Geoscience Reference
In-Depth Information
-120
-120∞
-100∞
-80∞
50∞
50∞
1993
45∞
45∞
40∞
40∞
o
St. Louis
35∞
35∞
1927
30∞
30∞
New Orleans
Gulf of Mexico
25∞
25∞
-120
-120∞
-100∞
-80∞
1000 km
0
km
The Mississippi River drainage basin and the extent of the 1927 and 1993 floods (based on Public Broadcasting Service, 2000).
Fig. 6.11
other in the Great Lakes lowlands. The latter network
has been developed only since the last glaciation, and
includes an area that can accumulate large, winter
snowfalls. Floods have been a constant feature of the
Mississippi River since historical records began
(Figure 6.12). Flooding can be divided into three
periods: before 1927, between 1927 and 1972, and
after 1972. Before 1927, flood control was considered
a local responsibility. The flood of 1927 brought the
realization that the Mississippi, which crosses state
boundaries, could only be controlled at the national
level. There were 246 fatalities, 137 000 buildings
flooded, and 700 000 people made homeless as a result
of this flood. Subsequently, the
Flood Control Act
of
1928 was passed and
levee
and reservoir maintenance
and management were put under the auspices of state
and federal authorities. Over 3000 km of
levees
and
floodwalls, averaging 9 m in height in the lower part of
the basin, were built to control flooding. Four floodways
were also built to divert excess floodwaters into large
storage areas, or into the Gulf of Mexico. Some parts of
the channel susceptible to erosion were also stabilized
with concrete mats, which cover the whole channel
boundary. Finally, reservoirs were constructed on
many of the tributaries to delay passage of floodwaters
into the Mississippi. The period between 1927 and 1972
represents one where floods on the Mississippi were
less severe and the mitigation works appeared to be
successful.
Since 1972, the Mississippi Basin has been subjected
to two periods of flooding that have tested these engi-
neering works to the limit. The 1973 flood was the
greatest to hit the basin to that point in time - 200 years
since records began. Total rainfall for the six-month
period exceeded the norm by 45 per cent in all parts
of the Basin. On 28 April, the river peaked at a
200-year historical high at St Louis. Damage exceeded
$US750 million, and 69 000 people were made
homeless for periods up to three months. Without
the flood mitigation works built since 1927, damage
would have been as high as $US10 000 million, and
12.5 million hectares of land would have been flooded.
40
35
30
25
20
15
10
5
1700
1750
1800
1850
1900
1950
2000
Year
Occurrence of major floods on the Mississippi River since
1700 versus interval between floods (based on Trotter
et al., 1998).
Fig. 6.12
