Few things are harder to describe than the shock of encountering a paddlefish on a central Minnesota river. The prehistoric giants, with long shovelnoses, look like they belong on the wall of a museum.

They were gone for more than 100 years, almost entirely extirpated from all of Minnesota's rivers except a few deep pools in the Mississippi and St. Croix.

Now they're back. One by one, the Minnesota Department of Natural Resources is removing the dams that killed them along with dozens of other all-but-forgotten species. Without dams blocking them from their breeding grounds, the state's river monsters are coming back to life.

"It's remarkable," said Luther Aadland, longtime DNR river ecologist. "I've done all sorts of river restorations throughout my career, but nothing has compared to removing a barrier in terms of the benefit to that river system."

During the past 150 years, river dams have caused the greatest loss of biodiversity in the state. As soon as a dam goes up, about half of the species above it die off, Aadland said. Those that need miles of uninterrupted, free-flowing river are penned into smaller and smaller areas.

Hundreds of Minnesota's dams were built more than a century ago, many for reasons long forgotten. Some were built to prop up ponds to rear game fish, others for long-closed sawmills or to create better duck hunting ponds.

Since then, communities have grown up around them and the calm lake-like waterfronts they created. Even with the ecological benefits, dam removal can be a difficult sell.

Many residents and city officials are upset by the DNR's plans to remove the Grindstone River dam in Hinckley. There's a fishing pier above the dam, and below it, a pool where many people learn how to swim, said Mayor Don Zeman. A DNR study showed that removing the dam could also lower the water table enough to harm nearby homeowners' wells.

"This is the only waterfront we have in Hinckley," he said. "And we just don't understand the purpose behind [the dam removal]. Even if they remove it, there's another dam 5 miles upstream that would still stop all the fish."

Ironically, many of the dams were built in the early 1900s in a wrongheaded belief that they would help native fish survive invasive common carp.

While the carp found their way around the dams, most native species did not. The dams in places such as Dry Wood Creek in western Minnesota and Highland Creek in northern Minnesota killed off more than 60% of the native fish species above the dams, Aadland said. The pooled water also became murkier, with less oxygen. The dams eliminated flathead catfish, which grow to 100 pounds and may be the only native fish large enough to eat a carp.

Once the carp got past the dams, they found the ideal murky, oxygen-poor habitat they prefer, with no predators to stop their takeover.

"So we kind of created a carp production area," Aadland said.

Now that the carp are established, it's virtually impossible to get them out.

Dam removals help attract more native predators to eat eggs and young carp, but they've done little to actually dent carp populations, said Grace Loppnow, invasive fish expert for the DNR.

"Common carp live up to 60 years," she said. "So it's a very long-term problem, and the tricky part is trying to keep a population down for that long."

The DNR's removal effort has primarily targeted small, low-head dams, which are especially dangerous to swimmers. Low-head dams, which are most abundant in the Midwest, create a powerful undertow that can become almost impossible to escape.

A 2010 Brigham Young University study found that such dams had caused hundreds of drownings throughout the Midwest between 1974 and 2009, including 53 in Minnesota.

In Granite Falls, the DNR removed a dam on the Minnesota River in 2013. Before they took it out, a dozen residents, the City Council and the Yellow Medicine County Board all raised concerns about losing waterfront. They worried that without a dam the river would run several feet lower much of the year, leaving a muddy, weedy shoreline.

Nearly a decade later, it's hard to say just how much the shore was affected by the dam removal, said John Berends, who represents the area on the Yellow Medicine County Board.

A boat launch no longer reaches the water above the former dam site, Brennan said. And some corners of the river now have weeds. On the other hand, more people seem to be fishing along the shoreline, he said.

When the dams come out, the biggest ecological benefits to the rivers happen out of sight, beneath the surface.

The DNR has removed more than 50 dams since the mid-1990s, either restoring rivers to a natural flow or replacing them with man-made rapids. After removing the dams, an average of 73% of the lost species return, according to DNR records.

It took just one year for paddlefish to make it back to the Minnesota River at Granite Falls after the dam was removed in 2013.

Sauger, sturgeon, bowfish, blue suckers and giant catfish, to list a few, have all returned to find their old spawning grounds on streams and creeks, areas they had been blocked from for decades. Common game fish such as walleye and northern pike that had been wiped out of some stretches of the Minnesota and Red rivers have also returned.

"We know how important it is to reconnect critical habitat in these systems and in watersheds around the state," Aadland said. "And we're slowly bringing them back."