As they leave the highlands and enter the north–south running Betsiriry Plain, the tributary rivers converge to form the Tsiribihina - the Mahajilo and Manandaza from the north, and the Mania and Sakeny from the south.

[4][5] The rich alluvial soils of the Betsiriry Plain make it one of the most suitable regions for agriculture in western Madgagascar.

[7] The Tsiribihina then flows westward, cutting a steep and winding gorge through the Bemaraha Plateau, a limestone formation that extends north and south along the western edge of the Betsiriry Plain.

The mangroves are generally 2–4 meters high, and the predominant trees are species of Avicennia, Rhizophora, Ceriops, Bruguiera, and Sonneratia.

The freshwater marshes and lakes of the lower Tsiribihina are dominated by the sedges Cyperus spp, the reed Phragmites, and non-native water-hyacinth Eichhornia.