Annual Ups and Downs
The most striking telltale of Amazonian seasonality is river-level fluctuation which can be considered seasonal pulses. Average annual river-level fluctuations range from approximately 4–15 m. River-level data has been collected in some parts of the Brazilian Amazon for more than a century. Because the Andean countries (Bolivia, Peru, Ecuador and Colombia) have been slow to invest in adequate infrastructure and enough personnel to maintain a permanent hydrological data logging, we have a relatively poor understanding of river-level fluctuation near and in the Andes. We do know that Andean headwaters can fluctuate radically on a daily basis because of heavy rainfall and narrow river valleys. It is not unusual, for example, for water levels in rivers in the Andean foothills to rise 4–9 m in a single day and then to drop again in a day or two.
In the Amazon Basin, average annual river-level fluctuations are most extreme (10–15 m) in an area stretching from the middle Madeira River in the east to the middle Juruá River in the west. Upstream of the mouth of the Madeira, average annual river-level fluctuations in the Amazon River range from approximately 8–10 m. Downstream of the Madeira, averages decrease progressively: in the estuary, daily tides reduce river-level fluctuation to less than 2 m. At Manaus, in the center of the Amazon, the average is approximately 10 m. The range between recorded high and low extremes of Amazonian river levels is approximately 1.5–2.0 times greater than the average annual fluctuation range. All of the large rivers appear to have years when floods are minimal and years when there is no pronounced low-water period. On average, however, floods are known as a “flood pulse” due to their relative predictability in the Amazon. In the estuary, the flood pulse is controlled by the tides though seasonal variability in Amazon River discharge results in an ocean-ward plume of freshwater during the high water season.