Page 109 - Fundamentals of Geomorphology

92 INTRODUCING LANDFORMS AND LANDSCAPES

Table 3.5 Natural and mining-induced erosion rates of the continents
Continent Natural erosion Hard coal, 1885 Brown coal and lignite, 1995 Iron ores, 1995 Copper ores, 1995
(Mt/yr) a (Mt) (Mt) (Mt) (Mt)
North and Central 2,996 4,413 1,139 348 1,314
America
South America 2,201 180 1 712 1,337
Europe 967 3,467 6,771 587 529
Asia 17,966 8,990 1,287 1,097 679
Africa 1,789 993 — 156 286
Australia 267 944 505 457 296
Total 26,156 18,987 9,703 3,357 4,442
a
Note: Mt = megatonnes (= 1 million tonnes)
Source: Adapted from Douglas and Lawson (2001)

river sediments, including reservoir construction, land by about 1.4 billion tonnes per years, although soil
clearance and land-use change, other forms of land dis- erosion and mining and construction activities have
turbance (such as mining activity), soil and water conser- increased it by about 2.3 billion tonnes per year (Syvitski
vation measures and sediment control programmes, and et al. 2005). The increased sediment can make coastal
climate change. Land-clearance, most land-use change, areas less vulnerable to erosion, even if it can adversely
and land disturbance cause an increase of sediment affect coastal ecosystems.The positive and negative influ-
loads; soil and water conservation, sediment control pro- ences of human activities on river flow could balance
grammes, and reservoir construction cause a decrease each other out, but the net global result at present
in sediment loads. A recent study provided a first is that rivers carry less sediment to the coastal zone,
assessment of current trends in the sediment loads of with considerable differences on the regional level.
the world’s rivers (Walling and Fang 2003). Analysis In Indonesia, where fewer dams have meant fewer
of longer-term records of annual sediment load and sediment-trapping reservoirs, more sediment is build-
runoff assembled for 145 major rivers revealed that ing up along the coastline because of human activities,
some 50 per cent of the sediment-load records contain chiefly deforestation. In general, Africa and Asia have
evidence of statistically significant upward or down- seen the largest reduction in sediment to the coast.
ward trends, although the majority display diminishing The effects of dams on rivers will be discussed in
loads. The evidence pointed to reservoir construction Chapter 9.
as probably the most important influence on land–
ocean sediment fluxes, although the influence of other
controls resulting in increasing sediment loads was SUMMARY
detectable.
Chemical, physical, and biological processes weather
rocks. The chief physical or (mechanical) weathering
Dam building
processes are unloading (the removal of surface cover),
The construction of dams, and other human activities, frost action, alternate heating and cooling, repeated wet-
alters the amount of sediment carried by rivers to coastal ting and drying, and the growth of salt crystals. The
environments, so affecting coastal geomorphology. chief chemical weathering processes are solution or dis-
Dams reduce the amount of sediment carried to coasts solution, hydration, oxidation, carbonation, hydrolysis,