The Minnesota River’s contribution of sediment to the Mississippi’s Lake Pepin has increased more than 12-fold since 1830. The Minnesota flows through an area composed of particularly fine-grained soils, which throughout geological history have been prone to erosion. But it is no accident that this relatively recent 12-fold increase in sedimentation parallels the development of intensive farming in the Minnesota River basin.
Humans can’t control the soil structure of the Minnesota River basin, or the slope of the land on a watershed-wide basis. But, says University of Minnesota stream ecologist Bruce Vondracek, we can change the hydrology of a particular area–the amount of water that flows over and under soil–and at what speed it makes that journey. Studies and anecdotal evidence show that land covered with perennial plants such as grasses, hay crops and trees is much less prone to erosion when compared to acres planted to annual crops such as corn and soybeans. Perennial plant cover slows down the water flow, provides year-around protection from the soil-loosening effects of rainstorms, and gives precipitation a chance to soak into the soil structure. What would happen if perennial plant systems were returned to an agricultural watershed? How much of a change in the landscape would it take to reduce sedimentation to more sustainable levels?
To find the answer to that question, Vondracek and two other researchers studied fish habitat in two Minnesota watersheds: Wells Creek and the Chippewa River. The study was part of the Multiple Benefits of Agriculture: An Economic, Environmental & Social Analysis research initiative.
Wells Creek flows through steep land in southeast Minnesota before draining directly into the Mississippi. The Chippewa flows through the flat former prairies of western Minnesota before dumping its load into the Minnesota River.
The researchers used modeling to predict what would happen to sediment loading in the two watersheds based on four land use scenarios. The scenarios ranged from extension of current farming trends in each watershed (Scenario A: fewer and larger farms, with increased acreage in row crops and the loss of small and medium-sized livestock farms) to conversion of row crop acres to year-round permanent plant cover such as grass, hay and trees (Scenario D). Under this last scenario, land would be rotationally grazed for livestock production, diverse cropping rotations would be implemented to build soil quality, and some prairies and wetlands would be restored. For the modeling study, all land use activities were simulated over a 50-year period (1950 through 1999).
What Vondracek and his colleagues found was that land use changes led to reductions in sediment loading of up to 84 percent in Wells Creek and 49 percent in the Chippewa River. These land use changes also produced other water quality benefits, such as a reduction in the amount of nitrogen and other nutrients leaving the land. (Keeping nitrogen in place could go a long ways toward dealing with the “dead zone” problem in the Gulf of Mexico.) How did the reductions come about? The presence of permanent, year-around vegetation on the land was the key. This has been the main thesis proposed by the multidisciplinary Green Lands, Blue Waters initiative.
By getting more perennial vegetation on the land in the form of grasses, hay crops and trees, water runoff was reduced as much as 35 percent in both watersheds. That meant more water was percolating into the soil and less was rushing to the waterways, carrying soil and other contaminants along the way. Restoring wetlands and other natural areas also helped reduce runoff considerably, according to modeling. The study only looked at sediment coming from farm fields, not the soil that erodes directly from riverbanks. But in theory less water rushing over fields should make for more stable riverbanks.
This is one of the first studies to look at the possible impacts the duration of sediment exposure can have on fish. The study indicated that a flush of huge amounts of suspended sediment during and after a storm event might not have as much of a negative impact on fish health as lower levels of suspended sediment present over a longer period of time. Fish can tolerate relatively high concentrations of sediment for a short time, but if the sediment lingers after a thunderstorm, the tolerance level drops dramatically.The critical factor is that fish become more sensitive the longer they are exposed to suspended sediment.
If the kind of chronic sedimentation that harms fish is to be controlled on a consistent basis, tweaking current farming practices using conservation measures called “best management practices” (BMPs) may not be enough in all watersheds. In Vondracek’s study, when BMPs such as conservation tillage and the establishment of strips of permanent vegetation (called riparian buffers) were used in the Wells Creek watershed, “lethal” concentrations of suspended sediment–levels high enough to kill fish–went down an astounding 63 percent. However, in the Chippewa River such practices did not significantly affect the negative impacts sediment levels had on fish. The Chippewa’s soil structure and the extent to which it is being farmed intensively makes reducing its sediment problems tougher, says Vondracek.
So does environmental protection in a place like the Chippewa River watershed mean idling massive tracts of land? Not necessarily, says the biologist. In the 1990s, Vondracek worked on the Monitoring Team, a research initiative that brought together farmers, scientists and government officials. A three-year Monitoring Team study of six farms practicing managed rotational grazing in southeast Minnesota found that this technique can significantly reduce the amount of sediment flowing into a waterway. The study also found that a stream degraded by overgrazing starts to recover as it flows through a rotationally grazed area. I’ve been on several of the farms involved with the Monitoring Team, and I can vouch for Vondracek’s research: the managed grazing systems are producing nice, durable streambanks, cleaner channels and healther streams in general. It’s an example of how sustainable farming methods can not only protect the environment, but actually improve it in a proactive way.
“Through my interaction with the Monitoring Team farmers, I saw how working farmland could have a positive impact on watershed health,” he says.