Geomorphic and ecological responses to human modification of the Fremont River, Capitol Reef National Park, Utah

Abstract

Listen

Translate article

This study documents impacts to the Fremont River that were initiated by the construction of Utah State Route 24 in 1962 within what is now Capitol Reef National Park (CARE). The highway and related river modifications led to an artificial abandonment of a 4000 ft long river meander, creation of a channel bed knickpoint (i.e., waterfall), and at least 9 ft of vertical incision into the underlying Navajo Sandstone. Channel incision also appears to have translated upstream, as we found a significant trend of a deeper channel immediately upstream of the knickpoint that transitioned to being wider and shallower upstream. The rate of upstream propagation of these geomorphic changes may be as high as 20 ft/yr, with ongoing and future incision threatening the stream-riparian system further. The knickpoint and its upstream propagation through a knickzone have also affected the Fremont River riparian vegetation community within the less frequently flooded incised reach by shifting it toward more dry-adapted species. In contrast, the channel in the less incised reference reach inundates its floodplain more frequently, has a higher mean vegetation abundance, and supports more wet-adapted species. Upstream propagation of a more deeply incised channel will likely continue through the Navajo Sandstone segment and into the more vulnerable alluvial segment upstream. Reactivation of the abandoned river meander to restore the Fremont River and its riparian corridor will require channel realignment to match elevations between the current and abandoned channels. Additionally, further upstream in a sub-reach where the highway has been repeatedly impacted by overbank flows, direct observations by park staff are supported by our hydraulic modeling indicating that the highway is inundated by flows of between 1000 and 1900 cubic feet per second, which are expected to occur every 2?5 years. Mitigation to improve flood resiliency and to alleviate river-highway conflicts may also be appropriate. Overall, this study?s findings can help inform current river management decisions, state highway corridor planning, and future river restoration efforts in CARE.