Divers find unexpected treasures and try to reintroduce others in a once nearly dead river.
By Dan Kelner
You wouldn’t think the first dive of the day would give someone who dives every day all day the willies, would you? It usually doesn’t–except when it’s in the mighty and unpredictable Mississippi River, notorious for its strong flows, near-zero visibility, and potential hazards such as jagged trash, drifting trees, and other debris.
Though we are trained to dive in low-visibility water, all of us on the dive crew are tired from diving the previous day. Suiting up in our cramped boat, we groan and laugh nervously at the prospect of the cold, dark water and utter lack of visibility. The interns are even more nervous than I am. Their previous diving experience was in the clear, warm water of the Bahamas.
Rush-hour traffic hums by on the State Highway 5 bridge. Jets take off from nearby Minneapolis—St. Paul International Airport. As I struggle down the dive ladder into the turbid water, my adrenalin kicks in and turns my apprehension to excitement. I’m suddenly thinking about what treasures we might find on this dive. Not the typical treasures one thinks about diving for, but treasures all the same.
I grab the dive rope and lower myself down–quickly, to keep from swinging in the current. The depthfinder shows us in only 12 feet of water, but it seems much deeper. Finally, I feel my feet and knees on the gravel and boulders. I tumble in the current before digging my feet in and lying on my stomach to keep a low profile in the swift water.
I sweep my hands back and forth across the riverbed and out in front to protect my head as I crawl upriver in complete darkness. My hand touches a familiar shape, and I pry a partially buried mussel out of the rocky bottom. This one is large, maybe the size of a softball, and smooth. Plain pocketbook, I think, and toss it into my bag.
Soon I come into mussels so dense that I kneel in one place and drop them into my bag, one after another. Within 20 minutes, I have more than 100 mussels of a dozen different species. Indeed, I have found a treasure!
Thirty years ago we would have been hard pressed to find a handful of live mussels from this stretch of the river, hard hit with municipal sewage and industrial pollution. Now native mussels are returning, a sign water quality has improved. In fact, the reach of river from the Twin Cities downstream to Red Wing, once nearly devoid of aquatic life, may now serve as one of the last big river mussel refuges in the Midwest.
Freshwater mussels–also known as naiads, clams, or unionids–are large, long-lived, bivalve mollusks that often form "beds" of many thousands of animals. Mussel beds are an important feature of healthy rivers and streams in the eastern two-thirds of the United States and Canada. In the upper Midwest, mussels are most diverse in the Mississippi River drainage below St. Anthony Falls.
Mussels are valuable animals. Fish and wildlife eat them. By filtering particles from the water for food, they clean it. And by stabilizing the substrate, they create habitat for vegetation, invertebrates, and fish.
The sedentary and filter-feeding lifestyle of mussels makes them sensitive to changes in the environment and, therefore, excellent monitors of a river’s health. Unfortunately, many of our rivers and streams have been dammed, channelized, dredged, and used as dumping grounds for toxic wastes and chemicals. These changes, coupled with the exploitation of mussels for the pearl button industry, have resulted in the extinction or dramatic decline of many mussel populations throughout North America.
More recently, the spread of the exotic zebra mussel further threatens the survival of native mussels. This invasive species can colonize the bottom of lakes and rivers in such numbers that native mussels and other bottom-dwelling animals can no longer obtain food and oxygen from the water because they are literally buried beneath the zebra mussels.
As a result of all of these pressures, groups such as the American Fisheries Society and The Nature Conservancy have declared freshwater mussels the most imperiled group of animals in North America. In Minnesota alone, two of the 48 native species have been extirpated and 25 are listed for legal protection. Among the state’s largest rivers, only the St. Croix appears to have retained its full complement of about 40 mussel species. In 1999 the DNR began to survey the state’s streams, to provide baseline data on the distribution and abundance of mussels.
As with most major rivers in North America, the Mississippi has not been immune to human neglect. Since at least 1880, the river and its mussels have been affected by overharvest, impoundment, erosional silt, and pollution. By the 1930s pollution was so bad that fish kills in the Mississippi River below the Twin Cities were common. Overharvest of mussels in the late 1800s and early 1900s decimated many mussel beds.
