Geoscience Reference
In-Depth Information
River blindness occurs in parts of tropical Africa, Latin America, and the Arabian penin-
sula. The presence of the parasite in Latin America is almost certainly a result of infected
people moving to the Americas, probably as part of the slave trade. The highest prevalence
and the most serious manifestations of the disease still occur in West Africa despite the
significant success of a huge programme initiated in the early 1970s to control the disease.
The Onchocerciasis Control Programme in West Africa focused on controlling the black fly
that transmits the disease by spraying vast stretches of West African rivers with insecticide.
At the peak of the programme, this involved more than 50,000 kilometres of river over an
area of more than a million square kilometres in 11 countries. Spraying was frequent, al-
most weekly for 10 to 12 months each year, in some cases over a period of 20 years. The
idea was to stop transmission of the parasite for the duration of the life span of the worm in
humans, considered to be more than a decade.
This ambitious programme is thought to have protected some 40 million people in West
Africa from river blindness and opened up 250,000 square kilometres of land in previously
infected river valleys to resettlement and cultivation. Monitoring of other insects, and fish,
in the treated rivers indicated few deleterious effects, and the current view of river ecolo-
gists is that permanent damage to other creatures in these rivers is unlikely.
Global warming
The human-induced warming of the global climate has issued in a new era of society's in-
fluence on rivers. An overall increase in temperature will melt snow and ice and translate
into a greater loss of moisture from soils due to higher evaporation and transpiration from
plants. River flows will also be affected by changes in precipitation amounts, the intens-
ity and duration of storms, their timing, and the type of precipitation involved. Climatolo-
gists agree that extreme weather events (examples include tropical cyclones, droughts, heat
waves, and heavy rainstorms) are likely to become more frequent, more widespread, and/or
more intense in many parts of the world as the 21st century progresses. All will inevitably
result in changes to rivers. Less direct, but potentially no less significant, changes will also
occur due to the ways in which plant communities respond to climatic warming. Societies
too can be expected to increase their influence on some rivers in response to other aspects
of climate change; expanding irrigation systems, for example, in regions subject to more
droughts.
Detecting the impact of global warming on rivers is by no means always straightforward
because of the difficulties of separating the effect of climate change from the natural vari-
