The term várzea refers to land that is inundated by overflow from sediment-rich whitewater rivers. Amazonian várzea forests are characterized by a high diversity of species and adaptations against extended flooding. From the lowest levels to the highest, the rivers of the Amazon basin can expect a yearly fluctuation of around 7 to 15 meters (25 to 50 feet). On land, waterlogging and submergence of vegetation can last up to 210 days per year, with water rising up to 6-7 metres (19-22 feet) around the base of trees.
The substrate of the várzea region is composed of alluvial and fluvial sediments less than 10,000 years old, loosed from the eastern slopes of the Andes Mountains. Downriver from Iquitos, the Amazon River floods twice a year as the rivers from the Peruvian Andes and then the Ecuadorian Andes swell and then exceed their banks after dramatic amounts of seasonal rainfall in these watersheds. Upriver in the Pacaya-Samiria National Reserve where we spend time on our Amazon cruise, the flooding season is once a year, usually between the months of December and June. The river water, heavily laden with sediments and thus called whitewater, floods the surrounding forest for periods up to 10 months each year, affecting a swath of land up to 20 kilometers (12 miles) wide. This is where the wealth of the river is transferred to the forest, rendering the ground remarkably fertile, a trait unequaled on the adjacent Amazon uplands.
In lowland Amazonia, thousands of miles west from the Atlantic Ocean, the rivers are barely 100 meters above sea level and have ample space and time to find the path of least resistance towards the east and the open ocean. Over decades to millennia, these river courses meander across the floodplain, resulting in an ever-changing geomorphology and heterogeneous landscape with a diversity of elements such as oxbow lakes, levees, meander swales, and point bars. Alternating erosion and deposition on the riverbanks occur with every flood, resulting in a vast number of temporary “young” islands and extensive bands of successional forest with a diversity of vegetation types. The youngest vegetation is dominated by the grasses Gynerium sagittatum, Paspalum repens, and Echinocloa polystachya. This is followed by woody shrubs, such as Adenaria floribunda, Alchornea castanaefolia, and Salix martiana, then by early successional trees, such as Annona hypoglauca, Astrocaryum jauari, and Cecropia latiloba. Extensive stands of the economically important buriti palm (Mauritia flexuosa) and Jessenia batuaua are characteristic in this region as well in permanently inundated areas. The waters edge is sometimes dominated by shrubs of the camu-camu fruit (Myrciaria dubia) that not only survive the long flood season, but seem to flourish. A strong component of monocots in the Heliconiaceae, Zingiberaceae, and Marantaceae dominate parts of the understory, along with palms and epiphytes. Trees that stand out in mature várzea forests are Calycophyllum spruceanum, Ceiba samauma, Inga sp., Cedrela oderata, Copaifera reticulata, and Phytelephas macrocarpa. Stands characterized by Parkia inundabilis, Septotheca tessmannii, Coumarouna micrantha, Ceiba burchellii, Ochroma lagopus, Manilkara inundata, and Iryanthera tessmannii are abundant near Iquitos.
There are many critical ecological functions in the várzea forest, such as capturing and rapidly cycling nutrients, hosting a great diversity of freshwater fish and aquatic mammals and stabilizing flooded soils and landscapes. Something that is not often recognized is the importance of the humid tropical floodplain forests in hosting a diversity of trees that produce fleshy fruit, critical to the survival of fruit-eating fish that enter the forest understory during the flood. The várzea region is in fact a critical habitat for reproductive and nursery grounds for fishes and many invertebrates.
Some of the flora and fauna of the várzea is unique, restricted to this seasonally flooded habitat and distinctive from the adjacent upland forest (terra firme). While the species richness of the flooded forests tends to be lower than the upland evergreen moist forests, the flooded forests tend toward a higher turnover rate in species composition across habitats or among communities because of its diverse landform composition. Here in the Upper Amazon region, the delineation of várzea and terra firme forest is less distinct than on the lower várzea, rendering these várzeas slightly richer than those on the central and lower Amazon.
The Iquitos várzea ecoregion as a whole claims 227 species of mammals and 624 species of birds. The following are common terrestrial mammals and some reptiles, though some are more easily seen than others: jaguars (Panthera onca), ocelots (Leopardus pardalis), tapirs (Tapirus terrestris), capybaras (Hydrochaeris hydrochaeris), kinkajous (Potos flavus), and white-lipped peccaries (Tayassu pecari). The endangered giant river otter (Pteronura brasiliensis) endemic to South America is found in the Pacaya-Samiria National Reserve and surrounding rivers. Primates that live in young floodplain forests include various species of spider monkeys (Ateles spp.), two capuchins (Cebus albifrons, C. apuella), red howler monkeys (Alouatta seniculus), squirrel monkeys (Saimiri sciureus), and the Monk saki monkeys (Pithecia monachus). Amazon bamboo rats (Dactylomys dactylinus) and various species of arboreal spiney rats (Echimys spp.) are also found in the floodplain forests in upper Amazonia. Aquatic mammals include the pink and grey Amazon River dolphins (Inia geoffrensis and Sotalia fluviatilis) and the vulnerable manatees (Trichechus inunguis). Large reptiles in the area include black caimans (Melanosuchus niger), Spectacled caiman (Caiman crocodilus), endangered yellow-headed sideneck turtles or yellow-spotted Amazon River turtle (Podocnemis unifilis), and anacondas (Eunectes murinus).
There are 4,700 freshwater fish species in South America right now. On average, over 100 species per year have been discovered and seen during Amazon travel. This year alone, 69 new species have been described thus far. These numbers show very clearly that we are far from knowing the number of freshwater species in South America. Some of these very large fish live in these whitewater rivers, and during the flood season they roam through the flooded forest eating and dispersing fruits from the floodplain trees. These fish include the pacu (Metynnis and Mylossoma), tambaqui (Colossoma macropomum), pirarucu (Arapaima gigas), sardinha (Triportheus angulatus), and the smaller piranha (Serrasalmus spp.), a carnivorous characin. Many beautiful aquarium fish come from these rivers and black-water tributaries and lakes in this region. These include the angelfish (Pterophyllum scalare), the discus fish (Symphsodon aequifasciata sp.), hundreds of cichlids such as in the genus Cichlasoma, assorted characins (family Anostomidae) such as tetras in the genera Hemigrammus and Hyphessobrycon and the famous neon tetra (Paracheirodon innesi), and many small catfish (families Aspredinidae, Corydoradinae, Doradidae, and Loricariidae). Some of these are the unusual “armored catfish,” unique to South America and something to see on our Amazon cruise. They’re the most diverse group of catfish in South America with probably close to 800 species. They’re a fairly evolved, and a very specialized group within catfish.
Amazon várzeas have a long history of human occupation because of their high productivity and accessibility, making for a fundamental difference between the peoples of terra firma and várzea. Principal activities in this region are subsistence agriculture, fishing, and selective logging. On the upper Amazon, a common practice among locals is to sow rice (and more) once a year on the nutrient-rich banks the moment the river levels descend, and harvest a few months later before the rise. Overall, land degradation does not generally occur where these small-scale farmers live. At times of exceptional high river levels, the people decamp to temporary residencies. This inconvenience is more than made up for by the advantages the rich soils provide for agriculture the rest of the year.
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