Submitted to: Soil and Water Conservation Society Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 3/1/2009
Publication Date: 7/16/2009
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. Soil and Water Conservation Society Proceedings. Available: http://www.swcs.org
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 the 1930s 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 agricultural 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 10%, decreasing channel storage by another 260,100 m3 (211 ac-ft). We interpret a 1930s runoff hydrograph peaking at bank full would today create 211 ac-ft of floodwater, and an event originally peaking at the 2nd terrace would now be exacerbated by 4334 ac-ft of flood water. River restoration should be conducted recognizing the nature of these changes and how fluvial processes respond to them.