QUANTIFYING SEDIMENT TRANSPORT IN MODIFIED STREAMS IN THE UPPER MACKINAW RIVER, IL

Stream modification, the widening, deepening, straightening, and alteration of channel geometry, is a common practice in central Illinois. Stream modification helps to drain water from fields to enhance agricultural production. Natural stream restoration begins immediately following stream modification as sediment is deposited and the stream reverts to a natural form. The objective of this work was to study how sediment transport is affected by stream modification and to determine what implications this will have on the hyporheic zone, the zone beneath a stream where stream water and groundwater mix. Three stream segments within the Mackinaw River watershed (IL) that have been modified at different intervals were chosen to assess how sediment transport varies as streams return to a natural form. Frog Alley (FA) was modified 11 years ago, Bray Creek (BC) was modified 34 years ago, and Crooked Creek (CC) has not been modified. Scour markers were installed along a 100 meter stretch at each stream to measure the amount of sediment deposited or scoured away. Suspended sediment samples were collected and suspended sediment concentrations were calculated from the samples. Stream cross sections were measured to examine how stream geometries were changing. The results from the scour markers indicate that the more recently modified streams have lower amounts of scour and sedimentation due to a lack of sediment in the stream. FA had the lowest amount of scour and sedimentation, BC had slightly more scour and sedimentation, and CC had the highest levels of scour and sedimentation. The average scour per recording for FA, BC, and CC was 0.143, 0.257, and 0.7 cm respectively. The average sedimentation per recording for FA, BC, and CC was 0.086, 0.071, and 0.6 cm respectively. The suspended sediment concentrations indicated that FA had less suspended sediment than BC and CC. Even though modified streams have higher energy stream flow, sediment deposition and scour is more closely related to available sediment in a stream rather than the strength of stream flow.