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Extension > Agriculture > Tillage > Sediment problems and solutions for the Minnesota River

Sediment problems and solutions for the Minnesota River

Norman B. Senjem, Minnesota River Basin Coordinator, Minnesota Pollution Control Agency
John F. Moncrief, Extension Specialist, Department of Soil, Water, and Climate, University of Minnesota
Gyles W. Randall, Soil Scientist, Southern Experiment Station, University of Minnesota
Samuel D. Evans, Soil Scientist, West Central Experiment Station, University of Minnesota (now Emeritus Professor at Morris, Minnesota)

A Minnesota River prognosis

The Minnesota River is in poor health. Like a heart patient's veins and arteries, its tributaries and main stem are clogged--not with cholesterol, but with sediment, nutrients and excessive algae growth. The river, like a heart patient, needs to go on a strict diet before its health can be restored.

The river's water, clouded by sediment and contaminated by bacteria, frequently is unfit for fishing, swimming, and other uses. In fact, the lower stretch of the river has seen violations of federal and state water quality standards for turbidity, dissolved oxygen, and fecal coliform. At periods of low flow, pollution-generated algae growth and subsequent decay remove so much dissolved oxygen from the water that many aquatic life forms can't survive. At other times, a steady supply of suspended sediments keeps the river turbid--cloudy--and far below its potential as a water resource that Minnesotans can use and enjoy. This is true of the main stem, and many of the major tributaries that drain into the Minnesota River.

The good news is, the Minnesota River can be restored to health. Measures already underway promise to result in reductions of the major pollutants spoiling the river--bacteria, sediment, phosphorus, and nitrogen. Farmers are upgrading feedlots and leaving more residue on their fields. Towns and cities are reducing storm water runoff and industrial discharges while upgrading wastewater treatment plants and septic systems. New techniques for reducing sediment losses through open tile inlets are also being tested.

Sediment: A priority pollutant

A significant part of the Minnesota River's water quality problems comes from sediment that enters the river and its tributaries throughout its 10-million-acre watershed. Approximately 625,000 tons per year of total suspended solids, largely sediment, are transported by the Minnesota River at its mouth at Fort Snelling, according to the Metropolitan Council. That's 86 20-ton truckloads a day.

Sediment is a pollutant in its own right, causing turbidity in the water that limits light penetration and prohibits healthy plant growth on the river bed. Sediment also covers much of the river bed with a blanket of silt that smothers life. By covering up gravel and cobble, sediment destroys the spawning grounds and habitat of desirable fish species such as bass and bluegills. Instead, less desirable species such as carp are favored by the sediment-enriched habitat.

Finally, sediment is an important carrier of a critical pollutant: phosphorus. This nutrient stimulates excessive algae growth in the water column. When the algae decomposes, it depletes dissolved oxygen from the water, reducing the quality of life forms that are able to survive.

Sediment sources

Sediment originates from many sources within the Minnesota River basin: stream banks, construction sites, lawns and streets, and agricultural fields. Each of these sources is being addressed as part of the river restoration effort. However, given the prevalence of row-crop agriculture throughout the basin, this source generally outweighs the others, and must be significantly reduced to improve the river's water quality.

Substantial water quality improvements can result from the use of economically achievable sediment-reduction practices on farmland. One of these practices is conservation tillage, defined as tillage systems that leave at least 30 percent of the field surface area covered by crop residue after planting. Leaving the surface rough and partially covered with crop residue reduces sedimentation at its origin by preventing the detachment of soil particles by raindrops, and retarding their transport across the field surface by water runoff. Soil erosion reduction does not translate into equal reductions of sediment entering surface waters, however. Erosion refers to the transport of soil over the field by water or wind, while sedimentation refers to the deposition of soil particles into surface water. Reductions in erosion usually correspond to much smaller reductions in sedimentation.

