CHAMPAIGN, Ill., 1/14/20: State Hydrologist Laura Keefer has waited her entire career at the Illinois State Water Survey (ISWS) to be able to talk about trends in the amount of sediment carried in Illinois’ rivers and streams. That’s because it takes many years of data to glean some answers.
Her results: the news looks promising, as noted in the new ISWS Report of Investigation 124, “Benchmark Sediment Monitoring Program for Illinois Streams: Analysis of 36 Years of Sediment Data.”
Excessive sediment is a water quality issue affecting the environment and the state’s pocketbook. Shipping lanes and reservoirs must be dredged periodically when sediment becomes too thick, and fish habitats can become unhealthy.
In the 1970s, state and federal agencies recognized that the loss of cropland soil productivity was caused by erosion and that a long-term statewide monitoring network was needed.
In 1980, the ISWS initiated the Benchmark Sediment Monitoring Program to develop a long-term sediment network with samples taken weekly at stations around the state. This was a coordinated effort with several state agencies to build a database of sufficient breadth, in time and space, to facilitate research, and to produce reliable information for land and water resource planning.
At a national scale, the 1985 Farm Bill established the Conservation Reserve Program with agricultural practices to help reduce land erosion, improve water quality, and protect wildlife.
“The strength of this database is its frequency of samples and duration,” Keefer said. “We needed a long-term dataset to know what’s going on. You can’t just sample for a year or two because sediment doesn’t work that way.”
Keefer likened sediment movement to that of a conveyor belt, in which the sediment is carried and then dropped repeatedly, mainly by wind, rainstorms, and water flow. It can take years for sediment to move from the hillside into a stream and then to a river.
Every week, rain or shine, a citizen observer from each sediment monitoring network station navigates bridges to take a water sample of suspended sediment, which is sediment that is floating in water near the surface and above the muddy current at the stream bottom.
The suspended sediment concentration data are used to compute sediment loads. This is done using streamflow data from U.S. Geological Survey gaging stations where ISWS Benchmark Sediment Monitoring Network stations are co-located.
Sediment loads are divided by the area of the land that is draining to that station to get sediment yield. This allows a comparison of sediment between stations.
Using data from 1981 to 2016 at 14 stations, the scientists computed annual discharge (streamflow), annual mean sediment concentration, and annual sediment load. There were no trends for annual discharge at all 14 stations.
The annual mean sediment concentrations had no trend for most stations, but decreased at a few, showing that from 1980 to 2016, there was either no change detected or the concentration slightly improved. Similar results occurred for annual sediment load.
“If the streamflow stays steady or has no trend, but some stations have a decrease in sediment concentrations, as in this case, it means that the sediment in the system may not be as available, which supports the argument that something good may be happening on the land,” Keefer said.
The greatest long-term sediment yields, 36-year total loads per square mile, are mostly concentrated along the western portion of the Illinois, along the Mississippi River.
Although the information doesn’t explain the reason for the results, Keefer believes that watershed land use and conservation programs that aim to slow erosion and sedimentation are having an effect. The ISWS is performing further analysis to detect trends between years and seasons and combining information from other research databases.
“It’s taken several decades to get the data needed to say something about the sediment in Illinois,” Keefer said. “We have this rich database because of the forward-thinking, fortitude, and will power of staff at the ISWS to keep it going year after year.”
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Media contact: Laura Keefer, 217-333-3468, lkeefer@illinois.edu
Tricia Barker, Associate Director for Strategic Communications, 217-300-2327, tlbarker@illinois.edu
