Lakes and Floodplain Habitats

Lakes and Floodplain Habitats

The landscape of the Amazon flood plain is a palimpsest written by the recent erosions and depositions of the rivers. Land is cut and re-laid to maintain an intricate mosaic of forest, scrub, marshes, lakes and channels. Due to the annual flood regime, all habitats flood occasionally. Most habitats flood every year, some submerging by up to 11 metres. Some floating communities do not submerge but are still subject to mixing and disruption by the floods.

All habitats are dominated by the physical activities of the river. Much of the 'land' begins as ridges alternated with swamps or lakes. The ridges are created by annual floods and sedimentation at the sides of major channels. As land distances from the main channels, fine deposits in-fill this relief and poor drainage in low-water season forms backswamps. In such interior areas the vegetation cannot progress to the high forest of levees and sand-bar ridges but forms lower swamp forest (chavascal). Subsequently, this land is recut by the return of the previous channel or another. Therefore in sharp contrast to the adjacent unflooded forest no climax vegetation type is ever reached (Salo et al., 1986). Likewise, no old water bodies develop, most lacks and channels are less than 1000 years old.

The Andean rocks supply both the sediments from which the floodplains of the Amazon river are formed and the dissolved nutrients which allow their high biomass productivity. Lower productivity blackwater lakes which are fed by nutrient-poor forest streams exist on the margin of these floodplains. These two types of seasonally flooded habitat occurring inside and outside of the outermost levees of the river and known respectively as várzea and igapó are often sharply contrasted even though spatially close (Prance, 1979). Igapó is not confined to the neighbourhood of the floodplain; it is also found around the edges of the sometimes very large impeded ('blackwater') lakes and tributaries (the whole lower stretch of the Rio Negro is one example) and is scattered as poorly drained and temporarily flooding patches within the matrix of non-flooding terra firme forest. In contrast, várzea habitats and vegetation are strictly confined to silty 'white-water' floodplains.

The River Amazon floodplain - whitewater habitats
Within the general category of várzea it will be useful to distinguish the habitat sub-types described in the following table:

Floodplain habitats, equivalent local names in italics.
Mainly terrestrial:	Wholly or mainly aquatic:
Island and point sand bars = praias	Main river channels =Rios.
Levees = restingas (silt-heightened riverbanks and bars, on which grows tall forest)	Lesser river channels = paranas (usually flowing but sometimes static during low water)
Low swampy woodland = chavascal (usually behind levees)	Lake channels = canos (as above but connecting to lakes rather than to other channels)
Whitewater floodplain = várzea Blackwater floodplain = igapó (i.e. around lakes and streams outside the flanking levees of a várzea)	Lakes = lagos (i.e. water bodies of variable shape and size but usually fluvial in origin holding static or slow moving water)
Never inundated land = terra firme (land flanking the floodplain)

It is inevitable that all the named landscape features intergrade and contrasting the simplicity of the table to the complexity of the actual floodplain as shown by satellite and aerial photographs shows this classificatory system to be too simple to describe all water bodies. A rapid-flowing parana at flood season may transform to a series of lakes at low water; lakes are often elongated like paranas and may be not much wider than their own channels; and so on. At high water the entire floodplain can be under water that is for the most part nearly static. At such a time only the pattern of the emergent vegetation allows the different types of water bodies to be distinguished. Even when not all is covered, it is useful to remember that all land has been deposited by the river and therefore várzea includes nothing, excepting upper parts of substantial trees, that is never flooded.

The time scales of different process involved in the formation of the floodplain (varzea)

