Geoscience Reference
In-Depth Information
10
5
Ousel Cr., Montana
Teton Dam
Katherine Gorge
10
4
Pecos R., Texas
Yangtze R.
10
3
10
2
Mississippi R.
10
1
Amazon R.
10
0
10
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
Drainage area (km
2
)
Plot of stream power per unit boundary area versus drainage basin area. The curve delineates the upper limit in flood power (adapted from
Baker & Costa, 1987).
Fig. 6.3
example, built across an undetected fault line), or mis-
chance. The greatest stream power calculated for any
flood in the United States was for the 8 June 1964 Ousel
Creek flood in Montana. During this flood, stream
powers reached 18 600 W m
-2
. Most of the powerful
floods in the United States have occurred on quite small
drainage basins because they have steeper channels. In
comparison to the Ousel Creek flood, the large flood on
the Mississippi River that occurred in 1973, had a
stream power of 12 W m
-2
.
For larger drainage basins, two conditions must be
met to attain high stream powers. Firstly, the channel
must be constrained and incised in bedrock. This
ensures that high velocities are reached because water
cannot spill out across a wide floodplain. Secondly, the
upstream drainage basin must be subject either to
high-magnitude rainfall events, or to high discharges.
These two features severely limit the number of high
stream power events occurring on large rivers. During
the Teton Dam failure in Idaho in 1976, a stream
power was reached in excess of 10 000 W m
-2
because
the river flowed through an incised channel. Similar
magnitudes have occurred through the narrow Kather-
ine River gorge in northern Australia. The Pecos River
in Texas is also incised into a limestone escarpment
subject to flash flooding, mainly during tropical
cyclones such as Hurricane Alice, which penetrated
inland in 1954. Finally, the Chang Jiang (Yangtze)
River produces stream power values as high as flash
floods on streams with drainage basins less than
1000 km
2
. At the Three Gorges section of the Chang
Jiang River, snowmelt and rainfall from the Tibetan
Plateau concentrates in a narrow gorge. This river,
in the 1870 flood, flowed at a depth of 85 m with
velocities of 11.8 m s
-1
.
Figure 6.3 does not plot the largest known floods.
Stream power values for cataclysmic prehistoric floods
associated with Lake Missoula at the margin of the
Laurentian icesheet on the Great Plains, and with Lake
Bonneville, were an order of magnitude larger. These
resulted from the sudden bursting of
ice-dammed lakes
as meltwater built up in valleys temporarily blocked
by glacier ice during the Wisconsin glaciation. The
Missoula floods reached velocities of 30 m s
-1
and
depths of 175 m, producing stream powers in excess of
100 000 W m
-2
, the largest yet calculated on Earth.
Similar types of floods may have emptied from the
Laurentian icesheet into the Mississippi River and
from Lake Agassiz eastward into the Great Lakes-
St Lawrence drainage system. Equivalent large floods
also existed on Mars at some time in its geological
history; however, if the high stream powers recorded
on Earth were reached, the flows may have been as
deep as 500 m - as gravity is lower on Mars.
The ability of these catastrophic floods to erode and
transport sediment is enormous. Figure 6.4 plots mean
water depth against the mean velocity for these flood
events. Also plotted on this diagram is the point where
flow becomes supercritical and begins to jet.
Super-
critical flow
is highly erosive, and cannot be sustained
in alluvial channels because the bed will be eroded so
rapidly. It is also rare in bedrock channels because,