Erosional silt from poor land-use practices over the past century destroyed habitat, suffocated mussels by clogging their gills, and smothered entire mussel beds. Pollution from sewage and industrial wastes, associated with increased urbanization along the river, threatened to finish off mussels in the reach between the Twin Cities and Lake Pepin. (Lake Pepin serves as a natural catch basin and spared the mussels in the lower reaches of the Mississippi River from much of the pollution generated upstream.)
Furthermore, efforts to aid navigation transformed the free-flowing river into a canal by narrowing its channel, and later, into a series of reservoirs with the construction of locks and dams, from the 1910s through the 1960s. The changes to the river degraded habitat and interfered with the many mussel species’ unusual reproductive strategy. Mussel larvae encyst (hitchhike) on fish, transform into juvenile mussels, then drop to the river bottom. This relationship allows mussels to be distributed to wherever their hosts live or can swim. Dams altered river habitat and restricted movement of mussels’ host fish between pools.
In the 1970s, the U.S. Army Corps of Engineers conducted a mussel survey and found only nine live species at one site immediately below the Ford Dam and only a few scattered mussels down to Red Wing. No live mussels were found between the Ford Dam in St. Paul and St. Anthony Falls in Minneapolis. "The outlook for a mussel renaissance in this troubled reach is extremely poor" and would remain so "until radical improvement in water quality is accomplished," the corps reported at the time.
Fortunately, there have been radical improvements in the quality of water entering the Mississippi River. In the late 1970s, citizens organized themselves into advocacy groups such as Citizens for a Clean Mississippi and developed slogans such as "we can’t all live upstream." These groups began to demand that Twin Cities area storm sewers and sanitary sewers be separated to prevent a flood of raw sewage from entering the river during rainstorms. Pressure from citizen groups and a threat of a lawsuit by the state of Wisconsin eventually led to the separation of these systems, beginning in the early 1980s. The work is nearly complete. As a result, water quality has improved substantially.
River biologists first suspected mussels were returning to the area with the arrival of fish. Walleye, sauger, and smallmouth bass began to appear in test nets. Even a paddlefish showed up during a survey below the Ford Dam. Theoretically, larval mussels would be able to arrive on fish of various species migrating from lower reaches of the Mississippi and from tributaries such as the St. Croix. Fish could gain access to upstream pools during floods when the dams’ control gates are raised clear of the water. Some fish could have arrived from stocking activities in lakes with outflow to the Mississippi, or by passing through the locks. Regardless of the mode of recolonization, fish were back. But to what degree, if any, had mussels accompanied them? A hint came when a biologist hired to survey river life for a proposed pipeline crossing found a number of mussels near Grey Cloud Island.
To document the potential recovery, the DNR mussel survey team took on the colossal task of surveying the Mississippi River. Armed with a half-dozen people and enough SCUBA gear and compressed air to outfit a dive shop, we planned to obtain base-line data on mussel distribution, abundance, and diversity. We also hoped to determine the extent of zebra mussel infestation in the area.
We dove at many sites for up to two hours at a site. That way we could survey a wide area in a short time and get a picture of what species were present and compare how abundant they were from site to site. By counting empty shells, which can remain for decades after the animal dies, we could even determine which species once lived at each site. We will return to areas that support an abundant and diverse mussel community and conduct a more thorough investigation to get a more accurate description.
We selected survey sites based on our knowledge of mussel habitat preferences along the banks, side channels, eddies, and wing dams. We avoided the main channel, riprap, eroded or unstable banks, and areas with high human activity such as barge-loading facilities.
The first dives were done in spring 2000 near Lower Grey Cloud Island downstream from St. Paul. We had been accustomed to working in small, relatively clear streams, often wading and snorkeling in them. Here, we dove in complete darkness, sifting through soft sand and mud for the hard outlines of a mussel. We’d fill our bags and dump the mussels out on the bow of the boat before sorting them into piles by species.
