Depositional response of a headwater stream to channelization, East Central Illinois, USA

Channelization typically modifies the energy regime and sediment transport capacity of rivers, triggering morphologic adjustments. Most past studies of channelization have focused on erosional responses involving channel incision and widening. Depositional adjustments to channelization, although noted in previous work, have not been documented in detail. This study investigates the depositional response of the Spoon River, a headwater agricultural stream in Illinois, USA, to channelization. Historical aerial and ground-based photography show that channelization of the Spoon River in the early 20th century produced a wide, deep trapezoidal drainage channel. Following this channelization, unvegetated alternate and mid-channel bars developed on the bottom of the ditch. Sedimentological analysis of bar stratigraphy indicates that the bars grew through vertical accretion of horizontal sheets of sand-and-gravel bedload and organic-rich drapes of fine-grained suspended load. The horizontal sheets of sand and gravel are consistent with the braided conditions shown on historical photographs. Late-stage bar growth appears to have been dominated by progressive overbank deposition of suspended load as indicated by the presence of a thick, fine-grained organic-rich A horizon immediately below the surface of each bar. The development of a soil layer also suggests that the bars are stable-an inference supported by the thick grass cover on the bar surfaces. The net result of the depositional response is the formation of a meandering channel flanked by a discontinuous floodplain on the bottom of the ditch. The construction of a wide ditch relative to original stream size is a key factor promoting a depositional response to channelization. Allowing or actively promoting floodplain elements to form in overwidened ditches may be a viable management option for improving the environmental quality of channelized agricultural streams. The Spoon River has a diverse fish community compared to channelized streams in East central Illinois that lack a meandering low-flow channel. This morphological configuration apparently enhances geomorphological and ecological variability while sustaining the drainage function of the ditch.