An example illustrates the point. Switching from clean tillage to a system that leaves at least 30 percent residue cover on the surface after planting can reduce soil erosion by 50 to 65 percent. Thus, if the original average annual erosion rate on a field was 4 tons per acre, conservation tillage would result in a 2-ton reduction in soil erosion. But only a fraction of this 2 tons would translate into reduced sedimentation. On a gently sloping, typical row-cropped field in the south-central part of the Minnesota River basin, the Natural Resources Conservation Service (NRCS) estimates that about 10 to 20 percent of eroded soil typically is delivered to a surface water channel such as a drainage ditch or surface tile intake or directly into a stream. Thus, a 2-ton per acre reduction in erosion may translate into a reduction of 400 to 800 pounds of sediment entering surface water.

These sediment losses are much lower than the so-called "tolerable" level of soil loss that can be sustained without sacrificing long-term productivity--the "T" level of approximately 5 tons per acre. But sediment losses of several hundred pounds multiplied over hundreds of thousands of acres of cropland can contribute to chronic water quality problems in the Minnesota River system. This is especially true of sediment composed of fine particles from the clayey soils so prevalent in the basin. These particles may stay in suspension for days, degrading water quality for hundreds of miles before settling out.

The relative proportions of the total sediment in the river contributed by cropland, stream banks and other sources still are not well understood. Very likely, these proportions vary widely within the basin. But according to an evaluation by the NRCS, the widespread adoption of conservation tillage practices within the south-central part of the basin could reduce sediment losses by approximately 45 percent. Since conservation tillage can be practiced with minimal effect on crop yields and often at lower production costs than conventional tillage, it offers a low-risk means of achieving substantial reductions in sediment losses from cropland.

Minnesotans can aim high in their efforts to restore the quality of their namesake river. However, we need to be realistic. Even substantial reductions in sediment and other pollutants won't restore the Minnesota River to its pre-settlement state of quality, as described by early explorers and settlers. For one thing, highly productive row-crop agriculture is the dominant land use throughout most of the basin. This intensive form of land use inevitably entails sediment losses. In addition, the prairie soils throughout the Minnesota River basin are generally of a fine texture that dislodge easily, carry substantial quantities of phosphorus, and cause a high degree of turbidity.

The benefits of a 40 percent sediment reduction

The Minnesota Pollution Control Agency has established a basin-wide goal of a 40 percent reduction in sediment losses. This is economically achievable throughout much of the basin, and would lead to substantial improvements in water quality.

During high-flow periods, typically in the spring, the river would begin to cleanse itself by scouring deposits from years of sediment loading. As the small spaces between pebbles on the river bed cleared out, spawning by popular game fish would increase. Species such as bluegills and largemouth bass would begin to increase in population in the lower reaches of major tributaries and in parts of the main stem.

During the rest of the year, at medium and low flow, the river and tributaries would become less turbid. A child could see his or her feet after wading in knee-deep, for example, versus only shin-deep today. But actually, light would penetrate much farther than that, all the way to the river bed of tributary streams, and up to a grown man's height in the main stem. As a result, plant growth would shift from algae at the surface to large aquatic plants at the bottom, becoming a healthy part of the biological community rather than a nuisance. Dissolved oxygen levels would rise as phosphorus levels and surface algae growth declined. The Minnesota River would be on its way to achieving the level of quality that Minnesotans expect from their major water resources.

Achieving these water quality gains through land-use changes that are consistent with a productive, profitable agriculture is the goal of the Sediment Reduction Initiative, a basinwide effort involving state and local government agencies, private businesses, landowners, and nonprofit organizations. As the first stage of this initiative, the University of Minnesota has developed conservation tillage guidelines for specific soils, climate zones, and crop rotations within the basin.

Reduced tillage guidelines for the Minnesota River Basin

Sediment from cropland can be reduced through a variety of measures including reduced tillage, crop rotation, waterways and terraces, grassed buffers at the field edge, and catch basins. In many situations, residue management through reduced tillage can be the primary means of sediment reduction. By preventing sediment losses at the source, surface residue management reduces the need for secondary measures. Where such measures are required for secondary protection, residue management makes them less costly, more effective, and longer lasting.

