Madagascar's soils are lateritic - tropical weathering has leached away most of their nutrients over the ages, leaving behind iron compounds that stain the earth a deep rust red. When rain strips that soil from the land and the rivers carry it seaward, the water itself turns the color of blood. The Betsiboka begins far inland, in the central highlands north of the capital Antananarivo, and gathers the runoff of a basin spanning tens of thousands of square kilometers before reaching this bay. By the time it arrives, it is among the most sediment-laden rivers on the planet. The redder it runs, the more of Madagascar is being lost upstream - a slow hemorrhage of the very ground the island stands on, made visible in a single dramatic stain spreading across the estuary.

The bleeding has worsened within living memory, and human hands are the cause. Since the 1950s, vast tracts of Madagascar's forests have been cleared for farming and fuel - by some estimates, roughly two-fifths of the island's tree cover gone. Forests once held the soil in place with their roots; stripped bare, the highlands began to wash away with every rainy season. Studies tracking the bay over recent decades record a dramatic surge in the sediment pouring into it. New islands and sandbars rise where the river drops its load, reshaping the estuary year by year. What looks from orbit like a timeless natural wonder is in fact a landscape transforming with alarming speed.

Threaded through all that sediment is one of Madagascar's great survivors: its mangroves. Bombetoka Bay shelters some of the island's largest remaining mangrove forests, their tangled roots gripping the shifting mud and trapping river sediment that would otherwise smother the coral reefs offshore. The reefs return the favor, breaking the surf that would tear at the mangroves. Within this shifting maze live dense communities of mollusks and crustaceans, sea turtles, and seabirds - and dugongs, the gentle marine grazers that have nearly vanished from Madagascar's waters but still browse the shallows here, in one of their last refuges along the coast.

For all its wildness, the bay is also a place where people make their living. Mahajanga, one of Madagascar's largest ports, sits at the estuary's mouth, and the surrounding lands hum with rice paddies, shrimp ponds, and coffee plantations climbing the hills. The bay's name lives in the city itself - Mahajanga was long known by its French name, Majunga, a trading port that for centuries linked Madagascar to the wider Indian Ocean world. But the same flood of sediment that builds the islands also fills the channels, and the changes of recent decades have cut hard against the people here - silting the harbor, fouling the fisheries, and complicating the shipping lanes that the port depends on. The blood-red water that astronauts admire from orbit is, for the families who live beside it, a daily reckoning with a landscape washing out from under them.