Environmental Monitoring of Bombetoka Bay and the Betsiboka Estuary, Madagascar, Using Multi-temporal Satellite Data

The causes and consequences of soil erosion in Madagascar have been the focus of recent environmental debates. The Betsiboka, Madagascar's largest river stretching 600 km from the high central plateau to the northwest coast on the Mozambique Channel, is a major conduit for transporting lateritic soils and sediments derived from the highlands of Central Madagascar to the sea. These entrained lateritic sediments color the river a blood-red hue, as if the life of the island is being drained away. In this study, Landsat visible and near infrared spectral bands are used to map the underwater sediments especially under the sea and in the interface between seawater and freshwater. Band color composites, single bands and band ratios are used to improve the detection of underwater sediments including sand bars and delta lobes. We show the evolution of the bay, coastline, delta, and change detection results derived from Landsat satellite images recorded in 1973, 1989, 1999, 2000 and 2003, respectively. Results indicate that sedimentary transport and suspension in Bombetoka Bay has significantly changed during the past 30 years, with a dramatic increase in the amount of sediment moved by the river, and deposited in the estuary and in offshore delta lobes. These changes have adversely affected agriculture, fisheries, and transportation for one of Madagascar's largest ports. The changes are attributed to increased erosion following large-scale deforestation, bush fires, and overgrazing in the river basin.