Big River is an independent magazine about the people, places, life and events on the Upper Mississippi River.

This was the summer of weeds on the Mississippi from lower Lake Pepin to Prairie du Chien, Wis. Some of the backwaters and lakes were choked with wildcelery, coontail, Canadian waterweed and sago pondweed. Boaters and anglers couldn’t get to some of their favorite places unless they used an airboat or a boat with a tilt motor. Weeds clogged motor water intakes and wound around propellers. It was even tough to paddle into many places.

Then in late August the winds blew and the waters rose, sending thick mats of rotting vegetation into the Main Channel where they snagged on trees or docks, or found their way into harbors or marinas.

Ever since a mysterious crash of submerged and emergent plants in the same stretch of the river during the drought years of 1987 through 1989, biologists have been trying to re-establish the plants. Healthy plant beds are important to wildlife and water quality, because they protect shorelines from erosion, provide food for waterfowl, and provide cover for small fish and other animals. When the plants came back like gangbusters this summer, many people blamed the agencies that have been staging drawdowns and other projects to encourage plant growth.

Seeing Cycles
Plants need sunlight, soil, nutrients and water to grow. There is always plenty of water in the river, and that water has plenty of nutrients in it, because lots of fertilizer washes from farm fields in the spring. The soil is on the river bottom. So, plant growth in the river is limited mainly by sunlight that may be blocked by the algae and silt in the water.

Understanding what triggered the dramatic plant growth involves many possible factors, including a relatively new one: zebra mussels, small invasive mussels that reached the Mississippi after the plant crash of the late 1980s, according to John Sullivan, Mississippi River water quality specialist at the Wisconsin Department of Natural Resources (DNR) La Crosse, Wis., office.

Cycles are key to many of the processes that Sullivan studies. For instance, as plants grow along a shoreline — whether they grow above the water or just below the water — they calm the water, reducing erosion along the shoreline and reducing the power of wind and current to stir up silt from the bottom. In calmer water silt settles to the bottom. The clearer water allows more light to penetrate, stimulating more plant growth. As the plant beds expand they create more calm water, and so on.

Zebra mussels may have started the current cycle. Because there are very many of them, they collectively filter huge volumes of algae and silt out of the water, making it clearer, which stimulates plant growth. Unlike most of the native mussels, zebra mussels need to attach themselves to something, such as bigger mussels, rocks, docks or submerged plants. Expanding plant beds create more habitat for zebra mussels. More zebra mussels filter more water, allowing more light to reach plants, creating bigger plant beds, and so on.

After the lock-and-dam system was built in the 1930s, the dams slowed the river’s flow for much of the year, allowing sand and silt to settle out, filling the backwaters, some claim at about an inch a year on average. This shallower water allows more light to reach the bottom, allowing plants to grow faster. Calmer waters around the plants allow more silt to settle out of the water, making it shallower, allowing more light to reach the bottom, and so on.

These cycles continue for years, because healthy plants leave bigger tubers or more seeds to overwinter to the next season. A graph showing light penetration at lock and dams 8 and 9 (Genoa, Wis., and Ferryville, Wis.) shows that the water was unusually clear in 2009, on average allowing one percent of the light to penetrate a bit more than 2.5 meters (8.2 feet). In 2010 it averaged about 2.2 meters (7.2 feet), still unusually clear. So this plant boom probably started last year.

The big question, of course, is what will happen next? Sullivan and other river scientists keep looking back to the crash in the late 1980s for answers.

The Big Crash
“We never really established why the vegetation crashed in the late 80s during the drought,” he admitted.

The water was clear, probably because less silt and nutrients washed into the river during the long drought. On average, light penetrated about nearly 2 meters (6.6 feet) in 1988. From 1989 through 1992 —when there were few plants in the river — light penetration was very low, never averaging more than one meter (3.3 feet).

The plant crash of 1989 may indicate that cycles can turn both ways: once plant beds begin to decline, the presence of fewer plants may accelerate the decline. Several things happened in 1988 that may have reversed the cycle: wild celery plants did not produce large tubers, high water temperatures may have killed many plants, and a type of algae that attaches to submerged plants was common and may have stunted plants by blocking light to their leaves.

Late summer floods this year might have cut the growing season short for submerged plants, which may reduce next summer’s crop. However, stay tuned. If light snow cover this winter allows more light through the ice, then plants will get an early and good start.

Sullivan wonders whether last summer’s plant explosion might be the peak of a cycle. He and other river scientists will be watching, hoping to learn more. Many boaters will be watching and hoping, too.

Looking Upriver and Downriver
Last summer the stretch of river where Megan Moore usually works showed a broad range of conditions. In the Mississippi stretch from the Minnesota River confluence, in the Twin Cities, to the St. Croix River confluence at Prescott, Wis., “It was as turbid as I’ve ever seen it,” noted Moore, aquatic vegetation specialist at the Minnesota DNR Lake City office.

She couldn’t find any rooted plants there. The Minnesota River flooded in late summer and contributed a lot of the water to the Mississippi flood. The Minnesota has been under scrutiny for years, because it carries so much silt and agricultural chemicals into the Mississippi.

Downriver, from the cleaner St. Croix to Lake Pepin, she found improved plant growth, but not so much as to cause complaints. The variety of submerged plants increased, too. Previously Moore had found mostly sago pondweed and curly leaf pondweed, plants that tolerate turbid water. This summer she also found wild celery, Canadian waterweed and coontail, indicating a healthier river.

From Lake Pepin to Alma, Wis., plants were a problem for boaters and anglers. During the early summer, Moore said she saw “some of the cleanest high water I’ve ever seen.”

She thinks that the late flood probably scoured parts of the river and may make for a better situation next summer, though excessive nutrients in the water may make it hard for the ecosystem to find a balance.

On Pool 13, from Bellevue, Iowa, to Fulton, Ill., the plants and water clarity looked pretty good, but the water was about three or four feet ­higher than usual. In Iowa, the Dehli Dam on the Maquoketa River failed in late July, sending a tremendous load of silt and water into Pool 13. The emergent plants were underwater for about a week in late summer. When the water fell, they were wilted and coated with dirt, but they bounced back, according to Josh Petersen, fisheries technician 2 at the Iowa DNR office in Bellevue.

Submerged plant beds were thicker than usual, but the high water overtopped them, so they didn’t block sloughs and channels. Healthy submerged and emergent plant beds are especially important in the lower part of Pool 13, which is the widest spot on the Upper Mississippi. Without a lot of plants, the wind really stirs up the silt in that stretch.

Unless the spring flood is dirty, the plants should look pretty good next year, Petersen said.

Mark Twain observed that “history never repeats itself, at best it sometimes rhymes.” We’ll just have to wait and see how the river writes the next verse.

Reggie McLeod is editor of Big River Magazine.