Mountain environments - Environmental Sedimentology - page 41

Table 2.4 Pearson correlation
coefficients for mechanical denudation
versus morphometric, hydrological and
climatic variables. (Source: Summerfield
& Hulton 1994.)

Variable Log. mechanical

denudation rate

Morphometric Area − 0.11

Mean trunk channel gradient 0.67

Basin relief 0.80

Relief ratio 0.78

Mean model elevation 0.66

Mean local relief 0.68

Hypsometric integral − 0.03

Hydrological Mean annual runoff 0.45

Runoff variability − 0.04

Climatic Mean annual temperature 0.41

Mean annual precipitation 0.42

Jansson (1988) is typical of this approach in
that the research poses the question, 'How does
the magnitude of erosion vary globally under
present human influence?' The methodological
approach used in this study is a statistical analysis
of sediment yield based on climatic conditions.
No attempt is made to classify rivers in terms
of topography. Alternatively, some studies have
explicitly examined relief as a controlling vari-
able on sediment discharge (e.g. Milliman &
Syvitski 1992), whereas others have used a multi-
variate approach incorporating morphometric,
hydrological and climatic variables (Summerfield
& Hulton 1994). The importance of drainage-
basin topography in influencing mechanical
denudation rates is clearly demonstrated in the
study of Summerfield & Hulton (1994). Results
(Table 2.4) show a relatively strong statistical
association between basin relief and mechan-
ical denudation, albeit partly a function of other
factors related to relief such as seismicity and
weak rock structure.
Dedkov & Moszherin (1992) using suspended
sediment yield data from 1872 mountain rivers
assessed variations in erosion intensity and
attempted to determine the significance of human
impacts on mountain sediment systems. Their
data (Fig. 2.7) show an interesting pattern that
suggests all mountain areas are affected to some
degree by human activity but it is the areas of
lower relief that are most greatly impacted. Fac-
tors that promote erosion, such as forest removal,
overgrazing, cultivation of slopes and road con-
struction, occur in virtually all mountain regions

500

Landscapes close to natural
Modern landscapes

400

300

200

100

0

Lowlands Hills Rivers -

source in the
mountains

Mountains
low

Mountains
middle

Mountains
high

Fig. 2.7 Diagram showing the dependence of suspended
sediment yield on relief and the significance of human
interference in enhancing erosion (Source: Dedkov & Moszherin
1992; reproduced with permission of IAHS Press, from Dedkov,
A.P. & Moszherin, V.I. (1992) Erosion and sediment yield in
mountain regions of the world. In Walling, D.E., Davies, T.R.
& Hasholt, B. (Eds.) Erosion, Debris Flows and Environment in
Mountain Regions . IAHS Publication 209, 29-36.)

but the intensity of these activities varies with
the degree of economic development and popu-
lation pressure. However, it is in the lower relief
mountains where these pressures are greatest.
Milliman & Syvitski (1992) in their analysis
of 280 rivers discharging to the ocean found that
sediment loads/yields were a log-linear function
of basin area and maximum elevation of the
river basin. Other factors controlling sediment
discharge, such as climate and runoff, were of
secondary importance. In particular, sediment
fluxes from small mountainous rivers have been
greatly underestimated in previous global sedi-
ment budgets, possibly by as much as a factor of
three. Figure 2.8 shows the subdivision of river