Geoscience Reference
In-Depth Information
ways. The accidental release of polluted water from a pond at the Aznalcóllar pyrite mine
in southwest Spain in 1998 caused huge damage to birds, fish, and other aquatic species in
the Guadiamar River and the Coto Doñana wetland. The water was acidic and contained
arsenic, lead, and zinc at concentrations that were lethal for wildlife. Mining has long been
associated with impacts on rivers. The Romans developed techniques of hydraulic mining,
diverting large volumes of river water to break up and flush away soil and rock and expose
minerals. The techniques were widely used to produce gold from alluvial deposits in north-
west Spain.
The innumerable links between a river and human activities in its surrounding landscape,
and consequently the importance of managing an entire basin, have been recognized for
centuries. In Japan, for example, government regulation of timber harvesting along moun-
tain streams in order to maintain channel stability dates back 1,200 years. Similarly, the tra-
ditional Hawaiian systems of
ahupua'a
involved managing drainage basins as an integrated
whole to safeguard food production from agriculture and fish ponds. Upland forests were
protected by taboo in order to supply rivers with nutrients for downstream fields and fish
ponds. In modern parlance, the approach is embodied in 'catchment management plans'
which in the countries of the European Union have become mandatory for all major river
basins.
The Mississippi
The Mississippi River which, together with the Missouri River, drains two-thirds of the
continental USA, has been significantly modified by numerous human activities over the
last 200 years or so. A rapid rise in river traffic dating from the beginning of steam boats
in the early 1800s spurred the large-scale felling of forests to fire the boats' boilers, and the
loss of trees in turn destabilized river banks and contributed to unpredictable migration of
the channel. Deforestation and the expansion of commercial agriculture in the Mississippi
Basin also resulted in more soil erosion and more sediment reaching the river. Sandbars,
a menace to navigation, were one result. As settlements expanded on to low-lying river
banks, the Mississippi's floods became a greater danger.
Attempts to manage these problems on the Mississippi in a systematic way began in the
19th century and continue today. Throughout the 1800s, the US Army Corps of Engineers
cleared rock and made the channel deeper on particular stretches of the river in an effort to
assist navigation. A major programme of river engineering was initiated after a disastrous
flood in 1927 in the Lower Mississippi Valley which cost more than 200 lives and dis-
