EROSIONAL IMPACT OF HIKERS, HORSES, MOTORCYCLES, AND OFF-ROAD BICYCLES ON MOUNTAIN TRAILS IN MONTANA

This study examined the relative impact of hikers, horses, motorcycles , and off-road bicycles in terms of water runoff and sediment yield fr om 108 sample plots on existing trails in or near Gallatin National Fo rest, Montana. A modified Meeuwig drip-type rainfall simulator was use d to reproduce natural rainstorm events. Treatments of 100 passes were applied to each plot. The results confirmed the complex interactions that occur between topographic, soil, and geomorphic variables noted b y others, and the difficulty of interpreting their impact on existing trails. None of the hypothesized relationships between water runoff an d slope, soil texture, antecedent soil moisture, trail roughness, and soil resistance was statistically significant. Five independent variab les or cross-products explained 42% of the variability in sediment yie ld when soil texture was added as a series of indicator variables. Ten variables combined to explain 70% of the variability in sediment yiel d when trail user was added as a second series of indicator variables. Terms incorporating soil texture (37%), slope (35%), and user treatme nt (35%) accounted for the largest contributions. Multiple comparisons test results showed that horses and hikers (hooves and feet) made mor e sediment available than wheels (motorcycles and off-road bicycles) a nd that this effect was most pronounced on prewetted trails.