Wetlands are often drained in many regions to facilitate human use of the land. This happens a lot within the Pairie Provinces of Canada, where wetlands are drained to make way for agriculture. Wetlands are also often drained so as to use the land for building houses. Humans have also altered the flow of rivers through constructing dams and over-abstracting water. In many regions, depressions that would have been flooded in the past to form wetlands are no longer saturated. Wetlands also act as a 'sink' for many pollutants, and much of the pollution released into upstream rivers by humans may settle into the relatively stagnant waters of wetlands, to be absorbed into the sediments, where often it acts as a chronic pollutant, negatively effecting the aquatic ecosystem and water quality downstream.