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A free-flowing river with sandy banks and large, green, mature trees.
Information icon Site of the former Green River lock and dam No. 6 across from Mammoth Cave National Park. Photo by Dan Chapman, USFWS.

With dam removed, Green River goes with the flow

Kentucky residents and wildlife benefit from improved water quality

Brownsville, Kentucky — The blacktop, like so many others, ends at the Green River. A few fire rings and Bud Light cans litter the banks. Swallowtail butterflies flit among the oaks, poplars and sycamores. The river ambles unobstructed this warm summer day.

There’s not much to see at the popular fishing spot across from Mammoth Cave National Park. It’s what you don’t see that matters.

Two years ago a decrepit concrete lock and dam straddled the river posing a serious hazard to swimmers, kayakers and underwater creatures. The dam was pile-driven into oblivion, courtesy of the U.S. Fish and Wildlife Service, and now the real river is coming back to life. Gone is the warm, sluggish water that backed up to the dam and created a pseudo-reservoir clogged with sediment and tree debris. In its place runs a cooler, swifter current that carries silt downstream in a more natural fashion.

Aquatic wildlife benefits mightily from the river’s return to its near-former self. Fish migrate more easily upstream to spawn and find food. Mussels — the unsung heroes of riverine ecology — thrive in faster-running rivers. They filter the water of impurities creating a cleaner, healthier ecosystem for animals, outdoor enthusiasts and municipal water drinkers (including those in Brownsville whose water comes from an intake pipe a few hundred yards downstream).

“There’s been a dramatic change in the Green River with the dam gone,” said Rick Toomey, a cave specialist who runs the park’s International Center for Science and Learning. “We’ve now got eight additional miles of free-flowing river. It’s really helped improve the biodiversity. It’s made the Green River a better river.”

No. 6 was the first of four dams scheduled for removal along the Green. Next up: Lock and Dam No. 5, about 14 miles downstream from No. 6. It’s due to come down in 2020. Once they’re all gone, the clean-water benefits will extend the length of the Green River to its confluence with the Ohio River where a new national wildlife refuge is being born.

“When we get all those dams out, we’ll see a lot of improvements to the entire river,” said Monte McGregor, the malacologist, or mussel expert, with the Kentucky Department of Fish and Wildlife Resources. “More fish will be able to migrate further and more nutrients will be able to flow further. And a new refuge downstream becomes that much more worthy with all that natural stuff going on upstream.”

Removing a safety hazard

Lock and Dam No. 6 opened in 1906 to allow wooden barges hauling asphalt and sandstone to run the upper reaches of the Green River and the Nolin River, a tributary. The U.S. Army Corps of Engineers shuttered the dam in 1951. Floods, sinkholes and the constant pressure of sediment, rocks and tree limbs slamming against concrete took its structural toll. Cracks appeared. Lock walls tilted. The underbelly of the dam was breached in November 2016. Water levels behind the dam dropped nearly seven feet in a matter of hours. The Corps labeled the breach “a very dangerous safety hazard,” particularly for canoers and kayakers.

On March 28, 2017, the Fish and Aquatic Conservation team of the Fish and Wildlife Service in the Southeast began demolishing the 220-foot-long dam. Sixteen days, and $870,000 later, the dam was gone. The demolition crew placed most of the debris into the lock’s chamber. The river’s slopes were graded and seeded. Chunks of concrete on either side of the river remain the only tangible sign that something once substantial crossed the Green at Brownsville.

Water levels upstream were now permanently altered, lowered. The river runs faster now, too, mimicking its natural flow to the benefit of aquatic critters.

“The entire river through the park is now mostly free-flowing,” the park service’s Toomey said. “The mussel populations are improving. The mussel beds are better developed. That allows us to re-colonize a large number of the mollusks.”

A small grassy island in the middle of a shallow river surrounded by mature trees.
Ring pink and fanshell mussel habitat in the Green River in Kentucky. Photo by Monte McGregor, Center for Mollusk Conservation, Kentucky DFWR.

The Ohio River basin, which includes the Green River, is still considered home to the richest diversity of mussels in the world. The Green itself once harbored 71 types of mollusks. Seventeen federally threatened or endangered mussels plied the river’s riffles and shoals. Ten of those mussels, including the fanshell, snuffbox and pink mucket, live in the 23 miles of river that flows through Mammoth Cave. Few places can match the above-ground, below-ground ecological punch of what Ralph Waldo Emerson in 1850 called “the great hole in the ground in Kentucky.” Which is why the United Nations labeled the Kentucky treasure a World Heritage Site in 1981 and, nine years later, an International Biosphere Reserve.

The importance of the mussel

A young boy wearing a life jacket and a snorkeling mask holds a mussel on the bank of a river
Clayton Bey, a very young malacologist, displays his mussel discoveries from the Green River. Photo by Dan Chapman, USFWS.

Clayton Bey, mask firmly in place and tethered to a scuba tank floating on an inner tube, emerged from the Green River to drop a few mussels into a mesh bag.

“Look how many I’ve got?” Clayton announced to nobody in particular.

He climbed the river bank and, one by one, unloaded his treasure. A pink heelsplitter. A kidneyshell. Two winged mapleleafs. A threeridge. It wasn’t a particularly remarkable haul. None of the mussels were threatened or endangered. But Clayton’s work was nonetheless notable on two accounts. He and fellow divers uncovered 2,500 mussels in one week’s time, a sizable amount even for one of the most biodiverse stretches of the Green River. And Clayton was only five years old.

“I reckon he’s the youngest malacologist around,” said Clarissa Lawlis, his mother and a true mussel biologist. “I pay him in Oreos.”

