fresh water scarcity has negative effects on ecology, biodiversity, agriculture and human health. it has also led to armed conflicts in several cases. some countries upriver have built dams to satisfy their needs for drinking water, irrigation and electric power. these needs have grown as populations has grown, and as climate changes has reduced rain in some places. down river from the dam, supply is usually reduced. this creates tension between countries. the river jordan in the middle east, and the nile are two good examples of where this is happening.