Legacy of the Dam

To restore a former reservoir, CALS scientists battle history in the soil.

FOR MOST OF THE PAST 150 YEARS, Wisconsin’s proud Badgers might as well have been beavers, so busy were they damming the state’s rivers and streams. To produce electricity, provide water for livestock and control floods, Wisconsin built some 3,800 dams by official estimates-or as many as 10,000 by unofficial counts-more than any other state.

As those structures grow old, obsolete and hazardous, however, many dam owners are embracing a new, free-flowing era, dismantling the dams and returning waterways to their natural course. But as long-submerged lands spring back to life, the consequences of dams can linger long after the structure is gone.

That’s what soil scientists Nick Balster and Ana Wells and restoration expert John Harrington MS’83 are finding at the UW’s Franbrook Farm, where they are trying to restore a native prairie at the site of a 43-year-old dam removed in 2003. But the scientists have found that soils flooded for decades hold stubborn traces of their past. For one, they’ve uncovered a striking lack of variety in the knee-deep sediments that piled up during the dam’s lifetime and buried the more diverse soils underneath. This uniformity could explain why former reservoirs usually cultivate monotonous blankets of invasive weeds after they are drained, confounding attempts to establish native plantings.

At the same time, the researchers have discovered swirling patterns of nutrients, bulk density and other soil properties that were laid down when the dam was breached and water surged through the break. How these patterns might influence the distribution and growth of native species-including their ability to stand up to weedy invaders-is now a major thrust of the trio’s work.

Surprisingly, the researchers say no one else has really done this before. “We’re asking the question, ‘How much do soils matter in the restoration of these basins?'” says Balster. “As people who love to study soil we’re going to say, ‘A lot! Soils likely drive the whole thing.’ But as scientists, we don’t know yet.”

In the meantime, they’ve been fascinated to watch the development of land that had been underwater and devoid of terrestrial life for decades. Thousands of earthworms have wiggled down into the fresh dirt, for example, while waves of different invasive plants have washed over the ground each year. And over time, the researchers expect these plant and animal pioneers will feed and mix the nascent soils, transforming the site yet again.

“That’s what has been fun for me,” says Balster. “You rarely get the opportunity as an ecologist to study and watch primary succession of a plant community into new soils. But we have it here.”