A dam good idea

The removal or modification of dozens of low-head dams has expanded the freedom of fishes and improved the health of Iowa’s natural environment.

The state has helped communities remove or modify three dozen low-head dams since 2008, when the Legislature established the Low-head Dam Public Hazard program. As the name implies, the primary impetus was to improve public safety by mitigating hazardous dams, which have contributed to about 60 deaths since the 1990s.

But those same projects also have reconnected formerly disconnected river reaches, much to the benefit of aquatic species and to the people who enjoy river recreation.

Most of the state's nearly 200 low-head dams, known as "drowning machines" for their powerful recirculating currents, were built to provide power to grist and lumber mills or to generate hydroelectric power. Not only do they no longer serve those purposes, but they also impede fish movement and otherwise impair the natural environment.

Many fish species migrate at least twice a year — a spring run to reach their preferred spawning areas and a fall run to areas with deeper water and slower flows where they can safely and comfortably spend the winter. Dams often interrupt those migrations.

The Department of Natural Ressources’ Interior River Research Team has monitored fish and fish habitat upstream and downstream of several dam modification and removal projects.

Before-and-after research has documented “new species showing up where they hadn’t been before,” said Greg Gelwicks, a DNR interior river research biologist.

After the dam removal at Rockford, researchers observed a dramatic transformation.

“It was mostly rough fish before, but now it’s game fish,” Gelwicks said.

At Manchester on the Maquoketa River, the DNR marked 22,000 fish of 20 species and recaptured 939 of them above the site of the dam. The dam removal “reconnected 14 miles of river and tributaries with the best overwintering habitat,” he said.

Mitigation of dams allows natural processes to resume, as has been especially evident with the removal of the dam at Rockford. Gelwicks said most of the silt that had accumulated in the impoundment has been washed downstream, exposing gravel, cobble and boulders, yielding improved game fish habitat. Streambank restoration further improved aquatic habitat and natural beauty, he said

Following word-of-mouth buzz about improved angling, “there’s a lot more fishing going on in what used to be the impoundment,” he said.

Improved fish passage also benefits mussels, which depend upon fish hosts to transport juvenile mussels within rivers.

“Most mussel species also prefer the free-flowing water fostered by dam removal,” said DNR biologist Jen Kurth, a mussel expert.

Seven years after the dam removal at Rockford, Kurth said researchers upstream of the site found many mussels age 5 or younger as well as examples of two species that hadn’t been there before.

In addition to the 36 completed dam mitigation projects, several more are under way, according to Nate Hoogeveen, DNR river programs director.

Among them, he said, is an innovative project on the Wapsipinicon at Central City that should be finished later this year. To preserve the upstream characteristics in Linn County’s Pinicon Ridge Park, the dam will not be removed. Rather, half the channel will become a rock arch rapids, while the other half will become a recreational passage for paddle sport enthusiasts.

Upon its completion, with earlier projects at Troy Mills and Quasqueton, fish passage in the Wapsie will be unobstructed from Anamosa to Independence.

Hoogeveen listed several other planned projects with expected fish passage improvements, including the Iowa River dam at Steamboat Rock, two dams on the Upper Iowa River below Decorah, the Hannum Mill dam on the South Skunk in Story County and the Lenon Mill dam on the Middle Raccoon at Panora.