The Amazon's annual flood pulse can change water levels by ten metres or more. During high water, Amazonian manatees move into flooded forest and feed heavily on aquatic macrophytes. When the water drops, they retreat to deep lakes and channels where that vegetation is scarce or absent.
The accepted explanation is that they fast, or feed at greatly reduced rates, for a substantial part of the low-water season and draw on fat reserves accumulated during the flood. Supporting observations include an unusually low metabolic rate for a mammal of their size and marked seasonal changes in body condition.
The details are thin. The species is difficult to observe in turbid water, and most of what is known comes from captive animals, hunted specimens and a modest amount of telemetry. How long individuals actually go without food, and whether some populations manage to continue feeding through the dry season, has not been established, which is one reason its conservation status is hard to assess precisely.