The riverscape of Western Amazonia – a quantitative approach to the fluvial biogeography of the region
Authors: Toivonen Tuuli , Mäki Sanna , Kalliola Risto 
Published: July 2007

Aim  To provide a quantitative spatial analysis of the riverscape (open‐water bodies and their surrounding areas) of the Western Amazonian lowlands using a consistent surface of remotely sensed imagery. Taking into account the essential significance of fluvial environments for the Amazonian biota, we propose that an enhanced understanding of the Amazonian riverscape will provide new insight for biogeographical studies in the region and contribute to the understanding of these megadiverse tropical lowlands.

Location  An area of 2.2 million km2 covering the Western Amazonian lowlands of the Andean foreland region, i.e. the upper reaches of the Amazon river system. Areas in Colombia, Venezuela, Ecuador, Peru, Brazil and Bolivia between longitudes 83 °W and 65 °W and latitudes 5 °N and 12 °S are included.

Methods  A mosaic of 120 Landsat TM satellite images was created with 100‐m resolution, and water areas of over 1 ha in size or c. 60 m in width were extracted using a simple ratio threshold applicable to a large set of data. With this method, 99.1% of the water areas present in 30‐m imagery were mapped with images with 100‐m resolution. Water pixels of distinct river segments were assigned to river classes on the basis of their channel properties, and islands and lakes were distinguished separately and classified. Measures of water patterns such as structure, composition, richness and remoteness were provided for various spatial units. Riverine corridors were computed from the open‐water mask by outer limits of active channels and floodplain lakes. Analytical results are shown as both thematic maps and statistics.

Results  A total of 1.1% of Western Amazonia is covered by open‐water bodies over 1 ha in size or 60 m in width. River‐bound waters comprise 98% of the total water surface. Whilst isolated lakes are scarce, river‐bound oxbow and backchannel lakes are plentiful, comprising 17.5% of all waters. They are particularly frequent along meandering channels, which dominate both in area and length. The riverine corridors including active channels and floodplain lakes cover 17% of the land area. The average distance from any point of land to the nearest water is 12 km. Geographically speaking, the distribution of waters is uneven across the region, and the detailed characteristics of the riverscape are geographically highly variable. Three major, fluvially distinct regions can be identified: central Western Amazonia, the south, and the north‐east. The proportional surface areas of the riverine corridors, numbers of lakes, sizes of islands and their distributions depend largely on the types and sizes of the rivers.

Main conclusions  Our results support the notion of Western Amazonia as a dynamic, highly fluvial environment, highlighting and quantifying considerable internal variation within the region in terms of fluvial patterns and the processes that they reflect and control. Biogeographically, the variety of types of fluvial environments and their characteristics are important constituents of what influences the distribution of species and dynamics of terrestrial habitats. Spatially consistent riverscape data can serve as a consistent and scalable source of relevant information for other biogeographical approaches in the region.