Flooding

The Amazon region is generally flat with little elevation.  Because of this, large tropical rivers have little gradient and flow at a relatively slow rate. The Amazon River, for example, only descends 105 m (345 feet) from the Peruvian river port of Iquitos to the Atlantic Ocean, a full 3700 km (2,300 miles) away.

The relative flatness of the region also results in a continuous cycle of flooding of surrounding forest areas and the creation of unique ecosystems.  The area covered by water from the Amazon River and its tributaries varies throughout the year.  In the dry season about 110, 000 sq km (42,000 sq miles) of land is covered by water while in the rainy season the flooded area more than triples to 350,000 sq km (135,000 sq mi).   Every year the Amazon River rises more than 9 m (30 feet) and replenishes nutrients in the floodplain area.

Freshwater Systems

The Amazon Basin has several freshwater systems:

The muddy brown appearance of the water of this type of river is due to its high sediment content. Each day, heavy tropical rains wash tons of rich soil into rivers from the mountains and the surrounding forest areas. This nutrient-rich, largely pH neutral (pH of 6.3-7) water helps fertilize surrounding land and sustains an abundance of wildlife.

The Amazon River is dominated by whitewater along its southern bank and blackwater in the northern bank.

This type of river gets its name from its clear dark amber colored water (like black tea) which results from the leaching of tannins from the decaying leaves of the surrounding vegetation. Blackwater rivers are directly fed by run-off from surrounding rainforests which have nutrient-deficient soil.  The result is mineral poor, almost sterile, soft, acidic water  (pH of 3.5-6).  This keeps parasite and bacterial populations to a minimum as well as inhibits the proliferation of insect larvae. Blackwater rivers are considered some of the cleanest natural waters in the world and the forests around them tend to be less "buggy". Because these rivers are nutrient poor, however, they do not have a lot of wildlife compared to surrounding nutrient rich whitewater rivers. These rivers however, have a tremendous variety of fishes, which have adapted to blackwater conditions.  Many are known for their popularity as aquarium fish; for example, discus, angelfish, arowana, elephantnose fish, many gouramis, and cardinal tetras.

The nearby Rio Negro is one of the world's largest rivers (8 km/5 miles at its mouth) and the most famous blackwater river.  The Urubu and Preto da Eva Rivers are blackwater rivers near Amazonat.

Mixed Waters

The zones where the two waters mix are particularly attractive to small organisms and fish and thus tend to have high numbers of animals. This occurs near Amazonat where the Amazon River meets the Preto da Eva and Urubú Rivers.

At Meeting of the Waters (also known as “Wedding of the Waters”), near Manaus, the differences between the whitewater Solimoes River and the blackwater of the Rio Negro are very clear. The rivers run side-by-side for several miles, the white and black water clearly distinct, before blending together to form the Amazon River.

Clearwater or bluewater rivers are named after the color of the water. These rivers are not common in lowland tropical rainforests but are mostly found in the highlands where mountain and cloud forests grow. Because of their elevation and tendency to run over rock, clearwater rivers are usually rapid or fast-flowing.  Clearwater rivers have a higher pH and tend to have some dissolved minerals, making the waters harder than in both white and blackwater rivers. The waters are clear because the rock formations are ancient and no longer erode in the current. Clearwater rivers generally support abundant plant growth due to their clarity and mineral content.