Harmful consequences of use of watercourses
There is only a superficial difference between emergencies and dangerous situations involving pollution and those involving floods and floating ice. The former are often the result of human activity, whereas the causes of the latter are frequently natural. Phenomena which are often purely natural may in some instances be exacerbated, or even wholly caused by human intervention. Floods, for example, may be caused or rendered more harmful by such human activities as the construction of canals or dams or land use practices, such as deforestation, which causes unnaturally rapid run-off. Conversely nature may interact with human activities to produce disastrous consequences, as in the case of flooding caused by earthquake damage to dams. There is therefore a continuum of possibilities ranging from the wholly natural to the wholly man-made.
The conditions most often giving rise to complaints and creating the need for deliberate international planning are those that cause in other countries: shortage of surface or ground-water supply; flooding; siltation; salinization; depletion of fish and elimination of breeding areas; eutrophication; excess vegetation; concentration of salts or other chemicals, untreated sewage, radioactive substances, oil or waste products (introduced from ship or shore); changes in temperature; blockage of passage (fish, vessels, timber); diminishing of scouring; and changes of flow. Even the otherwise innocent and beneficial use of fertilizers, the attempt to control the invasive water hyacinth, the construction of weirs for water storage and flood control, the drainage of a swamp, the cooling of a thermoelectric plant, or the return of irrigation water to the river may produce damaging consequences in other parts of the basin. Although the harm occurs most often downstream, among the numerous exceptions to this general rule are the effects in boundary streams and lakes. Some conditions are likely to be felt both upstream and downstream, particularly when fishing, navigation or timber floating is involved. Another factor which may contribute to water-related dangers is the phenomenon of global warming resulting in either excess or diminished rainfall with the consequent flooding or droughts.