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Wisconsin Natural Resources magazine

Wisconsin Natural Resources magazine

August 1999

The big river gives up another sample for analysis. © Terry Dukerschein

Taking the Great River's pulse

Assessing the health of the Upper Mississippi means drawing a lot of samples from Ol' Man River.

Terry Dukerschein


The big river gives up another sample for analysis.

© Terry Dukerschein

Searching the shallows
Pools on the Upper Mississippi (map)

On these hot mornings, the airboat fans us with sticky, humid air on the long trip to the main channel of the Mississippi River. If it's foggy, we temper our speed to look out for obstructions until the morning haze lifts. Some days, the fog just hangs on between the river bluffs; we can barely see the channel markers. When we stop to collect a sample, the still air distorts sounds – splashing fish, a rumbling train and a droning boat may all be closer or further away than we think.

Tucked into a cooler are about a dozen large bottles we will use to sample the Mississippi at preset monitoring spots. Only the extremes – thunderstorms, hail, blizzards, high winds, and Coast Guard closings – keep us off the river.

By "us," I mean the biologists, fisheries managers and researchers at DNR's Onalaska Field Station who collect Mississippi River water samples every other week, all year. Today we are doing our quarterly sampling of Navigation Pool 8 (the river segment between Dam 7 at Dakota, Minn., and Dam 8 at Genoa, Wis.). It's 23 miles long and up to five miles wide. We will take 150 or more water samples from random locations during five to seven days.

Our work is part of the Long Term Resource Monitoring Program, a federal and state partnership with other field stations in Illinois, Iowa, Minnesota, and Missouri to document environmental conditions along the upper Mississippi and Illinois rivers. We analyze water samples, fish, vegetation, and riverbed organisms to detect trends or changes in these large river systems. Our monitoring work is coupled with a program to enhance and rehabilitate river habitat for animals, fish, plants and people. (See "Mississippi River rehab," in our August, 1998 issue.) Together, these habitat and monitoring programs comprise the U.S. Arnmy Corps of Engineers' Environmental Management Program (EMP), which was authorized under the Water Resources Development Act of 1986. The information the team collects helps river managers and the public gauge when the rivers are being used wisely and when human activities harm natural river communities.

Pools on the Upper Mississippi

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Interpreting the data we gather can be a challenge. For example, water clarity trends from upstream to downstream differ in different pools. In Pool 4 (between Red Wing, Minn. and Alma, Wis.) water clarity is not as good in the upper portion as it is downstream. In Pool 8, downstream from Pool 4, the water is clearer upstream and gets cloudier downstream.

Walt Popp, team leader for Minnesota's field station, speculates that suspended particles carried by the Minnesota River to Pool 4 settle out in deep Lake Pepin. Jim Fischer, Wisconsin field station water quality specialist, posits that because Pool 8 does not have deep, slack water like Lake Pepin, there's nowhere for the silt to settle on the journey downsteam.

It's all part of tracking the life signs of the Big River.

Searching the shallows

Wide rivers like the Mississippi tend to naturally fill-in backwater bays where turbid water slows down and soil particles settle out. Eric Kramer, DNR fisheries technician, takes samples each spring from 150 sites in the Mississippi backwaters looking for bottom-dwelling animals like young mayflies, midges and fingernail clams – important foods for fish and waterfowl. The bottom-dwellers also indicate if the environment has been disturbed and polluted. For instance, if mayflies are absent from an area that would otherwise be suitable for them, biologists suspect certain types of pollution and investigate further.

Changing water depths are also recorded. Riverbed sampling sites that consistently were in a couple feet of water in the mid-1970s now have just a few inches of water above the mud.

"It's work to reach them," Kramer says. "If we don't have high water, we have to get out and push the boat to some of the monitoring sites."

Mayflies feed fish, which in turn feed hungry herons and other water birds.

