U of M study: Majority of river and lake sediment generated by stream bank erosion, not farming

Written by Jonathan Eisenthal

A number of scientists have focused their research on unlocking the puzzle of what causes the sediment-laden cloudiness of the Minnesota and Mississippi Rivers and the apparent increasing rate of sedimentation in Lake Pepin.

By measuring the exact topography of the river banks along the tributaries of the Minnesota River in Blue Earth County, University of Minnesota Soil Physicist Satish Gupta and GIS Specialist Drew Kessler have determined that most of the sediment comes from stream bank erosion.

Their results have been published in a report titled “Natural vs. Anthropogenic Factors Affecting Sediment Production and Transport from the Minnesota River Basin to Lake Pepin.”

Depending on the set of rivers, anywhere from 56 to 86 percent of the measured sediment load is generated from river banks, according to the paper. The research team found little evidence that agriculture is a major culprit in the issue of the cloudiness of these waters.

The findings do not rule out human actions when it comes to factors influencing increased sedimentation in Lake Pepin, however. In particular, are engineering measures (actions) that straightened the river, dredged channels for river transportation, placed dams for raising water levels, and increased area under impervious surfaces especially near the Twin Cities metropolitan area (now upwards of 30 percent consists of surfaces more or less impervious—driveways, roads, parking facilities, malls, and roofs to mention the most common). All these actions have undoubtedly increased the volume of water and the flowing energy in these water courses. With larger volumes of water flowing directly into streams and less water being absorbed in the ground, Gupta hypothesizes that the stream bank erosion as well as downstream transport has been accelerated.

Gupta’s research has raised serious questions about the findings of other scientists that the apparent increased rate of sedimentation in Lake Pepin, based on core samples, can be attributed to agriculture.

The report by Gupta states: The period between 1940-present has been shown to have the largest increase in sedimentation rates in Lake Pepin. It appears that higher sedimentation rates in Lake Pepin from 1940-present may be attributable to a combination of sediment production and sediment transport factors i.e. (1) increased precipitation resulting in more bank failure as well as in more lateral migration of the tributaries resulting in more sediment production, and (2) transport changes including dredging, widening, and straightening of the Minnesota River channel; increased impervious surfaces; and construction of levees along the main channel and the tributaries resulting in increased water and sediment transport.

Another question raised by the Gupta research is the physics of sediment deposits in Lake Pepin because of a delta in the Mississippi River that is moving downstream and shrinking lake volume. A delta–where the confined waters of a river open into a wider expanse, causes sediments to drop out as the current slows. As the length or volume of the lake shrinks, a constant amount of sediment would result in an increasing rate of sedimentation, the annual layer of deposition becomes thicker in any given spot.

“We simply cannot say with certainty anything about the rate of sediment delivery until this phenomenon is more clearly understood,” said Gupta.

Imagine a simple experiment in which the same amount of chocolate syrup is poured into a large tumbler and a narrow vial. Take a depth reading in both containers with a toothpick, measuring how far up the side of the toothpick the chocolate syrup marks. Such a ‘core sample’ would lead one to conclude that the small vial appears to have more syrup. Likewise, as the delta moves down stream and the northern reaches of Pepin fills and narrows the lake, the thickness of sedimentation layers as determined by core samples, would increase, but not necessarily reveal with any accuracy a change in the rate of sediment being carried by the water.

The Gupta report notes that Lake Pepin has been filling continually since the retreat of the last glacier a little more than 11,000 years ago. At that time, the northern shore of Pepin was in Saint Paul. Since then it has slowly, steadily moved downriver and is now about 45 miles south of the Cities, outside the city of Red Wing. Needless to say, this process began long before modern agriculture or agricultural drainage was put in place. 

“Since sediment cores taken from Lake Pepin are a repository of many effects, we further conclude that lake cores data, by themselves, are insufficient to single out sources (fields or banks), physical processes (bank failure or river migration), or agricultural management practice (cultivation or drainage) as the cause of recent increased sedimentation, especially from a large basin such as the Minnesota River Basin. We suggest that regulatory agencies undertake focused research on developing techniques that can more accurately measure the impact of channel modifications, impervious surfaces, climate variation, natural landscape processes (seepage and lateral channel movement), and the migrating river delta on lake cores data to quantify the role of landscape modifications (cultivation) and agricultural drainage on sediment production in the Minnesota River Basin.”

