CHANNEL MORPHOLOGY IN THE CONTEXT OF LARGE-SCALE LANDSCAPE RESTORATION IN CENTRAL IOWA, USA

Since 1991, hundreds of hectares of former row-crop agricultural land have been restored to native tallgrass prairie within central Iowa’s Neal Smith National Wildlife Refuge. Bisecting the Refuge is Walnut Creek, a perennial, third-order stream that drains a 5218 hectare, heavily-altered watershed. Extensive historic land cover change, channelization, and subsurface drainage have increased stream gradient and flow velocities, produced “flashy” hydrographs, and have accelerated streambank and bed erosion. Since the start of restoration, a wide variety of channel morphological data have been collected in association with a number of studies. These data sets provide an opportunity to observe nearly 20 years of channel morphological change in the context of large-scale landscape restoration.

Repeated measurements of channel cross-sectional area, erosive channel length, lateral streambank recession, and streambed sediment storage have occurred between 1998 and 2016. Channel cross-sectional measurements suggest the majority of channel length has continued to degrade and widen over the entire period, with a mean annual degradation of 2.5 m³ m^-1. However, as of 2015, the extreme lower reaches of the watershed have moved into the aggrading and widening stage of channel evolution. Storage of streambed sediment in 2015 was estimated at 0.49 m³ m^-1, a 15% reduction from the 1998 estimate of 0.58 m³ m^-1. Lateral streambank recession rate has exhibited no observable directional trend since restoration, fluctuating within a range of -0.64 (i.e., deposition) to 34.2 cm yr^-1. Similarly, erosive channel length (i.e., % total streambank length classified as severely or very severely erosive via USDA-NRCS protocol) has also not exhibited an observable directional trend since restoration, fluctuating between 14.5 and 40.2%.

Although large-scale prairie restoration has significantly reduced upland and overland flow contributions of sediment to waterways, reduction in total sediment load exiting the watershed has not been observed. Thus, the reduction in upland sediment contribution may be masked by in-channel sources as the channel continues to respond to historic channelization and hydrologic alteration.