Every year, huge stretches of the Amazon rainforest vanish beneath rising floodwaters, turning treetops into fish habitat and forest floors into riverbed. These flooded forests, and the eerie standing-dead forests created by modern dams, are among the most unusual ecosystems on Earth.
Here's what's actually known about how they form, who lives in them, and why they matter.
Each year, seasonal floods submerge vast areas of the Amazon rainforest for months at a time, a cycle known as the flood pulse.
These seasonally flooded forests come in two main types: varzea, flooded by nutrient-rich "whitewater" rivers carrying Andean sediment, and igapo, flooded by nutrient-poor "blackwater" or clearwater rivers.
In parts of the Amazon, floodwaters can rise more than 30-40 feet, submerging trees up to their canopies for much of the year.
Fish such as the tambaqui and the arapaima swim directly into the flooded forest to feed on fallen fruit and seeds from the trees.
The tambaqui has powerful, nut-cracking jaws adapted to crush hard fruit and seed pods that fall into the water.
The arapaima, or pirarucu, is one of the largest freshwater fish in the world and can grow beyond 6 feet in length.
By eating fruit and later excreting the seeds elsewhere, fish in the flooded forest act as seed dispersers, a role usually played by birds and monkeys in dry forest.
A single tambaqui can carry and disperse intact seeds from dozens of different plant species across long distances as it swims between flooded areas.
Roughly half to 90% of Amazonian tree species rely on animals, including fish, to disperse their seeds.
Flooded forests also support a rich web of algae, invertebrates, and microorganisms that form the base of the aquatic food chain.
Not all of the Amazon's underwater forests are seasonal. Brazil's Balbina Dam, completed in 1989, permanently flooded roughly 2,360 square kilometers of rainforest to create its reservoir.
Because the forest was not cleared before flooding, the skeletons of dead trees still stand throughout the shallow, ship-hazarding waters of the Balbina reservoir.
The Balbina reservoir contains around 1,500 forested islands, remnants of hilltops that stayed above the rising water.
Decomposing submerged vegetation in reservoirs like Balbina's produces acidic, low-oxygen water and releases methane, a potent greenhouse gas.
Changes to the natural flood cycle below Balbina Dam have killed floodplain trees for roughly 100 kilometers downstream, since those trees are adapted to a very specific pattern of seasonal flooding.
Healthy, naturally flooded forests act as important carbon sinks, and disrupting their flood cycles can undermine that role rather than enhance it.
Flooded forests serve as nurseries for many commercially and ecologically important Amazon fish species.
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