Streambank Erosion and Restoration

Streambank Erosion in the Illinois River Watershed

In the Illinois River Watershed, many farmers, ranchers, and families are losing acres of land every year from streambank erosion; the estimated loss of productive land in our watershed is 20 acres each year. This loss in productive and valuable land results in financial losses to landowners today as well as future generations. Here at IRWP, we are monitoring streambank erosion and providing grants and technical assistance to landowners to reduce erosion and improve land productivity.

Streambank Erosion Assessment Study

Illinois River and Tributaries:  Streambank Erosion Sites and Analysis 2020 Report 

Since 2017, the Illinois River Watershed Partnership (IRWP) and partners have monitored streambank erosion in the Upper Illinois River Watershed. Through multi-year field measurements and analysis at 15 sites representing around 5% of the watershed, we find that erosion is projected to contribute 102,822 tons of sediment and 154,233 lbs of phosphorus annually into the watershed. Observed streambank erosion is driven primarily by changing land-use as well as increasing precipitation.

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Illinois River and Tributaries:  Streambank Erosion Sites and Analysis  2020 Report  - Brief

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Background

Precipitation has increased and flooding is increasing in frequency and volume over the last decade in much of the watershed. IRWP and partners recognized the need to regularly monitor how the stream system is responding to landscape changes including rapid urbanization and population growth. Annual surveys on 49 miles of streambank were conducted in consecutive years spanning from 2017-2020. Utilizing established methods, we measured yearly erosion rates and calculated Near Bank Shear Stress (NBSS) at each site to develop a graphical prediction of annual streambank erosion rates. 

Findings

Streambanks in the upper Illinois River Watershed have eroded on average 3.88 feet of bank annually. Of the 15 study sites, eight have High, Very High or Extreme NBSS which means that high velocity water is concentrated along the streambank during high flows. Erosion is very likely to continue at accelerated rates at these locations. Based on modeling developed from these field measurements, erosion rates throughout the Upper Illinois River Watershed are projected to be 1.01 feet per year across the watershed. Observed streambank erosion is driven by a combination of factors including natural processes, changing precipitation, urban stormwater runoff, deforestation of the riparian corridor, construction in the floodplain, past attempts to alter the stream channel, debris jams, and gravel deposits from upstream bank erosion. Targeting restoration measures that reduce erosion at the most extreme sites could have a measurable positive impact on the watershed. In fact, restoring just the 4 highest priority banks of the 15 study sites could reduce the total sediment loading within the 49-mile study area by 11%.

This image shows a cross-section of Muddy Fork, a tributary of the Illinois River, where erosion has left a steep cut bank. The bank and the vegetation are sheared away over time from the volume and velocity of water hitting the channel during rain events.

Conclusion

This research demonstrates the need to address the primary drivers of streambank erosion by developing more comprehensive stormwater management solutions, and maintaining generous riparian buffers in both headwaters and downstream. Maintaining and restoring an extensive riparian and floodplain buffer of native deep-rooted vegetation is critical to the long-term stability of streambanks throughout the watershed. It is our hope that this study will help inform planning as we strive to understand, protect, and restore the water quality and habitat within the Illinois River watershed for generations to come. 

According to data collected by IRWP, the average landowner lost 3.8 feet of land annually since the study started and approximately 29% of streambanks will continue to lose at least one foot per year throughout the Upper Illinois River Watershed. These data indicate that we are in the middle of system-wide change that will not only impact the river, but may also impact the economic development potential of Northwest Arkansas as we expand westward into areas of the watershed that are at greater risk for flooding and erosion.

All rivers, creeks, and streams move and erode, but the question is how much of that erosion--and subsequent movement-- is a result of increased water velocity and volume that is preventable. Erosion creates a snowball effect: Loss of vegetation further weakens the banks, making even small amounts of stormwater destructive, loading even more sediment. Trees are wiped out and their branches collect downstream, causing further disruption. With little or no streamside vegetation and with so few roots to keep the soil on the banks in place, the water is exposed to direct sunlight, increasing the likelihood of algae blooms, pathogenic breeding grounds, and subsequently creating safety issues for animals and people. Infrastructure also becomes threatened by the increase in volume and velocity of stormwater, creating public safety hazards and costing landowners and municipalities tax dollars.

Resource Hub

Streambank Erosion and Restoration Video Learning Center

Streambank Erosion: Many of us assume it is a natural process for any river or stream. But that eroded streambank or moving river channel may be due to upstream land use changes. This series of resources discusses why it's happening and what you can do about it. 

Overview of Streambank Erosion and Restoration

Natural Stream Restoration: Streams in Nature (Part I)

Natural Stream Restoration: Good Stream Gone Bad (Part II)

Natural Stream Restoration: Restoring Streams (Part III)

 

Streambank Stabilization and Restoration Case Studies

Why Should You Stabilize a Streambank

Stream Restoration - Illinois EPA

Streambank Restoration Timelapse

Streambank Stabilization- Giving a Stream a Place to Flood

Using Rootballs to Stabilize an Eroding Streambank