Along the Peruvian Amazon River near Iquitos, the fluctuation of water levels is one of the Neotropics’ most amazing natural history events. The ebbing and flooding of water dictates the way of life for so many species including plants, fish, amphibians, reptiles, mammals and the local people — the ribereños. During high water times and low water extremes, the difference in water levels may change over 45 feet in one year in the Iquitos area.
When traveling along the Amazon  during low water, it is very obvious that the water gets much higher. Water marks are apparent on trees and the exposed mud banks, especially at ribereños villages, may result in a very steep bank, with mud stairs carved into them that allow people to get from the river to their village.
But the rise and fall of the river is not entirely due to rainfall.
Even though local people will frequently call high water the rainy season, there is not an appreciably greater amount of rain during high water. Instead, the high water is caused by snow melt from the Andes as high water coincides with the Austral summer as well as rain falling on the Guyana Shield. High water is basically from December through May, and low water is from June through November. Of course this varies considerably and some years the water level may not fluctuate greatly at all. This may be due to less snowfall in the Andes part of the water shed and therefore the high water mark does not reach the extreme levels that may occur during snow melt of high snowfall years.
The high water allows many species of fish to move into the flooded forest where they may spawn and some, like pacu enter the forest to feed on the plentiful fruits, which make up much of their diet. In addition, even the Amazon pink river dolphin enters the flooded forest to feed on crabs and fish that are more easily captured within the forest than in the deep turbid waters of the main river. As the high water begins to ebb, nutrients are deposited on the inside curves of the river where large areas of soil are exposed. It is during low water that the local people plant rice, a main staple in the diet of people of the area. Additionally, low water allows for tremendous numbers of fish to be stranded in pools of water that gradually get smaller and smaller. In those areas, herons, egrets, storks, kingfishers, caiman and people take advantage of the surplus fish supply and around those areas there may be a tremendous diversity of species capitalizing on the cornucopia of food.
The Amazon River’s changing water level is ecologically important and let’s hope that climate change does not affect this system too much as great changes may result — good or bad, and it will affect the system from the Andean highlands all the way to the river’s mouth and hundreds of miles into the marine environments in the Atlantic Ocean.
Naturalist Greg Greer  is a favorite among IE travelers, and has gained a reputation for his friendliness and good humor, along with his incomparable knowledge of natural history, photos and articles have been widely published in books and magazines, including Georgia Outdoor News, Bird Watcher’s Digest, Alabama Outdoor News, Riversedge and Southern Wildlife.