Fall 2015

Mentor: Dr. Jonathan Phillips

Quantifying Sediment Storage in the Red River Gorge Geological Area, Ky using Field Methods

Introduction

Involvement in recreational activities is growing in popularity. In 1999, a national survey revealed that hiking was the most popular form of outdoor recreation engaged by 67% of Americans (Cordell, 1999). In 2012, the outdoor recreation economy grew 5% annually between 2005-2011, a time of economic recession when many industries contracted. The importance of understanding the connection of land use and presence in the environment is increasing as the efforts to preserve forested and wild lands are impacted by encroaching development and rising tourism. Characterized by its geology and sandstone arches, Red River Gorge Geological Area is internationally recognized for its rock climbing opportunities. Red River Gorge is a National Natural Landmark and Historic District of Daniel Boone National Forest, comprising several congressional designations including a National Wild and Scenic River, National Scenic Byway, and Wilderness. Described by the geographic distribution of geologic formations, the overall rugged terrain and erodible shale and siltstone units support high levels of natural erosion without human disturbance. The purpose of this project was to estimate the bed load sediment storage of point bars (Figure 4) along the most popular stretch of the river and be able to make interpretations on the intensity unregulated hiking and camping activities have on the river’s channel stability and erosion.

Methods

The site for the study was located in the Gladie Creek sub-watershed, along a 3 kilometer stretch of the river in the most popular and visited part of the forest given the proximity to KY 77, Sheltowee Trace trail, Jump Rock swimming hole, and Gladie Visitor Center (Figure 7). The site was selected in an effort to portray large local sediment storages as being a function of visitor occupancy. Determining bed load sediment storage of the river was done by measuring the length, mean width, and approximate height above the channel bed on point bar exposures along the banks (Figure 1). In addition to these notes, the presence of a log jam was noted if it appeared support causation of the point bar formation (Figure 2). This data was later compared to the daily mean discharge values provided by the USGS Red River Gage Site, approximately 27 miles upstream (horizontal distance of 9 miles) to account for the extent flow related point bars are exposed. For both data collection periods, the stretch of river covered was approximately equal in upstream and downstream distance of the Sheltowee Trace suspension bridge.

Results

A range of grains sizes from silt to large gravels 4-6 inches in diameter are evident along all parts of the studied portion of river, representing a sediment transport capacity that frequently varies as a function of stream discharge. Numerous log jams are evident along the river, collecting other woody debris and diverting flow and providing an environment for sediment accumulation. Also along the river are several extensive displays of tree root exposure (Figure 5), soil creep, and recent slope failures (Figure 3). For both data collection periods, the stretch of river covered was approximately equal in upstream and downstream distance of the Sheltowee Trace suspension bridge. Surface area of point bars during the weekend of low discharge expressed a cumulative 31,983 sq. ft., while the higher discharge data collection period was displayed a meager 11,749 sq. ft. surface area. Upstream values characterized by the 41 cfs mean daily discharge represented 215,257 ft^3 of sediment storage, while downstream values characterized by 6.1 cfs of discharge represented 242,200 ft^3. Volume of sediment for the upstream segment does not include Site 13, as it was determined to be a low lying floodplain given the large amounts of shrubby vegetation.

Discussion

The resulting data and the surrounding steep topography indicates a sensitive relationship between discharge and sediment storage exposure, a product of the wide channel planform. Further, rare intermittent periods of high or peak discharge that occupy greater portions of channel width erode the bank sides intensively promoting bank failures, transmitting large woody debris and accumulating large volumes of sediment settle in point bars and along the bed as the river returns to lower mean discharges. Bank slope instability is caused by soil compaction from heavy trail and campsite visitation. Soil compaction, especially along river banks, extremely degrades slope stability by damaging riparian vegetation that requires less compacted soils to support the root structure that ultimately preserves and stabilizes channel widening.  

Inaccuracy of the data presented can be attributed to difficulty in estimating river depth at each site and constrained data given by the short segment of river measured, both of which are due to lack of proper equipment for river navigation. The season during which field work occurred also limited amount of data by the duration of daylight. Discerning between river transported sediment and slope failure present at a site was difficult. While they are both representations of upslope sediment sources, the study emphasizes the importance of sediment transport capacity and sediment storage that has been suspended and deposited by the river. During the course of the study, the method for measuring storage height was questioned on the basis that sediment (belonging to a point bar) along the wall of the current flow, or beneath the water surface should serve as a feature belonging to the cross-sectional channel form instead of being included into the height measurement. This doubt of sediment function was abandoned once it was realized that sediment belonging to a storage body or channel bed are both river transported deposits and products of channel aggradation.

Conclusion

Intensity of erosion and sedimentation along the stream will decrease with increasing distance from hiking trails and the KY 77 scenic byway, as use and frequency of visitation are strongly correlated with accessibility (Figure 6). Given the observed topographic and geologic relationships of the region as well as measurements of bed load sediment storage and erosional features, the Red River evaluated in the study exhibits widening and aggradation, two stages of channel adjustment. And finally, despite the data being collected from a restricted 3 km stretch of the river, there are no major local variations in geology or land use giving rise to the generalizations made about erosional impacts of hiking trails are likely to be relevant to all portions of the river in proximity to recreational practice.