A major part of the Gupta project involved LiDAR (light detection and ranging) — equipment mounted in the belly of an aircraft. These are lasers used to make very precise measurements on the elevation of the land surface. Comparisons of scans over time has revealed just how much river bank slumping is going on–it is extensive.

Some scientists have suggested that most of the sediment is generated by agricultural activity, but, according to Gupta, basic physical laws raise challenges to this assertion. First, the landscape of the Greater Blue Earth River Basin is incredibly flat–one-half of the landscape is less than two percent slope, and almost none of the topography is greater than six percent slope.

The amount of sediment is beyond question. Gupta’s report states that approximately 625,000 tons per year of total suspended solids are transported by the Minnesota River at Fort Snelling. United States Geological Survey studies show sediment loads in the Minnesota River at Mankato range from 0.2 to 3.6 million tons. Over 55% of these sediments and 46% of the water flow in the Minnesota River at Mankato originates from the Greater Blue Earth River Basin, a relatively flat area with 54% of the land less than 2% slope and 93% of the land less than 6% slope.

The question is where does the sediment come from? When the volume of soil in the lost stream banks is calculated using the LiDAR data, it becomes clear that almost all of these sediment volumes must come from the unstable bluffs and stream banks crumbling into river and carried by its water.

The report describes the phenomenon: “Bank failure occurs, not only at the base, but also higher up in the middle and near the top of the bank. Increased wetness in the middle or top of the bank is due to seepage from a perched water table, while river water uptake by capillary action increases the wetness at the base.  Other mechanisms of bank failure include freeze and thaw, wetting and drying, pore water pressure build up, undercutting, and rapid decrease in river water level.”

The Gupta team also found that the timeline of sedimentation does not prove a link between sedimentation and the arrival of European settlers or widespread cultivation. Evidence from the historical record, both scientific and anecdotal, does not support the idea of a sudden change in the condition of the rivers due to agricultural activities.

“As early as 1835, travelers’ logs indicated that the Blue Earth River was ‘loaded with mud’ and was the cause of turbidity of the Minnesota River,” the research team found. Their report goes on to state, “Subsequent writings in 1850s described the Minnesota River at Fort Snelling as ‘turbid’ and as a ‘dirty little creek’. USGS measurements in 1904-1905 showed that turbidity of the Minnesota River at Mankato went as high as 600-800 ppm (equivalent silica concentrations) during spring.”

As the Gupta paper points out, this turn of the 20th century period preceded both deep moldboard plow utilization and widespread cultivation of corn–both of which could elevate erosion without proper erosion control methods. Further, corn at the time was grown in three-to-five year rotations with other crops–a practice not as prone to erosion as continuous corn done without benefit of conservation tillage techniques.

“Some people believe that additional water from drained agricultural land is increasing river flows and contributing to sediment production,” Gupta said “Our data indicates that’s probably not true. We demonstrate the soil in the stream banks don’t have much strength. Water in the basin comes mostly from precipitation. That’s a natural fact. Soil properties don’t change radically in the course of a few hundred years–this is another natural factor. A little more water (from tile drainage systems on farmland) is not the source of half a million tons of sediment coming from each of the Blue Earth and the Le Sueur Rivers.”

Prior to the turn of the 20th Century, a sandbar existed at the mouth of the Minnesota River, where it joins the Mississippi. It allowed little more than 18 inches depth of water to flow out of the Minnesota. Gupta’s team hypothesize that previous to the dredging of that sandbar to allow river navigation, the slow moving Minnesota River dropped a great deal of its sediment before joining the Mississippi. Sediment cores taken during the development of highway bridges near the confluence of the rivers show layers of fine sediment to 160 feet depth. This supports the idea that stream bank erosion and high sediment load in the Minnesota River has been going on since the retreat of the glaciers.

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