© Scott Nielsen
Mayflies feed fish, which in turn feed hungry herons and other water brids. © Scott Nielsen

Aerial photos taken by the U.S. Geological Survey and the U.S. Fish and Wildlife Service are used to identify and electronically map floating, emergent, and terrestrial vegetation. These observations have to be verified on the ground – up close and personal. Heidi Langrehr, our field station's vegetation specialist, and her assistant spend a good part of their day mucking about in the river shallows. They often return from a day of sampling spattered from head to foot with black mud. On the other hand, the job has its rewards – for instance, when the two slowly motor through a sea of pale yellow lotus blossoms fringed by tall, feathery wild rice. They follow narrow channels winding into the silty backwaters preferred by algae, aquatic insects, breeding and migrating waterbirds, and fish such as sunfish, bluegills, largemouth bass, black crappies and northern pike. As the backwaters continue to fill with sediment, they become too shallow for fish.

Water lilies and other plants thrive in the Mississippi's sluggish backwaters.

© Robert Queen, DNR
Water lilies and other plants thrive in the Mississippi's sluggish backwaters. © Robert Queen, DNR

In late spring and mid-summer, Langrehr's crew collects plants from approximately 1,200 sites in eight different backwaters of Pool 8. They find common underwater vegetation such as coontail and waterweed, Eurasian watermilfoil (a non-native invader), various pondweeds (which are valuable food for wildlife), and floating plants such as lotus and water lilies. Nearer the main river channels, Langrehr finds wild celery, a valuable underwater food for diving ducks. Since aquatic animals ultimately depend on plants for food and shelter, it's important to track plant abundance throughout the growing season.

Our crews also assess river health by monitoring approximately 30 water quality variables. River anglers are always interested in our readings of dissolved oxygen content since waters high in oxygen generally support more fish species. Dissolved oxygen generally remains adequate in Pools 4 and 8 but occasionally dips lower in the backwaters. Because oxygen dissolves best in colder, moving water, the shallow, sluggish backwaters sometimes contain less oxygen for fish and other aquatic life. Fluctuating water level, the shape and size of a backwater and its connection to the main river channel, as well as the amount of ice and snow cover can also change dissolved oxygen and water temperature. Based on population data, river managers speculate that many young panfish never make it through their first winter because suitable overwintering areas in river backwaters are becoming scarce. Scientists hope to use computerized mapping and monitoring data to help managers improve winter habitat for fish.

Fish species that prefer more river-like conditions have other problems. When deeper water rises behind a dam, the water slows and some river-like areas disappear. Dams are also a barrier to migratory fish traveling to spawning or wintering areas. The monitoring crews seldom collect lake sturgeon, paddlefish, and blue suckers – commercially important migratory species that were once common throughout the Mississippi River. The scarce fish are difficult to collect in the deep, flowing waters they prefer.

That's not to say the Mississippi River fishery lacks variety. In our monitoring work in Pool 8, we capture 75-78 different fish species per year. Most Midwestern lakes contain fewer than 15 species of fish. Given the variety, researchers use a host of electroshocking and netting techniques in the 500 or so fish collection sites in Pool 8 each year.

The fisheries data, vegetation data, and other monitoring data can also be can be mapped and combined to provide clues for managing the Mississippi's wild resources. We now know that catfish are more abundant in the southern reaches of the Upper Mississippi. And catch data shows that catfish near La Crosse reproduce abundantly, but only in certain years under certain conditions.

Compared to the world's other temperate rivers of similar size, the Mississippi supports remarkably large numbers of animal species and the river remains an exceptional biological resource. It deserves comprehensive monitoring and habitat preservation plans, and it deserves attention along its entire length. Like a coordinated team of medical personnel, the partners in the Environmental Monitoring Program monitor the river's vital signs, ready to diagnose and treat this vast patient whose health determines the well-being of so many species, including our own.

Terry Dukerschein leads DNR's long-term monitoring subteam on the Mississippi River in Onalaska.