Written by Jonathan Eisenthal
Knowing where sediment comes from will ultimately help us devise ways to reduce the amount of sediment that impacts the biological health and recreational value of Minnesota’s waterways.
For the past five years, Patrick Belmont, a watershed scientist from Utah State University, has worked to ‘fingerprint’ the sediment in the Root River Watershed, a system of rivers and streams draining an area the size of Delaware in southeast Minnesota. Minnesota Department of Agriculture and Minnesota Corn Research & Promotion Council funded Belmont’s research.
Using calculations of the rate of decay of a lead isotope that naturally occurs in soil, researchers could determine the source of soil particles. Samples taken from throughout the watershed and at the mouth of the Root River (where it joins the Mississippi), found that 56 percent of the sediment in the system comes from forested hill slopes and stream banks, while the other 46 percent was washed from agricultural fields in the last 20-40 years.
“Many farmers are using buffers, grassed waterways and other best management practices. Good land management has helped, but there is more that can be done,” Belmont told a group of environmental agency workers and scientists at a meeting in Preston on Monday.
His research showed that 280,000 tons of sediment are carried away each year in the Root River. The system of winding stream banks and channels also temporarily stores twice as much soil. About 500,000 tons of sediment move from one spot to another within the watershed. Cutoffs, also called ox-bow lakes, serve as a major ‘sink’ that collects sediment. But floods can flush that sediment and put it right back into the watercourse.
The volume of the water in the system is a part of what creates the energy that erodes stream banks, Belmont said. Sub-surface drainage systems on farms, or tile drainage, has added to that volume in the last few decades.
Belmont said farmers don’t need to do away with tile lines, “They serve a very important economic function, allowing farmers to use agricultural land more productively.”
However, the water flowing from the land needs to be slowed down and retained, in order to reduce sedimentation.
“Targeted water retention should become the focus for watershed managers and farmers engaging in conservation best management practices,” said Belmont. “Slowing the flow of water, and holding more water on the land in wetlands and detention ponds would have a very positive effect.”
Water holding features would solve a number of problems at once, according to Belmont. Reducing the sediment flowing into streams would prevent some of the phosphorous loading that occurs. The vegetation in ponds and wetlands would act as a sink for nitrogen fertilizer that leaves farm fields in runoff.
Among the other findings of the research:
- In the tested subwatersheds, 7 percent of the annual precipitation leaves the fields as runoff.
- About half of all runoff occurs when the ground is frozen—much of the dissolved phosphorous is lost at this time. Incorporating fertilizer into the ground instead of spreading it on the surface could prevent this loss.
- 50 percent of annual nutrient and sediment losses take place during the one or two biggest rain events each year. May and June are the most dangerous times for runoff risk.