Regions where fresh water flowing in rivers and salt water from the seas and oceans mix are called estuaries, and they are among the most biologically productive ecosystems on Earth. This astronaut photograph, taken from the International Space Station, highlights two estuaries located along the northwestern coastline of the island of Madagascar. The Mozambique Channel (image top) separates Madagascar from the southeastern coast of Africa. Bombetoka Bay (image upper left) is fed by the Betsiboka River and is a frequent subject of astronaut photography due to its striking red floodplain sediments. Mahajamba Bay (image right) is fed by several rivers including the Mahajamba and Sofia Rivers; like the Betsiboka, the floodplains of these rivers also contain reddish sediments eroded from their basins upstream.
The brackish (mix of fresh and salty water) conditions found in most estuaries host unique plant and animal species adapted to live in such environments. Mangroves in particular are a common plant species found in and around Madagascar estuaries, and Bombetoka Bay contains some of the largest remaining stands. Estuaries also host abundant fish and shellfish species - many of which need access to freshwater for a portion of their life cycles - and these in turn support local and migratory bird species that prey on them. However, human activities such as urban development, overfishing, and increased sediment loading from erosion of upriver highlands threaten the ecosystem health of the estuaries. In particular, the silt deposits in Bombetoka Bay at the mouth of the Betsiboka River have been filling in the bay.