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"In terms of species extinction and threats, the greatest numbers of species affected and most severe threats are encountered in freshwater habitats. Freshwater ecosystems and biodiversity are the most vulnerable to human activities and environmental change because of the disproportionate richness of inland waters as a habitat for plants and animals. They currently support approximately 40% of the fish diversity and one quarter of the global vertebrate diversity. All this diversity is found within 0.1% of the world’s water, covering only about 0.8% of the Earth’s surface.

Causes of degradation of freshwater biodiversity are typically classified under five headings: overexploitation; water pollution; flow modification; destruction or degradation of habitat; and invasion by exotic species. Other environmental changes that are occurring on a global scale may cut across these categories. Examples include global warning and changing patterns of precipitation and stream flows and atmospheric deposition of pollutants.

Overexploitation primarily affects fish and some other vertebrates (reptiles and amphibians), whereas pollution, flow and habitat modification and exotic species are threats to all freshwater biodiversity. Most developed countries have made progress in controlling water pollution from domestic and industrial point sources but diffuse pollution from multiple sources in rural and urban areas remains a difficult practical and policy challenge. In particular, the problem of nutrient enrichment and other chemical pollutants used in farming or by rural and urban households.

Modifications to flows are common and almost universal, tending to be most severe in locations where the undisturbed or natural flow regime is itself highly variable."

-excerpt from course on water resource management