As a kid growing up in the 1960s in southwestern Minnesota, I spent most of my free time fishing, hunting, and watching wildlife along Dutch Charlie Creek in the Cottonwood River watershed. I loved to hear Alfred Erickson and other old-timers in the close-knit neighborhood talk about what the creek was like when they were young. Born in 1891, Alfred had a lifetime of experience fishing, hunting, and farming along the creek. He talked about the huge runs of northern pike and other fish species years ago and speculated that siltation or other habitat changes might have led to their decline.

In my childhood days, the creek had an abundance of pollution-tolerant fish like creek chubs, common carp, and black bullheads, but it was clearly less diverse and productive than it once had been. Much later, as a river scientist, I realized that a dam far downstream was preventing fish from spawning, migrating, and periodically recolonizing the creek.

The Flandreau Dam was built in 1938 just 6 miles upstream of the mouth of the Cottonwood River and roughly 100 stream miles downstream of where I lived. After that, 24 of the fish species documented in the watershed were not collected upstream of the dam.

Dams impede free-flowing waters and alter natural processes. Barriers restrict all kinds of movement, from the flow of sediments to the migration of fish. Freshwater species from mussels to game fish may decline or disappear from waterways due to various factors such as downstream migrations over the dam, droughts, harsh winters, periods of poor water quality, and inundation of spawning habitat. With access to upstream waters blocked by the dam, these species cannot recolonize. Since the 1990s, the Department of Natural Resources has been working on projects to remove dams from rivers, restore natural hydrology, and re-establish aquatic communities.

A single dam can affect many rivers. For example, the Flandreau Dam was removed in 1995 ("Dam Yanking," March–April 1994). Almost immediately, native species began to reappear upstream in the river and its tributaries. Seventeen of the 24 missing species have returned. The recovery occurred in roughly 2,000 miles of rivers in the 1,300-square-mile watershed.

Freshwater Biodiversity.

Many aquatic species need to be able to migrate in streams to maintain healthy populations. Fish migrate seasonally to reproduce, optimize foraging, and recolonize after droughts, severe winters, and other disturbances. Were it not for fish species immigrating to Minnesota waterways primarily from the Mississippi River, most of the state would be fishless. After glaciers receded from Minnesota some 10,000 years ago, the north-south alignment of the Mississippi allowed fish colonization from downstream.

Dams have been the primary reason that many aquatic species are no longer present. Upstream of dams, abrupt species losses follow construction. Loss of biodiversity is a global issue. A 1999 study by Anthony Ricciardi and Joseph Rasmussen projected the extinction rate of freshwater fauna at about five times that of terrestrial species. About 72 percent of North American freshwater mussel species, 36 percent of global freshwater fishes, and 85 percent of sturgeon species are imperiled. Twenty-two of the 26 mussel extinctions in North America have been attributed to dam construction.

Mussels play critical roles in stabilizing streambeds, increasing biodiversity of other invertebrates, filtering water, and removing harmful bacteria. Mussels depend on fish for reproduction and dispersal. Many use only one species of fish as a host. When Lock and Dam 19 was built on the Mississippi in Iowa in 1913, migratory skipjack herring disappeared from the watershed upstream of the dam. Previously found upstream to Big Stone Lake on the Minnesota River, skipjack herring are the sole hosts of ebonyshell and elephant ear mussels. The disappearance of these two mussel species followed suit. Ebonyshell had been the dominant mussel of the upper Mississippi and lower Minnesota rivers. Half of the roughly 40 species of native mussels that once existed in the Minnesota River can no longer be found.

A 1995 DNR report identified approximately 2,000 dams on Minnesota streams. We recently evaluated the effects on fish diversity of 32 barriers farthest downstream. On average, about 40 percent of a watershed's fish species are abruptly absent upstream of complete or near-complete barriers. Pollution-tolerant species like common carp, creek chubs, and black bullheads are abundant upstream of most barriers. These species have generalized habitat needs and can survive harsh winter conditions, low dissolved oxygen, and drought. Carp do well in reservoirs and actually benefit from fragmentation and the exclusion of native fish, which prey on their eggs and young. Two of the dams evaluated were built as carp barriers, but nonnative carp remained abundant while 60 percent of the watershed's native fish species were absent upstream of the dams. In the study, desirable native species unable to tolerate low oxygen levels and pollution were most likely to be missing.

