DEFINITION OF LEVEE

A natural or man-made topographically high embankment

on the side of a river, stream, or body of water. Natural

levees are built by stream systems during floods, where the

water leaves the fast-moving channel and loses velocity as it

expands onto the floodplain. When the water loses velocity, it

has a lower capacity to hold material in suspension and drops

much of its load on the side of the channel, forming the levee.

Levees are also commonly built or enhanced along riverbanks

to protect towns and farmlands from river floods. These levees

are usually successful at the job they were intended to do,

but they also cause some other collateral effects. First, the levees

do not allow waters to spill onto the floodplains, so the

floodplains do not receive the annual fertilization by thin layers

of silt, and they may begin to deflate and slowly degrade

as a result of this loss of nourishment by the river. The ancient

Egyptians relied on such yearly floods to maintain their fields’

productivity, which has declined since the Nile has been

dammed and altered in recent times. Another effect of levees

is that they constrict the river to a narrow channel, so that

floodwaters that once spread slowly over a large region are

now focused into a narrow space. This causes floods to rise

faster, reach greater heights, have a greater velocity, and reach

downstream areas faster than rivers without levees. The extra

speed of the river is in many cases enough to erode the levees

and return the river to its natural state.

One of the less appreciated effects of building levees on

the sides of rivers is that they sometimes cause the river to

slowly rise above the height of the floodplain. Many rivers

naturally aggrade or accumulate sediment along their bottoms.

In a natural system without levees, this aggradation is

accompanied by lateral or sideways migration of the channel

so that the river stays at the same height with time. However,

if a levee is constructed and maintained, the river is forced to

stay in the same location as it builds up its bottom. As the

bottom rises, the river naturally adds to the height of the

levee, and people will build up the height of the levee as well

as the river rises, to prevent further flooding. The net result is

that the river may gradually rise above the floodplain, until

some catastrophic flood causes the levee to break, and the

river establishes a new course.

The process of breaking through a levee is known as

avulsion. Avulsion has occurred seven times in the last 6,000

years along the lower Mississippi River. Each time, the river

has broken through a levee a few hundred miles from the

mouth of the river and has found a new shorter route to the

Gulf of Mexico. The old river channel and delta are then

abandoned, and the delta subsides below sea level, as the

river no longer replenishes it. A new channel is established

and this gradually builds up a new delta until it too is abandoned

in favor of a younger shorter channel to the Gulf.

Some of the most tragic examples of the effects of rivers

rising above and breaking through their levees are from the

Yellow River in China. The Yellow River flows out of the

Kunlun Mountains across much of China into the wide lowland

basin between Beijing and Shanghai. The river has

switched courses in its lower reaches at least 10 times in the

last 2,500 years. It currently flows into Chihli (Bohai) Bay

and then into the Yellow Sea. The Chinese have attempted to

control and modify the course of the Yellow River since

dredging operations in 2356 B.C.E., and the construction of

levees in 602 B.C.E. One of the worst modern floods along the

Yellow River was in 1887 when the river rose over the top of

the 75-foot (22-m) high levees and covered the lowlands with

water. More than one million people died from the floods

and subsequent famine, along with widespread destruction of

crops and livestock.

The Yellow River was also the site of an unnatural disaster

in 1938. As part of the war effort, in 1938 Japan attacked

and bombed the levees along the Yellow River. The river

escaped and took another million lives. The Yellow River is

continuing its natural process of building up its bottom, and

the people along the river continue to raise the level of the

levees in an attempt to keep the river’s floods out of their

fields. Today, the river bottom rests an astounding 65 feet (20

m) above the surrounding floodplain, a testament to the

attempts of the river to find a new lower channel and to

abandon its current channel in the process of avulsion.

See also FLOOD; RIVER SYSTEM.

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