We found many specimens of a state endangered species called the wartyback. What a treasure! Wartybacks are not abundant anywhere. We also found several other state-listed species, including many juveniles of a species called the rock pocketbook. And very few zebra mussels! All this, from a stretch of river once written off as a dead zone. Our findings were soon big news among mussel researchers.
During 2000 and 2001, we sampled nearly 200 sites along an 83-mile stretch from Dayton south to Red Wing. We spent more than 200 hours scratching, crawling, and fighting the current and debris, most of the time in complete darkness. Rare was the dive site where we did not find mussels. Sometimes we’d fill our bags two or three times in a 20-minute dive.
We documented more than 12,000 specimens, representing 27 species once again living in the river. Because most individual mussels found were younger than 10 years, we could see that the recovery in the mussel population had begun recently. Given time and continued improvement in habitat conditions, older individuals should become more prevalent.
Significantly, we did not find live specimens of Higgins’ eye pearly mussel and winged mapleleaf, two federally endangered species. We found only shells, suggesting they were once present below St. Anthony Falls but have failed to naturally recolonize.
But we have tried to help speed nature along, and in the process provide refuge to several mussel species that are threatened by the exotic zebra mussel. Working with the U.S. Fish and Wildlife Service, the U.S. Army Corps of Engineers, and the natural resources departments of Wisconsin, Iowa, and Illinois, we rescued more than 2,000 rare mussels, as well as a few hundred Higgins’ eye mussels, from zebra-mussel-infested waters near Cassville, Wis., and Cordova, Ill., and relocated them below the Ford and Hastings dams. If ever the Mississippi is managed more like a flowing river and less like a series of lakes, zebra mussels may decline and native mussels may once again flourish in these downstream pools.
Efforts to artificially propagate Higgins’ eye mussels are also underway in an attempt to establish new populations. Fish are infected with the mussels’ larvae, then placed in cages in the St. Croix and Mississippi rivers for a few weeks until the larvae transform into juveniles and drop off. These Higgins’ eye juveniles will then be placed throughout this reach of the Mississippi River. This study has shown that the mussels are successfully maturing and dropping off the fish, so we may begin simply releasing the fish to disperse the developing mussels, as Iowa biologists have done.
We discovered few zebra mussels through the Twin Cities reach of the Mississippi, and found none above St. Anthony Falls. It’s a different story from Lake Pepin downstream. There, zebra mussels are extremely abundant and are decimating native mussel communities. Zebra mussels have recently invaded and appear to be reproducing in the lower St. Croix River, which had served as the chief refuge for the upper Midwest’s native mussels. Lake St. Croix might become a zebra mussel seed population.
Unlike native mussels, zebra mussels travel upstream on watercraft, not fish. Furthermore, they appear to do best in a large, placid, lakelike stretch where they multiply before releasing larva to drift downstream and mature. The Twin Cities portion of the Mississippi doesn’t seem to offer this habitat. Moreover, this reach may have an as-yet-unidentified toxin or other limiting factor, such as the soil bacteria recently shown to be toxic to zebra mussels but not to natives, that is inhibiting their colonization. Whatever the reason, zebra mussels have been slow to spread upstream to the Twin Cities. We hope they never do.
The first phase of the mussel survey is complete.
Next we will return to sites that supported an abundance and diversity of mussels to get an accurate assessment of age-class structure, density, and species richness. We will monitor our relocation areas and place juvenile Higgins’ eye mussels into the reach. Data will be used in conservation planning for protecting the species listed for legal protection in the Mississippi and other rivers.
Seventy-two miles of this reach fall within the Mississippi National River and Recreation Area. The National Park Service, which provided funds to the DNR for the 2001 survey effort, will continue funding our monitoring efforts to identify trends in populations and evaluate the health of the mussel fauna.
Because of improved water quality, lack of zebra mussels, and the natural and artificial recolonization of native mussels, this reach of the Mississippi River, running through one of the major metropolitan areas in the country, may now become one of the last big river mussel refuges in the Midwest–gratifying, to say the least, for a river once written off as dead.
Dan Kelner was DNR mussel research coordinator. He now works for the U.S. Army Corps of Engineers, St. Paul.