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United States Department of Agriculture

Agricultural Research Service

Research Project: WATER QUALITY IMPROVEMENT FROM MANAGEMENT PRACTICES IN AGRICULTURAL WATERSHEDS Title: Floodplain Sediment Accretion and Stream-course Straightening Along the South Fork of the Iowa River

Authors
item Tomer, Mark
item Yan, B -
item James, David
item Cole, Kevin

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: June 16, 2009
Publication Date: September 16, 2009
Citation: Tomer, M.D., Yan, B., James, D.E., Cole, K.J. 2009. Floodplain Sediment Accretion and Stream-course Straightening Along the South Fork of the Iowa River [abstract]. In: Dust Bowl to Mud Bowl: Sedimentation, Conservation Measures and the Future of Reservoirs. p. 49.

Technical Abstract: River valleys have been influenced by sediment derived from agricultural erosion and channel straightening intended to hasten flood routing. Post-settlement alluvium (PSA) has not been documented in tile-drained areas of the upper Midwest where agricultural settlement is relatively recent (<150 yrs). This study investigated changes to the river channel and PSA accumulation along the South Fork (SF) of the Iowa River. Channels of the SF and tributary Tipton (TC) were digitized using aerial photographs from 1939 and 2002. Soil cores were collected along cross-valley transects to determine PSA extent and thickness. Within 80 m of the SF, PSA averaged 0.78 m thick and 85% frequency of occurrence. Beyond 80 m, PSA occurred half the time and averaged 0.32 m thick. Within 43 m of TC, PSA averaging 0.58 m thick occurred with 75% frequency. We estimate >9.2x106 Mg (>10.2x106 tons) of PSA is stored along these two valleys, representing 156.6 Mg/ha (69.8 t/ac) of soil eroded from uplands since settlement. This PSA has reduced the valley’s flood-storage capacity by >5.1x106 m3 (4123 ac-ft), equivalent to 11 mm (0.44 in) runoff. Channel straightening reduced stream length by up to 15%, hastening routing of discharge. We interpret that flood hydrographs under the current regime are exacerbated by up to 4123 ac-ft compared to pre-settlement conditions, simply based on PSA accretion, without considering effects of channel straightening or of land-cover changes associated with agricultural settlement. River restoration should be conducted recognizing the nature of these changes and how fluvial processes respond to them.

Last Modified: 8/30/2014
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