Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 3, 2009
Publication Date: January 2, 2010
Citation: Yan, B., Tomer, M.D., James, D.E. 2010. Historical Channel Movement and Sediment Accretion Along the South Fork of the Iowa River. Journal of Soil and Water Conservation. 65(1):1-8. Interpretive Summary: Recent research has shown that sediment derived from soil erosion that occurred during the decades following agricultural settlement has accumulated in river valleys and continues to influence rivers and be a source of current sediment loads. Recent research pointed out unexpected large sediment loads from the South Fork of the Iowa River. This study aimed to determine whether historical sediment can even influence this tile-drained watershed in the upper Midwest, which is relatively flat and has only a short history of settlement (about 160 years). We found that historically accumulated sediment along the South Fork averages 2.6 ft thick and extends laterally to a distance of 260 ft from the channel. This is equivalent to 69.8 tons per acre of soil eroded from the uplands across this 158,000 acre watershed, and reduces the capacity of the floodplain to store floodwater by an estimated 4123 acre-ft. In addition, channel straightening of the South Fork and its tributary Tipton Creek has reduced channel length and hastened routing of water downstream to the Iowa River. Results highlight the impact of historical soil erosion on river conditions today; these impacts can include large sediment loads from bank erosion and exacerbated flooding. River restoration projects need to be conducted recognizing these historical influences on current conditions and processes in our river valleys. Results are of particular interest to water resource managers and aquatic ecologists who are developing and implementing river restoration and watershed management plans.
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 been little documented in tile-drained areas of the upper Midwest, where agricultural settlement began around 1850 and few soils are highly erodible. This study investigated channel movement and PSA accumulation along the South Fork (SF) of the Iowa River. Channels of the SF and tributary Tipton Creek (TC) were digitized using rectified aerial photographs taken in 1939 and 2002. Soil cores were collected along valley-crossing 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 decreased below 50%. Within 43 m of TC, PSA averaging 0.58 m thick occurred with 75% frequency. An estimated 9.2x106 Mg (10.2x106 t) of PSA is stored along these valleys, representing 156.6 Mg/ha (69.8 t/ac) of soil eroded from uplands since settlement. The volume of PSA is equivalent to 11 mm (0.44 in) runoff from the watershed. The valley’s flood-storage capacity has been reduced by 5.1x106 m3 (4123 ac-ft), considering pore space of the PSA. Modern flooding events are accordingly exacerbated by accretion of agricultural sediment, compared to pre-settlement river conditions. Channels were straightened in response to local flood events, which reduced stream length by 10% and hastening routing to the Iowa River. River restoration should be conducted recognizing the nature of these changes, ongoing impacts on hydrology, and how fluvial processes respond to them.