Geoscience Reference
In-Depth Information
Las Vegas and many other southwestern cities. As you can see
in Figure 20.9a, the average spring runoff in the Colorado was
generally almost half of the normal amount between 2000 and
2007. This decrease in runoff has caused water levels in Lake
Mead to drop to the point where it now contains only about
50% of its total capacity (Figure 20.9b). The extent of this de-
crease is visible on the bleached canyon walls on the edge of the
lake (Figure 20.9c).
The impact of drought on Las Vegas and other southwest-
ern cities is potentially significant, especially given the recent
16
14
30-year average 7.9 million acre feet
12
10
8
6
4
2
0
80 81 82
83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05
06 07 08
09 10
Annual spring runoff
(a)
1240
1220
1200
1180
Average (1173 feet)
Average (1173 ft)
1160
1140
1120
1100
1080
1060
1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
(b)
Year
Figure 20.9 Colorado River runoff and Lake Mead.
(a) Av-
erage spring runoff in the Colorado River since the 1980s has
been highly variable, with the first decade of the 21st century
particularly low. (
Source
: U.S. Bureau of Reclamation.) (b) Due to
reduced spring runoff, water levels in Lake Mead have dropped
significantly in the past two decades (
Source
: U.S. Bureau of
Reclamation.) (c) Current water level in Lake Mead. Since 2000,
the lake level has dropped about 30 m (100 ft), exposing a ring of
bleached rock around the edge of the lake.
(c)
