Pool 19: Old Niota
Hancock County, Illinois
“Later that day I stopped near Fort Madison at a place over looking the best, open sweeps of Pool 19. It was a spring waterfowl mecca. Most were lesser scaup in great rafts spread on the open pool like tattered fabric, here tightly knit, there open and torn. With them, but apart from them in their own bunches were redheads and the lordly canvasbacks–the canard chevals or “horse ducks,” named for their straight, throughbred profiles.
Some of the birds were resting; a few were engaged in nuptial flights. Many of the “cans” and redheads were feeding, diving deeply and reappearing briefly and then diving again. Some were nearby, but even with the powerful ‘scope I could not detect any rootstocks or other plant materials in their bills. They were obviously battening in Pool 19’s blueplate special–the fingernail clam. This fingernail-sized mussel is a succulent bit of rich nutrition relished by the diving ducks.” –John Madsen, 1985
The Iowa Hawk Eye had a story the other day about students and researchers collecting and tagging scaups migrating along the Upper Mississippi River.
John Madsen knew and loved the Upper Mississippi and described the thousands of scaups and other diving ducks who came to Pool 19 for the fingernail clam. Read Up on the River, it was written by a person who truly loved the Upper Ms.
During the first half of the twentieth century, the scaups and canvasback ducks migrated to the Illinois River to feast on its population of fingernail clams. As the Illinois filled with sediment and the mud flats became vegetated, the clam population declined. The scaups and the canvasbacks migrated to the Mississippi, where Pool 19 hosted a population of fingernail clams as dense a 2,800 individuals per square foot of mud along the channel border.
In 1913 when the Keokuk and Hamilton Water Power Company completed its dam, the Mississippi back up into a pool twenty-three miles long and up to thirty feet deep. Today, it is Pool 19, and, it, too, is filling with sediment. Mud, washing down the Skunk River and other streams that spill into Pool 19, has accumulated along the channel borders where submergent and emergent plants have taken root. In the eighty years between the completion of the dam in 1913 and 1993, Pool 19 has lost fifty-eight percent of its volume to sediment. A final note: Because Pool 19 has been accumulating sediment longer than the other pools, the volume of sediment in the pool is greater than in the other pools and it has lost more of its depth.
When John Madsen published his chronicle of the Upper Mississippi in 1985, Pool 19 provided a rich menu of macroinvertebrates for fish and ducks: fingernail clams, oligochaete worms, myraid snails, mayflies, leeches, and chironomid larvae. The lesser scaup is the most numerous duck in North America. It winters in the southeast and along the Gulf of Mexico. It breeds in the northwest and follows a migration pattern from the Yukon Flats in Alaska to Florida. It rests in Pool 19 and other pools on the Mississippi, where it feeds on fingernail clams.
The adult fingernail clam is both male and female. It self-fertilizes its embryos, carries them in its gills, and releases them fully formed into the river. Like the Higgins’ Eye and other mussels, its single foot drags it through the mud. And, it filters nutrients and toxins through its siphon. It’s the toxins, particularly ammonia, along with drought, that may have led to its decline in Pool 19 in the early 1990s.
Drought came to the Mississippi in the late 1980s. The pool level dropped, exposing the mud. Call it nature’s drawdown. Vegetation took root. The population of fingernail clams in Pool 19 dropped to zero by 1990. The ducks disappeared. The flood of 1993 flushed out the toxins and stripped the pool borders of vegetation. The fingernail clams reestablished themselves in the mud. The scaups and the canvasbacks returned to the Illinois side of the river near Niota and Nauvoo, but not in the numbers seen at mid-century. However, if sediment continues to accumulate at the rate of fifteen centimeters a year, Pool 19 will lost eighty percent of its volume by 2050 along with the fingernail clams and the diving ducks.
Madsen, 158-59; U.S. Fish and Wildlife Service, Mark Twain Wildlife Refuge Complex, Comprehensive Conservation Plan and Enviromental Assessemnt, Draft, 51; Upper Mississippi River Conservation Committee, “FACING THE THREAT: An Ecosystem Management Strategy for the Upper Mississippi River,”
http://www.mississippi-river.com/umrcc/Call-for-Action.html; Day, David M., Anderson, Richard V., Romano, Michael A., Canvasback and Lessor Scaup Activities and Habitat- Use in Pool 19, Upper Mississippi River, Transactions of the Illinois State Academy of Sciences, (1993), Volume 86, no. 2, 33-45, http://www.il-st-acad-sci.org/transactions/PDF/8604.pdf; Kennedy. Robert, Anderson, Richard V., and Morgan, Melissa, “ Current Status of the Benthic Community OF Pool 19, Mississippi River,” Triannual Unionid Report, Report No. 13, November, 1997; U.S. Fish and Wildlife Service, National Wetlands Research Center, “Habitat Suitability Index Models: Lesser Scaup,” Biological Report 82, April 1985, http://www.nwrc.usgs.gov/wdb/pub/hsi/hsi-091.pdf; Massachusetts Audubon Society, Wellfleet Bay Wildlife Sanctuary, “Fingernail Clam,” http://www.wellfleetbay.org/pond/fingernailclam.html; Personal Communication. Dick Steinbach, Manager, Mark Twain Wildlife National Management Complex, October 2004.