Historical channel movement and sediment accretion along the South Fork of the Iowa River

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 of the Iowa River. Channels of the South Fork and tributary Tipton Creek 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 (262 ft) of the South Fork, PSA averaged 0.78 m (30.7 in) thick and 85% frequency of occurrence. Beyond 80 m, PSA decreased below 50%. Within 43 m (141 ft) of Tipton Creek, PSA averaging 0.58 m thick occurred with 75% frequency. An estimated 9.2 × 10⁶ Mg (10.2 × 10⁶ tn) of PSA is stored along these valleys, representing 156.6 Mg ha⁻¹ (69.8 tn ac⁻¹) 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 5.1 × 10⁶ m³ (4,123 ac ft), considering pore space of the PSA. Modern flooding events are accordingly exacerbated by accretion of agricultural sediment, compared to presettlement river conditions. Channels were straightened in response to local flood events, which reduced stream length by 10% and hastened routing to the Iowa River. Design of river restoration projects should take account of fluvial processes and how these processes are responding to historical sedimentation and channel straightening.