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