FLOODING

Seasonal flooding is characteristic of many tropical rivers, although few compare to the so-called igapo (swamp forest) and varzea (flooded forest) of the Amazon River Basin, where large tracts of rainforest are inundated to depths of 40 feet during seasonal flooding. The lowest flood stage occurs in August and September, while the highest flood stage occurs in April and May. Tributaries that drain the Guyana Shield flood in June, while the tributaries that drain the Brazilian shield flood in March or April. Since the peak rainy seasons are out of phase, the peak discharges of left bank (Guyana shield) and right bank (Brazilian shield) rivers are somewhat offset having the effect of moderating high and low water levels on the main stream.

Rain and snow that fall in the Andes and other highland areas reach the Amazon through its tributaries and produce the high water season. Deforestation of foothills and upper basin may have caused a shift in rain levels during certain times of the year resulting in irregular high and low river levels.

Flooding has important functions for the surrounding forests including eradicating pests, enriching soils with nutrients from whitewater rivers (especially varzea forests), and dispersing seeds.

Varzea vs. Igapo Forest

LOW WATER

The contrasts between the low and high water season is some areas of the Amazon Basin are extreme. Low water leaves vast islands and sand bars exposed and the river banks high above water level. The steep muddy banks makes access to settlements a difficult and messy business. Smaller tributaries may become so shallow that travel by dugout canoe is not possible and travelers must get out and push the canoe. Creeks and streams, which are raging torrents when rainstorms come, may dry up all together.

Low water is a time of troubles for most Amazonian fish and a time of plenty for predators like Arapaima, large catfish, dolphins, and jaguars. With the dramatic decrease in water area, fish become trapped in tiny lakes and rivers shallows and are easy targets for predators.

In the floodplains, which during highwater are a continuous stretch of water, water bodies are reduced to floodplain lakes. These floodplain lakes are packed with fish and predators and dissolved oxygen levels are sharply reduced. During a few weeks each year, massive die offs are caused in these pools when cold Antarctic air passes over parts of the Amazon, cooling surface waters and causing them to sink to the bottom. The bottom of floodplain lakes is often a decaying anaerobic layer of organic sludge. As surface waters sink to bottom, methane and hydrogen sulfide from the bottom pushes toward surface causing tremendous die-offs. Vultures crowd by thousands to feed on carcasses.

Many fish have adapted to lack of oxygen by developing structures that enable them to take atmospheric oxygen from the air. Most famous are the lungfish of South America, Africa, and Australia, but also many catfish, labyrinth fish, and loaches are able to directly use atmospheric oxygen.

The best known predator of floodplain lakes is the Arapaima or Piracucu, one of the world's largest freshwater fish. The species attains a maximum of 16 feet, though today such large individuals are extremely rare because of overfishing.

HIGH WATER

High water is the time of the flooded forest when water levels rise 40 to 60 feet and flood the surrounding forest and floodplains, linking river branches as one massive body of water. The increase of water level makes the lower canopy accessible by boat. Many tree species depend on the floods for seed dispersal through animal or mechanical (floating down-river) means. It is a time of abundance for most herbivorous fish which can feed on the fruit and seeds that fall from fruiting trees. The Amazon is home to the vast majority of fish species dependent on fruits and seeds.

One famous fruit-eating fish is the Tambaqui, a large fish that crushes fallen seeds with its strong jaws. The Tambaqui waits beneath trees that are dropping seeds, congregating especially under its favorite: the rubber tree Hevea spruceana, which is widely scattered in the flooded forest. Humans take advantage of the tambaqui and other fish that wait for fallen seeds by imitating falling seeds using a pole with a seed attached by a line. When the fish is attracted within range, the hunter harpoons it. In Amazonian folklore, it is said that the jaguar hunts such seed-eating fish using its tail to mimic the "thud" of falling seeds.

The high water season is a difficult time for predators. The increased water area, gives potential prey a larger range and predators must rely on their fat stores from their heavy feeding during the dry season. Many omnivorous species east mostly seeds and fruit during this period.

High water also means difficulty for ground dwelling plant and animal species. Many ground-dwellers migrate to more elevated areas, while some species move up into the trees. Understory plants and shrubs may spend 6-10 months underwater where they are thought to continue some form of photosynthesis.