Water History.

Many Minnesota communities were named for falls and rapids that no longer exist because dams have been constructed on them. Ironically, many of these old dams are registered and protected as historic sites, while the falls and rapids themselves lack similar consideration despite their lengthy cultural and natural histories.

Removal of the Minnesota Falls Dam in 2013 was a restoration of Minnesota Falls itself. The historic rapids and those upstream at Granite Falls had been described as "an uninterrupted succession of violent rapids for three miles" by George William Featherstonhaugh, who took an expedition up the Minnesota River in 1835.

Minnesota Falls had been inundated by the dam since 1904 and as early as 1871 by other dams at the site. The dam was the upstream limit of 40 species of fish found in the Minnesota River. In the short period since removal, 12 fish species have returned upstream, including shovelnose sturgeon, sauger, flathead catfish, state threatened species paddlefish and black buffalo, and species of special concern lake sturgeon and blue sucker. While the length of river gained by the removal is short—Granite Falls Dam obstructs the river only 3.3 miles upstream—rapids are rare in the Minnesota River and provide critical spawning habitat for paddlefish, sturgeon, sauger, and many other species. It is especially gratifying to see a historic and ecologically important site like Minnesota Falls restored for the sake of fish, natural beauty, and recreation.

Floodplain Rebuilt.

The reservoirs formed by dams trap a large amount of sediment. When rivers below dams are starved of sediment, the riverbed may undergo down-cutting, which disconnects the floodplain and causes bank erosion.

When dams fail, sediment accumulated over the life of the dam is quickly passed downstream. Sometimes it temporarily fills downstream pools. But the free-flowing sediment also rebuilds habitat and floodplains.

In Minnesota, the DNR Stream Habitat Program has found ways to accelerate the recovery process in sediment-filled reservoirs following dam removal. Using special techniques, a river channel can be reconstructed within reservoir sediments. We first took this approach with the Appleton Dam, a 16-foot-high milldam built on the Pomme de Terre River in 1872. By 1997 the reservoir had filled with up to 15 feet of sediment. Carp and bullheads were the dominant fish species.

After failing, the dam was removed in 1998. We excavated a meandering river channel in the accumulated silt and stabilized it with wood, plantings, and boulder riffles. Planted trees grew in the newly exposed floodplain. Fish that hadn't been able to ascend the river for 120 years began to return. People soon began catching walleyes in the restored river. Of the 17 native fish species that came back, nine were important mussel hosts like channel catfish and freshwater drum. Of the seven mussel species that had been found only as dead shells upstream of the dam, three have re-established mussel beds 45 miles upstream to near Morris Dam, the next barrier. Canoeists and kayakers too could now travel the free-flowing river.

More Work.

For some dams, existing functions like water supply prevent removal. In those cases, we've replaced or converted the dams to rapids. While they do not restore all river functions, the rapids eliminate dangerous hydraulics, provide passage to fish and paddlers, and create spawning habitat for sturgeon and other species.

We worked with tribal biologists on the Red Lake Indian Reservation and engineers from the Department of Transportation to remove the Mud Lake Dam, restore the river, and facilitate construction of a new bridge. The sediment was contaminated with a natural source of arsenic, and removing it all would have been costly. Instead, we built rapids under the new bridge and restored the river channel, which allowed much of the sediment to remain in place.

Current projects include removal of the Lake Shady Dam and restoration of the Zumbro River, and conversion of four dams on the Sand Hill River to passable rapids. Plans also call for removal of the dam in Blue Mounds State Park and restoration of Blue Mounds Creek.

Projects that reconnect river systems are bringing back native fish and mussel communities across the state. Approximately two-thirds of the fish species that were absent upstream have returned after dams were removed or converted into rapids. Lake sturgeon have been reintroduced to several watersheds, and 5-foot-long fish are being caught where the species hadn't existed for generations. To bring back some species, we will also need to work on restoration of degraded rivers, riparian buffers, downstream barrier removal, wetland restoration, water quality improvements, or reintroduction.

This work is a team effort of biologists, engineers, hydrologists, citizen advocates, and officials from local, state, and federal government, Native American tribes, and nonprofit organizations. By working together we can ensure that our children and grandchildren will know the state's rivers as great places to fish and recreate.