Taming the Drowning Machines

Every few years someone drowns in the tailrace of an undershot dam somewhere in Iowa. Almost every river has one, and larger rivers usually have several. Most were built a century or more ago and served some practical purpose, but most are now just lurking there, waiting to kill the next unwary visitor.

The cross-section drawing (Figure 1) shows how they work, and it’s the backflow towards the dam that is deadly. People and small boats caught in this current get dragged under at the face of the dam and are run repeatedly through this loop. At high water flows, even small horsepower boats can be dragged into and under the loop. Attempts at rescue are almost always too late.

For those living in Iowa City, the nearest undershot dam for most of us is in town at the Burlington Street bridge.

Several decades ago, Luther Aadland began a program in Minnesota to remove these old dams and replace them with very carefully engineered structures, most descriptively called rock-arch rapids. These still hold back a pool of water for fishermen, but allow spawning native fish to migrate upstream. Instead of a dangerous dam to portage around, kayakers now have a rapids to shoot. And it is much more difficult to drown in these structures. Even if you flip your kayak or don’t swim very well, in these rapids the current will deliver you to the slower flow downstream, with no backflow to suck you under.

And the program has finally started to come to Iowa. The nearest one to us is in Manchester, on the Maquoketa River. Built in 2014-2015, it has become a tourist draw, now safe enough for tube riders to shoot the rapids regularly. The floods of summer 2016 tested it as an engineering structure, and my grandson, the engineer who inspected it at low water last summer, said these man-made rapids came through the floods with no damage.

So it is possible to tame the drowning machines and turn them into a win-win for conservation and aquatic recreation. The Manchester upgrade has revitalized the town, supports new businesses, and local folks now socialize in their new riverbank park while watching their kids and the tourists run the rapids. My suggestion? Keep after our politicians to continue funding these conversions.