Large-scale processes
All of the physical features mentioned range greatly in size reflecting the variety of processes which form them. Those on the largest scale gain their characteristic forms over time scales of more than 10⁴ years. The most important of these events for Amazonian floodplains have been the eustatic ocean changes of the Holocene (Irion, 1984). During periods of lower sea water level rapid channel incision occurred. Over the last 4000 years a rise in sea water level has widened the floodplain and aggrading silt has slowly limited the blocked-valley ('blackwater') lakes along the border of the reserve to their present shape. So-called 'blackwater' of such blocked valleys and sidestreams is characteristically red-tinted with forest-derived humic material. Margins of the blackwater lakes support the igapó flooded forest and have little floating meadow.
Medium scale processes
The patchy, often laminar, structure so conspicuous in radar and satellite photographs of the floodplain is created over time scales of 10² - 10⁴ years. Main channel migration over this time scale results in the general pattern of lakes and channels we see. Channel migration can be rapid. Puhakka et al. (1992) give an average erosion rate of 115m/year and a maximum 300m/year for the Amazon in Peru. Other estimates are 400 m/year near the Peruvian border with Brazil (Mertes, 1985; Kalliola et al., 1992) and about 200 m/year beach growth at the Solimőes side of the Mamirauá reserve ( Sr Pedro Santos , pers. comm.). Within the várzea, new water bodies are formed in various ways. Meander migration results in a scroll-swale topography within the meander loops. The low lying region between the meander bar and the inside bank initially forms a crescent lake which later becomes a back swamp because of overbank sedimentation. Many lakes within the reserve are crescent lakes of this kind and they offer series of successional habitats (Kalliola et al., 1991b). Larger oxbow lakes are formed by the cutting of whole meanders. These vary considerably in age and size. The largest result from the abandonment (avulsion) of a main river channel and may include a number of meanders. Within the reserve it is likely that its eponymous Lago Mamirauá is an abandoned main river channel, a notion supported by its maximum depth of about 40m which is similar to that of the adjacent main channel of the Rio Solimőes.

The continual reworking of the floodplain in the manner described produces a characteristic scaling of lake size. Hamilton et al. (1992) showed that for Amazonian floodplain lakes size distribution is self-similar over a size range from 0.74-1000 Km2 with an estimated fractal dimension of 1.8. Using 1: 50,000 maps for the Mamirauá reserve focal area, which is the triangle of floodplain bounded by the Rio Solimőes, Rio Japurá and the Parana Aranupu, the Pareto plots show that this self-similar structure extends to water bodies as small as 10 ha. Below this size the Pareto plot is non-linear probably reflecting census bias against smaller water bodies; over the approximately linear part of the curve the slope is -1.27, exactly the same as Hamilton et al. (1992) calculated for Orinoco channel lakes and confirming the fractal patchiness of water distribution at this scale.
Short-term processes
Over time scales of 1 to 10² years shear stress and sediment deposition channel structures such as undercut banks, sand bars and levees. Erosion and sedimentation create, change and destroy both aquatic and 'land' habitats. For example, bank erosion fells and sinks trees to create submerged wood habitats which are characteristic for many animal and plant species. Sedimentation changes flow patterns so that well oxygenated flowing channels transform to languid, oxygen-poor water.

At time scales of less than 1 year by far the most important process is the seasonal change in river flow which creates the annual flood. The graph shows seasonal variation in water depth.

As is the case for all rivers, the course of the annual flood cycle is highly variable. Variation within the annual cycle alters both the amount and duration of habitat availability. Forest dwelling fish must adapt to an ever changing habitat availability. The table shown earlier shows the frequency at which different low water minimum depths have been experienced this century.

Media browser - click on picture to select
The Pareto plot of lake area distribution within the triangle of floodplain within the Rios Solimőes and Japurá and the Parana Aranapu	The Pareto plot of lake area distribution within the triangle of floodplain within the Rios Solimőes and Japurá and the Parana Aranapu	Water depth at Tefe	Aerial view of the Rio Solimoes in the vicinity of Tefe showing varzea floodplain forest. Note the abandoned river channels and the scole-swale topography of the forest.	Aerial view of varzea forest within the Mamiraua reserve. The picture shows two of the most important types of water body, a channel running from top to bottom, and in the foreground, a varzea lake. The lake has floating meadow around its periphery.
Aerial view of Lago Mamiraua and surrounding varzea habitat. In this low lying area of the floodplain there are few trees and extensive areas of floating meadow.	False colour satellite image of the varzea floodplain in the Mamiraua reserve at the confluence of the Rios Solimoes and Japura.	False colour satellite image of central Amazonia.	Flowering trees on the edge of a floodplain whitewater lake in the vicinity of Tefe 345x401 50K	View towards the edge of a varzea lake in the floodplain of the Rio Japura
The edge of a varzea lake showing typical white water inundation forest	Vegetation along the edge of a white water inundation lake in the floodplain of the Rios Solimoes and Japura near Tefe	View towards the forest across Lago Mamiraua in the floodplain of the Rios Solimoes and Japura near Tefe	A caiman swimming across Lago Mamiraua during the low water season	A view across the Rio Japura at high water
View across a varzea lake at high water as a storm approaches	View across a varzea lake at high water	Storm clouds over a varzea lake	Storm clouds over a varzea lake	A varzea lake
		Sunset over a varzea lake