Overview of tillage systems

A variety of tillage systems can be used in a corn-soybean rotation, or rotations including a small grain, to achieve an average surface residue cover of approximately 30 percent after planting, the goal of conservation tillage.

A three-stage implementation sequence

The Natural Resources Conservation Service (formerly Soil Conservation Service) has recommended that sediment reductions on farmland in the Minnesota River basin be pursued by introducing residue management on cropland through a three-stage process:

Critical management factors

The performance of reduced tillage systems depends on a wide variety of management factors that differ from those used under clean-till systems. Crop rotation, equipment selection and adjustment, fertility management, weed control, and drainage are among the critical factors required for successful use of reduced tillage systems.

The degree of management changes required depends on the extent of tillage reduction. Farmers who substitute the chisel plow for the moldboard plow face minor management changes, while those adopting no-till face systematic adjustments touching many aspects of crop management. Farmers in the eastern part of the Minnesota River basin, where annual precipitation averages 28 inches or more, will often face higher management requirements than those in the western part of the basin where rainfall is lower. Similarly, farmers with poorly drained, fairly level fields face greater challenges than those farming better drained, sloping fields.

Crop rotation

Each crop rotation presents distinct opportunities and challenges for residue management:

Drainage

Planting

Fertilization

Weed control

Performance summary of tillage systems

For purposes of evaluating tillage systems, the Minnesota River basin can be divided into four regions based on soil parent material and rainfall. The two soil parent materials are lacustrine and glacial till. Lacustrine soils, which are fine-textured and poorly drained, are found south of Mankato, in the Blue Earth basin, as well as to the northwest of New Ulm in Renville, Chippewa, and northern Lac Qui Parle counties, and in isolated pockets elsewhere. Most of the rest of the soils are classed as glacial till. The east-west dividing line is formed by the 28-inch annual precipitation line, which runs roughly north-to-south between Highways 71 and 15. Based on these distinctions, the following four regions have been delineated:

Lacustrine, High Rainfall (L-HR)
Lacustrine, Low Rainfall (L-LR)
Glacial Till, High Rainfall (G-HR)
Glacial Till, Low Rainfall (G-LR)

 

Each of the five tillage systems identified earlier are evaluated using the following four performance indicators:

  1. Inadequate residue for sediment control -- considerably less than 30 percent of surface covered after planting. Highest yields may be obtained, however, on poorly drained, fine-textured, high organic matter soils.
  2. Recommended with good management -- If the above management guidelines are observed, no yield penalty is expected, and surface residue should be 30 percent or more.
  3. Excellent management required -- Surface residue should be adequate for erosion control, but a slight yield penalty is possible, even if all recommended management practices are observed. Above average crop management, especially weed control without excessive herbicide use, will be needed to ensure profitability.
  4. Reduced yield potential -- Surface residue should be adequate for erosion control, but the potential exists for substantially reduced yields in wet years on poorly drained sites.

Soybeans following corn

Lacustrine Soils Glacial Till Soils
HR LR HR LR
Moldboard Plow 1 1 1 1
Chisel Plus 2 2 2 2
One of Two Passes 3 3 2 2
Ridge-till 3 2 2 2
No-till 3 3 3 3

Corn following soybeans

Lacustrine Soils Glacial Till Soils
HR LR HR LR
Moldboard Plow 1 1 1 1
Chisel Plus* 2 2 2
One or Two Passes 2 2 2 2
Ridge-till 3 3 2 2
No-till 4 4 3/2 3/2
*Even if straight shanks are used, this system cannot reliably achieve the 30 percent surface residue target, and must be used in a rotation where corn residue levels are at least 40 percent after planting.

Continuous grain corn

Lacurstrine Soils Glacial Till Soils
HR LR HR LR
Moldboard Plow 1 1 1 1
Chisel Plus 3 2 2 2
One or Two Passes 4 4 4 4
Ridge-till 3 3 2 2
No-till 4 4 4 4

Small grain following soybeans

Glacial Till Soils Lacustrine Soils
Moldboard Plow 1 1
Chisel Plus 2 2
One or Two Passes 2 3
No-till 2 3

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