Richard Fetterly is a retired engineer living in Rice County. He has been a citizen stream monitor for the Minnesota Pollution Control Agency for more than 15 years. He reflects on his time spent monitoring the Straight River. In his words...
On a Clear Day You Can See Forever is the name of a song from the movie by the same name. Actually, according to the U.S. EPA, on a clear day, the visibility is about 15 to 90 miles, depending on the amount of air pollution.
In clear water, the molecules that refract and absorb light are much closer together so we can only see about 200 meters under perfect conditions. The ocean has some undissolved impurities called “turbidity” but the visibility is still in the 50 to 70 meter range. Southern Minnesota rivers, in contrast, can be extremely variable in their clarity due to impurities from runoff and erosion. On the high side, the water can be very clear, but probably never quite as clear as the ocean. On the low side, clarity (visibility) of 8 centimeters (cm) or less is not uncommon and less than 2 centimeters is not unheard of.
The Minnesota Pollution Control Agency monitors the clarity of rivers using both staff and volunteer monitors. Clarity or “transparency” readings are taken using a clear plastic tube called a Secchi tube. The tube is one meter in length and 4 centimeters in diameter, and has a disc that can be lowered into the tube on a string. If the water is relatively clear, the Secchi disc can be seen all the way to the bottom of the tube. Although there is no exact formula, the relationship between the transparency reading and the actual amount of suspended solids (dirt or soil) can be estimated using historical data.
In the summer of 2016, a study was done to determine the amount of soil that was lost due to erosion in Steele County, Minnesota. The Straight River leaves Steele County north of Medford (note that there are other “Straight” rivers in Minnesota). At this point, it drains nearly the entire county, but very little land outside of the county. By analyzing the water at this point, a gross estimate of the soil leaving the county can be determined. The estimated daily amount of soil loss was determined by converting the Secchi tube reading to a suspended solids estimate (parts per million by weight) and multiplying by the stream flow volume. The stream flow volume was taken from a USGS stream gage a few miles downstream of the Steele county line.
In order to better visualize the amount of soil loss, the quantity was converted to truckloads. A truckload for this study was considered to be 10 cubic yards. A cubic yard of dirt was considered to weigh 2,500 pounds. Readings were taken on 86 of the 152 days between May 2 and September 30. Generally, on days not recorded, the water was low and clear so there was little sediment (turbidity). The period from May 2 to July 6 also had little rain and little sediment. The lowest Secchi reading was 14 cm on June 9, resulting in a loss of about 7 truckloads of soil. The total loss for this first part of the summer was about 35 truckloads.
The second half of the summer, from June 7 through September 20, had significantly more rainfall. Secchi readings were generally below 50 cm, with two readings at 8 cm and two at 6 cm. The total soil loss for this period was about 478 truckloads.
Southern Minnesota had a serious rainfall event starting the night of September 20, which caused severe flooding throughout the region including most of Iowa and other areas. The Secchi reading on September 21 was 8 cm; it fell to 6 cm the next day. The stream flow was 7,500 cubic feet per second — that means 358 truckloads of soil went down the river. This single rainfall event caused a total soil loss of about 690 truckloads over the high water period of about 10 days.
In total, Steele County lost more than 1,200 truckloads of soil over the summer.
This is only one county and represents only a very small percentage of the soil erosion in Minnesota. Most of the sediment from the Straight River probably settles out in Lake Byllesby near Cannon Falls. What does not settle out there is added to the Lake Pepin sediment load. Lake Pepin sediment is estimated to be in the 800,000 ton-per-year range. In our truckload analogy, this would be 64,000 truckloads, or a line of trucks 300 miles long.
In addition to the direct damage from flooding, there are also many other factors not addressed here such as chemicals, pathogens, sewage, and other pollutants. Many of these pollutants are dissolved and do not settle out. They combine to alter the aquatic food chain of river systems and contribute to the creation of the dead zone in the Gulf of Mexico.
The state of Minnesota, counties, cities, environmental organizations, landowners, and citizens are becoming more aware and are working to solve this problem. By supporting this effort, we can create the environment that we want to pass along to future generations of both people and wildlife.
Support your local planet.
To find out more and become a citizen monitor, visit https://www.pca.state.mn.us/cmp/enroll.