The demolition of No. 6 lowered river levels throughout Mammoth Cave prompting the park service to extend the ramps that lead to the ferry crossing the river. First, though, the agency needed to adhere to the Endangered Species Act which requires federal agencies undertaking construction projects to make sure no harm comes to listed species. Clayton’s parents were hired by the park service to dig up, identify and re-plant mussels into a particularly fertile mussel bed a half-mile upstream from the ferry crossing.

They spent five days in May plucking the mussels from the muck lining the river bottom and carefully cataloguing their booty. They discovered 28 live mussel species of which two — the fanshell and the sheepnose — were federally endangered. The results weren’t startling. A preliminary assessment of the demolished dam’s impact on mussels at a dozen sites throughout Mammoth Cave two years earlier documented 27 species including the same two endangered ones. But the report by the Office of Kentucky Nature Preserves, contained one critical conclusion.

“Flowing habitat had the greatest richness and abundance,” wrote Mike Compton, who wrote the report for the U.S. Fish and Wildlife Service.

Later, in a phone call, the zoologist elaborated.

“The aquatic communities were really limited because the dam changed the habitat from a free-flowing river into more of an impounded lake-type situation,” Compton said. “With the removal, the anticipation is that the river will return to a more natural setting of habitats and communities of mussels.”

Dams turn free-flowing, highly oxygenated, shallow and cool upstream habitats into sluggish, oxygen-deprived and warmer pools of water. Mussels and other critters suffer. Today only 11 of 17 threatened or endangered species of mussels can be found in the Green River.

Why should we care? Relatively few people come in contact with mussels. They won’t win an endangered-species popularity contest. Leonardo DiCaprio isn’t likely to shine his star wattage on endangered orangefoot pimplebacks as he does Sumatran elephants.

A brown mussel with orangish brown striations
An orangefoot pimpleback mussel. Photo by Monte McGregor, Center for Mollusk Conservation, Kentucky DFWR.

Yet mollusca play a critical role in the natural interplay between water and mankind. Mussels are filter feeders: they suck up nutrients, sediments and contaminants before expelling the cleansed byproducts downstream. Overall water quality improves to the benefit of other critters and humans who rely on water treatment plants that stick straws in streams, rivers and reservoirs. Some mussels filter ten gallons of water daily. And some football field-sized riffles are home to tens of thousands of mollusks straining water like a sieve does flour.

“There are economic benefits, too, to the Green River if mussels were ever restored to their historical role,” said Michael Floyd, a U.S. Fish and Wildlife Service biologist in Frankfort, Kentucky. “You’d have cleaner water for communities that use the river for water supply. And more people will boat and fish on the river overtime as it gets restored.”

Mussels do another important job too. They’re the early-warning system telling us that something’s wrong with the water. Malacologist McGregor said “there is no other species that reflects what is going on in a river better than mussels.” A surge in dead mussels means bad stuff — toxic chemicals, sediment runoff, temperature changes — is harming water quality. If the mussels disappear, then other species are likely too also.

“It’s pretty simple, really,” said Rick Olson, the ecologist at Mammoth Cave. “Mussels clean the water. Humans need clean water. Mussels are like the canaries in the coal mine.”

A bizarre, blind shrimp

Olson, Toomey and Tim Pinion, the park’s chief scientist, descended deep into Mammoth Cave, through the Historic Entrance, past the Great Bat Room, down the Massive Dome and into the River Hall. They crossed the Natural Bridge over the Dead Sea, yet another offshoot of the Green River. Finally, 360 feet down, they stopped.

“Here,” Toomey the paleontologist said, “is the River Styx.”

Appropriate. The scientists were in search of the elusive, otherworldly, highly endangered and supremely bizarre Kentucky cave shrimp. The Service refers to the shrimp as a “nearly transparent decapod crustacean with only rudimentary eyestalks.” An adult is about an inch long with ten legs and two pincers of different sizes. It molts every 40 or so days, sloughing off its entire exoskeleton — antennae, mouth, legs, tail — in one piece which resembles a wholly intact cave shrimp. The shrimp deploys long antennae to find, smell and taste food, because it doesn’t have any eyes. In fact, it doesn’t need eyes, because it lives in a cave where the sun never shines.

A transluscent shrimp with visible organs
A Kentucky cave shrimp. Photo by John MacGregor, Kentucky Department of Fish and Wildlife Resources.

Once thought extinct, the shrimp was rediscovered in the early 1980s and listed as endangered in 1983. Today, maybe 7,000 survive. It is believed to reside in 10 streams or pools in, or around, Mammoth Cave, but nobody really knows.

“This is a species we need to know a lot more about,” Olson said.

One thing, though, is certain: the health of the Green River dictates if the shrimp lives or dies. Heavy rains raise water levels, move the shrimp around and create the pools and streams where they live. Lack of rain reduces the river’s flow and places where the shrimp can live. A low-flowing Green also allows the build-up of silt which buries the sand and gravel layers where shrimp feed.

The river also brings food in the form of tiny protozoans, insects, fungi and algae. Without the sun’s photosynthetic qualities, the shrimp must rely on take-out food delivered via upstream (or downstream whenever flooding backs up the river) currents.

More than two centuries of growth and development have imperiled the shrimp. Pesticides and sediment from farm fields pollute the Green. Logging strips the river of natural buffers. Coal, oil and gas extraction introduces toxic chemicals into the stream, as do rock quarries. Septic tanks leach sewage into the groundwater. Non-native trout eat the shrimp. Fish and Wildlife says Dam No. 6 was “responsible for the decline” of the shrimp between 1967 and 1981.

“The river should now, basically, go back to its historical condition,” said biologist Floyd. “The groundwater system won’t be underwater permanently. It’ll be just a seasonal, short-term thing. The entire ecosystem of that cave should recover.”


Dan Chapman, public affairs specialist, (404) 679-4028

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