Anosy - Geography, Geology & Hydrology - Hydrology

Hydrology

The 3 primary hydrologic drainage areas in Anosy are the Mountain Zone (30+% of the total catchment area), with steep slopes causing rapid, high runoff and clearly defined rivers; the Bedrock Plain which has rolling hills with low relief and several large rivers and the Coastal Sands, which can be found in bays ranging in size from more than 20 to only a few km2. These bays are clearly defined and are cut by meandering rivers. Along what are generally coastal dunes at ocean's edge are ribbon lakes from which water discharges into the ocean either through seepage or channels which cut through the dunes. These can be further subdivided into the Andriambe, Ebakika, Efaho, Fanjahira, Lakandava, Lanirano, Manampanihy, Mandromodromotra, Vatomena, Vatomirindry and Vatorendrika basins.

There are 3 major rivers in southern Anosy : the Mandrare along the southern border, the Efaho (formerly called the Fanjahira) just west of Tolagnaro and the Manampanihy which drains the Ranomafana valley, emptying into the ocean at Manantenina. Other rivers in the Anosy region include the Isoanala, Manambolo, Mangoky, Menarandra and Isoanala.