The major rivers of Madagascar include the Betsiboka, Tsiribihina, Mangoky and Onilahy rivers, all of which start in the eastern highlands and flow west towards the Mozambique Channel. Many of these rivers flow through fertile valleys and supply much of the island's irrigation for crops. The longest river in Madagascar is the Betsiboka, which flows east of Antananarivo, the capital and largest city.
The Betsiboka River floods regularly into the port of Mahajanga on the northern coast of Madgascar. Due to a century of extensive deforestation and heavy rains in the early 2000s, the estuary of Betsiboka suffered massive erosion. Red silt was carried from the mountains and hillsides down to the coast with tropical rains. Erosion intensified when Tropical Cyclone Gafilo hit Madagascar in 2004. Ocean-going vessels were once able to travel up the Betsiboka, but massive amounts of silt now restrict huge vessels from traveling upstream.
Madagascar's western rivers run more slowly and spread out among several river valleys. East coast rivers travel swiftly from highlands into the Indian Ocean. Several eastern rivers feature waterfalls that flow into the Indian Ocean.
Large rivers such as the Betsiboka, Ikopa and Mangoky drain several hundreds of thousands of square miles of basin. These basins are responsible for major crops, mangrove trees and tropical forests.