Stream channel responses and soil loss at off-highway vehicle stream crossings in the Ouachita National Forest

Abstract

This study investigates the geomorphic effects of ford-type stream crossings in an off-highway vehicle (OHV) trail complex in the Ouachita National Forest, Arkansas. At a total of 15 crossing sites, we used a disturbed vs. undisturbed study design to assess soil truncation and an upstream vs. downstream design to assess in-channel effects. The 15 sites ranged from OHV crossings active for over 20years to those on trails that have been closed to regular use for >5years. All of the sites designated for OHV use (14 sites) exhibit soil loss of ~30 to 45cm within the trail segments on either side of the crossings. In-channel responses attributable to the crossings were observed at 14 (93%) of the sites and include increased bank erosion, increased mud coatings on coarse channel clasts, increased in-channel fine-sediment accumulations, changes in the size distributions of coarse bed material, and occurrence of large channel-filling sediment plugs. However, while every site but one shows at least one channel impact, only one site exhibits all of them. Despite the relatively homogeneous geographic area sampled, only limited generalizations are evident. Sediment impacts seem to predominate over runoff impacts from the trails. Small channels (basin areas <0.4km2) show greater consistency in their response behavior than larger channels. Where OHV use is currently allowed, downstream increases in mud coatings and sediment deposition features are more common. What seems more certain is that individual effects are strongly contingent on local details of channel and valley geomorphology.