Wildly fluctuating water levels in the Amazon

Grey dolphin

The grey river dolphin left its habitats in the Samiria River to find refuge in the larger channels of the Amazon

Research is being conducted in the Peruvian Amazon on the effects of climate change to its wildlife and local people. In 2010 the water levels of the Amazon River were at a historic low resulting in extreme dry conditions. In 2009 the same river was at a historic high, flooding huge area of Amazonian forests.

The Amazon basin is going through dramatic climate changes that will impact the largest rainforest on Earth. Each year the Amazon River goes through seasonal changes between the flooding period from December to June and the low water period between July and November. However, these normal seasonal changes are now becoming more intense.


Research on wildlife populations is being conducted on the river dolphins, primates, fish, caimans, macaws, deer, peccaries, tapirs, jaguars, giant river otters and other species to understand how the ever increasing climatic changes are impacting their ecology, behaviour and populations. The research team is also working with the local Indian communities to see how the changes are affecting their fishing and bushmeat hunting that they depend on for their daily livelihood.

The research is being conducted in the Pacaya Samiria National Reserve in the Peruvian Amazon that spans over 20,000 km2 of seasonally flooded forests. The wildlife of the Samiria River lives in an ecosystem that is driven by the large seasonal fluctuations occurring between high and low water seasons. The ecology of the aquatic and terrestrial wildlife revolves around these seasonal changes in water level.

Chestnut fronted macaw

Chestnut fronted macaws, after the extreme low water, decreased by an average of 66%

The ecological conditions of long periods of flooding are very harsh on much of the floral and faunal community. Many plant species cannot withstand the long periods of inundation and the diversity of plants in the heavily flooded areas is lower than lightly and non-flooded levees.

Likewise, the terrestrial wildlife must seek out floodplain islands or levees during the high water season, which have increased competition and predation pressures. Even the arboreal wildlife is impacted by the flooding, since many of the fruit trees are quite seasonal in the flooded forests, resulting in seasons with low food production.

The aquatic wildlife is equally affected by the large seasonal inundations. During the flooded periods the fish enter the water-laden forests and feed on the abundance of vegetative and animal production. Many tree species fruit during this season and rely on the fish as their primary means of seed dispersal. During the flooded period many fish populations reproduce within the inundated forests.


The normal cycles in the Amazon forests are now being disrupted by the extreme flooding and drought events that are occurring. The flooded forests are particularly important in understanding the impacts of climate change in the Amazon, since the aquatic and terrestrial interface between high and low water seasons makes this habitat sensitive to greater seasonal variations.

Results from our research show some important consequences of the extremely low water levels that occurred in 2010, primarily lower dolphin numbers throughout the Samiria River. Overall, the pink river dolphin numbers decreased by 47% and the grey river dolphin by 49% in September and October 2010. The dolphins left their habitats in the Samiria River to find refuge in the larger channels of the Amazon.

The normal cycles in the Amazon forests are now being disrupted by the extreme flooding and drought events that are occurring

The decreases in dolphins numbers is directly related to the fish populations. The fish were also impacted by the extremely low water levels of the Samiria River. As the season progressed and the water levels sank below the long term levels the fish abundance became obviously lower than average with a 63% decrease from normal years.

The caimans are resident species and do not have migratory movements as the dolphins or fish. The spectacled caiman appeared to be impacted by the extreme low water levels, whereas the black caiman appeared to be less affected. The spectacled caiman had an overall lower abundance in the Samiria River during the drought than their six year average with the upper section having 56% fewer, the mid section having 27% fewer, and the lower section having 40% fewer.

The macaws are used as indicators of the terrestrial habitats. The low water appears to have impacted the smaller macaw species, especially the Chestnut-fronted Macaw, which after the extreme low water, decreased by an average of 66%. The larger Blue and Yellow Macaw did not show this same trend and had similar numbers to previous years.

Latest sightings

pink dolphin

Pink dolphin - in March 2011 sightings were 9% higher than the previous March

The drought ended in late October 2010. Recent research results show that in January and February 2011 both the pink and grey dolphins continued to be consistently lower (between 60-70% fewer sightings) than the same months in 2010. However, in March 2011 both species appear to have returned to the Samiria River, with pink dolphins being 9% higher in numbers than in March 2010 and the grey dolphin 30% higher than March 2010. This is a very good sign and suggests that the Samiria River is recovering from the drought of 2010.

The spectacled caiman continued to have lower numbers in 2011 than the same time the previous year. Between January to March 2011 the spectacled caiman was 61% lower than the same period in 2010. The black caiman numbers were also lower between January to March 2011 by 34%, than the same period the previous year. The spectacled caiman continues to show a greater impact from the drought than the black caiman.

The fish numbers appear to have recovered from the drought. Fish populations generally recover faster than larger, slower reproducing animals. Between January and March 2010 there was an average of 13.7 fish/hour in the survey nets, and during the same period in 2011 there was an average of 16.8 fish/hour in the nets.

In January and February 2011 the Chestnut-fronted macaw continued to be lower than the previous year by 76%. However, in March it returned to the Samiria and had numbers that were 59% greater than in March 2010.

The research will continue in order to better understand the impacts of the climate change occurring in the Amazon in hope of finding solutions for the local people in ways that will not deplete the fisheries and bushmeat species. Deforestation, over-hunting and over-fishing, contamination from the oil industry, and now climate change are all leading to the demise of the greatest rainforest on Earth.

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One Response to “Wildly fluctuating water levels in the Amazon”

  1. John Swindells Reply April 2012 at 10:26 am

    Is isn’t clear which of the animal density changes are simply movement and which are actual population number changes. The recovery numbers seem to indicate a return to deeper water rather than a reproductive increase. Which species appear to have permanent or long-term change from which recovery is not expected?