AIDS , which likely originated in primates and crossed over into humans, is thought to have accelerated its spread via the movement of infected people from rural to urban areas. Since antiquity, irrigation, drainage, and impoundment have been the three types of water control having a major impact on landscapes and water flows. Since the dawn of irrigated agriculture at least 5000 years ago, controlling water to grow crops has been the primary motivation for human alteration of freshwater supplies. Today, principal demands for fresh water are for irrigation, household and municipal water use, and industrial uses. Consumptive water use refers to water that is not returned to streams after use.
Irrigated agriculture is responsible for most consumptive water use, and decreases surface run-off.
An extreme example is the Colorado River, which has most of its water diverted to irrigated agriculture, so that in a normal year, no water at all reaches the river s mouth. A great deal of water use is non-consumptive, which means that the water is returned to surface runoff. Usually that water is contaminated however, whether used for agriculture, domestic consumption, or industry. Some believe that fresh water will be a critical limiting resource for many regions in the near future. Harvestable fish, an important source of protein for humans, and aquatic biodiversity, are also threatened.