Stream bank and sediment movement associated with 2008 flooding, South Fork Iowa River

Abstract

Stream bank erosion can damage 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 a changing climate. We assessed bank erosion within the South Fork Iowa River (SFIR) watershed caused by 2008 flooding, which set flood stage records in many eastern Iowa streams. Bank positions before and after 2008 were digitized using rectified aerial infrared imagery and overlaid. Differences in stream bank lines were mapped as polygons; those <4 m (13 ft) wide were deleted to eliminate polygons possibly formed by digitizing error. A high-resolution (2 m [6.6 ft] grid) elevation model was used to map elevation changes associated with both erosional and depositional polygons. We estimated that along three streams (SFIR, Tipton Creek, and Beaver Creek) with watershed areas of 20,000 to 36,500 ha (49,000 to 90,150 ac), the 2008 floods widened the channels by 0.5 to 1.1 m (1.6 to 3.6 ft), and about 8.5 ha (21 ac) of land was lost to become part of these channels. The volume and mass losses associated with this movement totaled 117,000 m3 (153,000 yd3), which comprised on average 1.1 Mg m-1 (1 Mg m-1 = 1.01 tn yd-1) of stream length. Below the Tipton Creek confluence, the middle SFIR (58,500 ha [144,500 ac]) was widened by more than 4.4 m (14.4 ft), with about 8.1 Mg (8.9 tn) of sediment lost per meter of stream. The 2008 floods substantially altered these channels, but using images from helicopter flights from late 2008 and 2009, we found evidence that where riparian buffers were kept in ungrazed, grassy vegetation, there was less bank erosion than where grazed pastures lined these channels. Riparian management systems may reduce stream bank erosion even under extreme flooding.