Geoscience Reference
In-Depth Information
erosion rates and sediment conveyance in river
systems. Increases in flooding and flood peaks,
river sediment loads and rates of valley-floor
alluviation following human disturbance, or dis-
turbance of vegetation, are now well documented
(Robinson & Lambrick 1984). This section dis-
cusses several types of anthropogenic impacts
on sedimentary processes in river systems.
3.4.1.1 Agriculture, deforestation and afforestation
Human settlement and cultivation have had a
drastic impact on river sediment budgets, largely
through deforestation and land clearance. These
activities result in increased soil erosion, soil
creep and landslide events, which in turn result
in increased sediment input to, and sedimenta-
tion rates in, rivers (Fig. 3.14) (Knox 2001).
Furthermore, the sediment composition is often
altered, because progressively less weathered
and less organic-rich terrigenous materials are
stripped and added to the suspended load dur-
ing clearance.
Fig. 3.13
Destruction of Armero, Columbia, by a lahar,
triggered by an eruption and melting snow, which emanated
from the Nevado del Ruiz volcano. More than 23,000 people
died during this event. (Image and information courtesy of
R.J. Janda, USGS/Cascades Volcano Observatory.)
sediment regime and ecology of many of the
world's rivers. Deforestation, agriculture, land-
use change, river regulation and mining are some
of the anthropogenic activities that affect soil
100
98
96
1820 1890 1925 1940 1990
94
Historical channel evolution
92
90
0 10 20 30 40 50 60
Distance (m)
Fig. 3.14
Historical changes in
channel morphology and hydraulics
on the Shullsburg Branch tributary,
Galena River system, USA.
Accelerated overbank sedimentation
from the beginning of Euro-American
settlement in the 1820s increased
progressively, resulting in increases of
bank heights and facilitation of deeper
flows with high shear stresses that
led to bank erosion and channel
explansion. After about 1950,
overbank sedimentation decreased
owing to smaller and less frequent
floods as a result of improved
upland land conservation practices.
(After Knox 2001, fig. 9.)
90
80
70
60
50
40
30
20
10
0
1820 1840 1860 1880 1900 1920 1940 1960 1980 2000
Year
Discharge (cm)
Thalweg shear stress (N m
−
2
)
Channel capacity (m
2
)
