Page 198 - Fundamentals of Geomorphology

HILLSLOPES 181

constructing log landings, and constructing logging Uphill Downhill
roads (Johns et al. 1996). 100
Thenatureoftrail useaffectsthedegreeofsoilerosion. cent)
The comparative impact of hikers, horses, motorcycles,
and off-road bicycles on water runoff and sediment yield (per 50
was investigated on two trails – the Emerald Lake Trail ground
and the New World Gulch Trail – in, and just outside, Motorcycle
Horse
respectively, the Gallatin National Forest, Montana, USA Bare Hiker
(Wilson and Seney 1994). The results revealed the com- 0
plex interactions that occur between topographic, soil, 90
and geomorphic variables, and the difficulty of interpret-
ing their impact on existing trails. In brief, horses and (cm) 60
hikers (hooves and feet) made more sediment available width
than wheels (motorcycles and off-road bicycles), with 30
horses producing the most sediment, and sediment pro- Trail
duction was greater on pre-wetted trails. In the northern
Rocky Mountains, Montana, USA, trails across meadow 0
15
vegetation bear signs of damage – bare soil and eroded
areas – through human use (Weaver and Dale 1978).
The meadows were principally Idaho fescue–Kentucky (cm) 10
bluegrass (Festuca idahoensis–Poa pratensis) communi- depth
ties. Experiments were run on meadows underlain by 5
deep sandy-loam soils at 2,070 m near Battle Ridge Trail
US Forest Ranger Service Station, in the Bridge Range.
They involved getting hikers, horse riders, and a motor- 0 0 0
cyclist to pass up and down slopes of 15 . The hikers 500 1,000 500 1,000
◦
weighed 82–91 kg and wore hiking boots with cleated Number of passes
soles; the horses weighed 500–79 kg and had uncleated
shoes; the motorcycle was a Honda 90 running in second Figure 7.10 Experimental damage done by hikers, bikers,
gear at speeds below 20 km/hr. The experiments showed and horses moving uphill and downhill on trails in Bridge
◦
that horses and motorcycles do more damage (as mea- Range, Montana, on a sloping 15 meadow site.
sured by per-cent-bare area, trail width, and trail depth) Source: Adapted from Weaver and Dale (1978)
on these trails than do hikers (Figure 7.10). Hikers,
horses, and motorcycles all do more damage on sloping
ground than on level ground. Hikers cause their greatest slope units, which may be slope segments (with a roughly
damage going downhill. Horses do more damage going constant gradient) or slope elements (with a roughly con-
uphill than downhill, but the difference is not that big. stant curvature). A common sequence of slope elements,
Motorcycles do much damage going downhill and uphill, starting at the hilltop, is convex–straight–concave.These
but cut deep trails when going uphill. elements form a geomorphic catena. Different geomor-
phic processes dominate different slope elements along
a catena. Landform elements are basic units of the two-
SUMMARY dimensional land surface. Properties such as slope angle,
slope curvature, and aspect define them. Land-surface
Hillslopes are the commonest landform. There are bare form is also the basis of landform classification schemes.
and soil-mantled varieties. A hillslope profile consists of Geomorphic processes that transport material over and