When freshwater rivers drain into the sea, the waters mix to form a backish estuary. The slowing river also starts to drop its load as its energy drops, depositing gravel and sediment into bars. This particular estuary is Bombetoka Bay, where Betsiboka River flows into the Mozambique Channel.

The Betsiboka River is cloudy with sediments, fading from sapphire blue to pink where the sediment is thicker. The river is ringed with the dark green of thick mangrove forests. The mangroves filter out sediment from the river, protecting offshore coral reefs, while the coral reefs buffer the mangroves from ocean waves. The rectangular blue ponds along the estuary are shrimp pens, while coffee plantations scatter the hills.

Natural-colour image from August 23, 2000. Image credit: NASA/GSFC/METI/ERSDAC/JAROS.

Heavy logging of the rainforests and mangroves clear the land, increasing erosion of bright red soil into the river. The government has instituted programs to control land-clearing, encourage reforestation, and limit development, but it's not entirely effective. Notice the ever-increasing number of shrimp farms along the lip of the estuary.


Natural-colour image from September 4, 2003. Image credit: NASA/Earth Observatory

Tropical Cyclone Gafilo, a major tropical storm hit Madagascar on March 7 and 8, 2004. In this false-colour image covers green visible light through infrared, the main river is green, other water is blue, and vegetation is red. The river flooded its banks during the storm, leaving behind pools of bright cyan stagnant water.


False-colour infrared image from March 20, 2004. Image credit: NASA/ASTER/Earth Observatory


Severe storms like Tropical Cyclone Gafilo in March 2004 dramatically increase erosion, staining the river estuary brilliant red even in a natural-colour image.

Natural-colour image from March 25, 2004. Image credit: NASA/Earth Observatory

For another look at pretty water flows, check out algae blooms in the ocean, or the first-light images released by the new ESA Sentinel telescope.