Skip to main content
ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Agroecosystems Management Research » Research » Publications at this Location » Publication #334584
Title: Stream bank and sediment movement associated with 2008 flooding, South Fork Iowa River

Author
item Tomer, Mark
item Van Horn, Jessica

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2017
Publication Date: 3/1/2018
Citation: Tomer, M.D., Van Horn, J.D. 2018. Stream bank and sediment movement associated with 2008 flooding, South Fork Iowa River. Journal of Soil and Water Conservation. 73(2):97-106.

Interpretive Summary: Stream bank erosion can cause substantial damage to riparian systems and impact the use of water downstream. Risks of bank erosion increase during extreme flood events, and frequencies of extreme events may be increasing. We assessed bank erosion within the South Fork Iowa River (SFIR) watershed caused by 2008 flooding, which set flood stage records in a number of eastern Iowa streams. We mapped areas of channel movement and estimated volumes and masses of erosion and deposition using high-resolution topographic data. We found that along three upper streams, the 2008 floods widened the channels by 1.7 to 3.5 ft, and with about 21 acres of land lost to the channels. This totaled 4.1 million cubic ft of bank erosion, or an average 1.1 tons for every yard of stream length. Below the Tipton Cr. confluence, impacts increased and the SFIR was widened by 14.5 ft along a length of 6.1 miles, with about 8.1 tons of sediment lost per yard of stream length. While the 2008 floods substantially altered these channels, we found evidence that riparian areas in permanent vegetation had less bank erosion along the smaller channels. Riparian management systems may benefit the maintenance of stream corridors even under extreme flooding. This information is of interest to the conservation community and those interested in reducing impacts of climate change and extreme events on surface waters and aquatic ecosystems.

Technical Abstract: Stream bank erosion can cause substantial damage to riparian systems and impact the use of water downstream. Risks of bank erosion increase during extreme flood events, and frequencies of extreme events may be increasing under changing climate. We assessed bank erosion within the South Fork Iowa River (SFIR) watershed caused by 2008 flooding, which set flood stage records in a number of eastern Iowa streams. Bank positions before and after 2008 were mapped using rectified aerial infrared imagery and overlaid. Differences in stream bank lines were mapped as polygons; those <4 m wide were deleted. A high-resolution (2-m grid) elevation model was used to map elevation changes associated with both erosional and depositional polygons. We estimated that along three upper streams (SFIR, Tipton and Beaver Creeks, 20,000-36,500 ha), the 2008 floods widened the channels by 0.5 to 1.1 m, and with about 8.5 ha of land being lost to become part of these channels. The volume and mass losses associated with this movement totaled 117,000 m3, which comprised on average 1.1 Mg m-1 of stream length. Below the Tipton Cr. confluence, the middle SFIR (58,500 ha) was widened by more than 4.4 m, with about 8.1 Mg of sediment lost per m of stream. The 2008 floods substantially altered these channels, but we found evidence that riparian areas in permanent vegetation had less bank erosion along the smaller channels. Riparian management systems may benefit the maintenance of stream corridors even under extreme flooding.