers pushed up against artificial or natural obstructions, according to DNR stream habitat supervisor Ian Chisholm. Many roads and homes built on sediment near unstable channels tumbled into the torrent after having the earth swept out from beneath them. But in Whitewater Wildlife Management Area, the success of a DNR stream restoration project became evident.
When floodwater entered the restored stretch, it spread out across the floodplain, dissipating force and distributing sediment over a wide area. When the water receded, roughly two inches of sediment had settled across the land nearly a mile from each bank.
The DNR project restored roughly 3.5 miles of the Whitewater River, where a straight, man-made channel built in the 1950s diverted water to several waterfowl impoundments.
Completed in 1999, the restoration returned the river to much of its meandering former channel. With the help of aerial photographs, the DNR completely reconstructed another 1.5 miles.
Once the course of the restored section was set, DNR river ecologists raised the height, or profile, of the river to bring the water level closer to the banks. Like adding height to the risers of a very long and gradual set of stairs, the DNR built up shallow rocky areas at the beginning and end of the restored section. Sediment also had to be removed to lower the banks in places to provide the channel access to its floodplain.
River ecologists determined that the river should be close enough to its banks to flow over them every 1 1/2 years -- the average interval between typical high-water events.
"Natural rivers are self-formed and self-maintained," Chisholm said. "What we tried to do was take the disturbed channel and put it back to a form that can handle the sediment and discharge of the watershed."
And assessments in the wake of what has been called a 1,000-year flood event indicate the effort succeeded.
Nevertheless, across the geographically unique driftless area, the nearly ubiquitous problem of unstable channels has its roots in accumulation of silt, which, over time, has confined and disconnected rivers from their floodplains. From the time of European settlement to today, as much as 10 feet of sediment has been deposited in the Whitewater River valley. Much of the sediment was mobilized near the turn of the 20th century as people removed trees and plowed the steep valley slopes. During a string of floods in 1938, sediment literally buried the town of Beaver.
Though modern agricultural practices have improved, the legacy of human activity has compounded the damage from the most recent floods. Finding the best ways to recover and minimize future damage will be a daunting task.
Michael A. Kallok, Editorial Assistant