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When fishing a mainstem river, you'll always catch something, but you're never sure what it will be.

Because of the tremendous importance of river transportation in the 1800s, most large midwestern cities developed along the banks of major waterways. Today, millions of anglers live within easy driving distance of these rivers.

A mainstem river could be defined as the major river into which all other rivers and streams in a given drainage system flow. As a rule, these rivers share the following characteristics:

- large size, usually 10,000 cubic feet per second or more.

- murky water, especially if located in farm country.

- pattern of severe flooding and dramatic changes in water level.

- dams along their course, to control floods and maintain water levels for navigation.

- a diverse fish population; fish can move into the river from a large tributary network.

Many of these rivers are still important navigation routes, accommodating barges carrying commodities such as grain, coal and oil. As such, the rivers are often subject to dredging, channelization and pollution. In some, the gamefish population has dwindled to the point where there is very little sport fishing.

But big rivers are remarkably resilient; despite man's disregard for these mainstem waterways, many still provide excellent multispecies fishing. Some support a dozen or more gamefish species and even more species of roughfish.

Case Study:
Upper Mississippi River,
Minnesota & Wisconsin

THE UPPER MISSISSIPPI has a complex network of backwater lakes, sloughs and connecting channels. Because of the diverse habitat, this portion of the river supports a variety of gamefish and roughfish.

THE LOWER MISSISSIPPI, from Alton, Illinois, downstream, has little habitat diversity because the river is confined between high banks or levees. Roughfish predominate, with few gamefish species present.

One of the best examples of a midwestern mainstem river is the Upper Mississippi where it splits the states of Minnesota and Wisconsin. The 118-mile stretch from Lock and Dam No. 3 upstream of Red Wing, Minnesota, to Lock and Dam No. 8 near Genoa, Wisconsin, is regarded as the river's most varied and productive zone.

When the U.S. Army Corps of Engineers built these dams in the 1930s, thousands of acres of marshland adjacent to the river were flooded, creating a maze of backwater lakes connected to the main river by narrow cuts.

"0l' Man River isn't really a river at all," wrote Mel Ellis in the Milwaukee Journal in 1949. "In fact, he's a hundred rivers and a thousand lakes and more sloughs than you could explore in a lifetime."

The navigation channel in this stretch averages 300 feet wide and 12 feet deep, although there are holes more than 30 feet deep. Along the channel is the main-channel border, a shallower zone extending from the edge of the channel to shore. In this area are numerous wing dams, structures made of rocks and sticks that deflect the current toward the center of the rivet Constructed in the late 1800s and early 1900s, the wing dams help keep the channel from filling in with sand.

Current speed in the main channel measures 1 to 2 mph at normal water stage; up to 6 in high water. Current-tolerant species, such as smallmouth bass, white bass, catfish, walleye, sauger and sturgeon, spend most of their time in the main channel or along the main-channel border. Man-made cover is very important in this zone. Besides wing dams, man-made cover includes riprapped shorelines, bridge pilings, and the rock piles that support channel markers.

Navigation dams are spaced at 10- to 44-mile intervals along the Upper Mississippi River. Each has a lock to allow passage of boats, including river barges. The dams are not high compared to dams on most other major rivers. They hold back only 6 to 9 feet of water at normal stage. Most of the dams are too low to create a lakelike zone upstream, although the river above a dam is nonetheless called a pool. Each dam has a number, and the pool extending upstream to the next dam has the same number. For example, Pool 5 encompasses all of the water area from Lock and Dam No. 5 upstream to Lock and Dam No. 4.

The dams provide good habitat for many kinds of gamefish, especially the current-tolerant species. The large eddies that form below the gates and along the edges of the fast-water zone provide refuge from the swift current.

Backwaters generally have little or no current, average less than 5 feet deep, and have excellent cover, including flooded trees and stumps, and lush stands of submerged and emergent vegetation. They make ideal habitat for slack-water species such as largemouth bass, northern pike, sunfish and crappies. Other species, such as smallmouth bass and catfish, use the backwaters for spawning.

Although backwaters may be found anywhere in a pool, the most extensive ones are usually at a pool's lower end, where the dam elevates water levels the most.

Some of the backwater areas are vast, covering several thousand acres. In these areas, the river (including the main channel and backwaters) may be more than 3 miles wide.

Another type of habitat found in the Upper Mississippi is a river lake. The best example is Lake Pepin, a 25,000-acre body of water formed by the delta of the Chippewa River. The delta acts somewhat like a dam, partially blocking the flow of the Mississippi and creating a lakelike environment upstream. The lake has very little current, except at the extreme upper and lower ends. The average depth is about 20 feet; the maximum, 65. Lake Pepin holds good populations of practically all gamefish found in the river.

One of the biggest challenges in fishing a big river, especially one with such diverse habitat, is locating fish at different water stages. Runoff generally peaks in mid- to late April, and the water level may rise as much as 20 feet above normal. As the water rises, fish abandon their normal haunts. They continue moving until the water level returns to normal. Rainy weather in summer and fall may cause the river to rise several feet, enough to move gamefish out of their usual habitat. But these movements are seldom as dramatic as those associated with spring runoff.

The primary food for most of the river's gamefish is the gizzard shad, a species found in few other waters in the region. This portion of the river is at the northern edge of the shad's range, so most of the annual crop dies off in winter because of the cold water. But some shad survive the winter by staying in spring holes and warmwater discharges
from power plants. At first glance, the shad dieoff would seem counterproductive to good fish production. But the reverse is actually true.

Most of the fishing on the Upper Mississippi is done from small boats, although there is a great deal of bank fishing and some fishing from commercially operated barges moored in the tailwaters of several of the dams. For a small daily fee, barge operators will ferry you out to fish from a floating platform anchored in a prime fishing area.

Jon boats and semi-V aluminum boats from 14 to 16 feet are popular in this area. Because they draw very little water, they can run cuts and sloughs where larger boats would bottom out.

One of the major environmental problems on the river is dredging. The main channel must be dredged frequently by the Corps of Engineers to maintain the minimum 9-foot channel depth necessary for barge traffic. The dredge spoil (10 percent sand, 90 percent water) is pumped onto land adjacent to the river, often spilling into backwater lakes and sloughs and channels leading into the backwaters. The sand smothers bottom organisms and plant life, greatly reducing the backwaters' capacity to produce gamefish.

The fishing season for most gamefish is open year around on this part of the Mississippi. Prime fishing times for each species vary considerably, but you can bet that something will be biting every month of the year.


YOUNG-OF-THE-YEAR SHAD are ideal for food. They predominate because of the annual die-off. If not for the die-off, there would be more older shad, too large for food, and fewer young ones.

GAMEFISH in the Upper Mississippi grow rapidly because of the abundant food supply. The highly fertile water teems with plankton that nourishes invertebrates and baitfish. The Mississippi's fast-growing gamefish have smaller heads and fatter bodies than fish from most other waters in the region. Notice the difference in body shape between a Mississippi River walleye and a slower-growing walleye from a nearby lake.

Upper Mississippi River Physical Data (at Wabasha)

Average width 1,000 ft
Average depth 12 ft
Gradient low
Clarity 1.5 ft
Color brown
Discharge (cubic feet per second)  33,300
Winter low temperature 32° F
Summer high temperature 85° F