Fourteen. The Vaupes Arch and Casiquiare Canal Barriers and Passages

Abstract

This chapter examines the relationship between the fi sh faunas of the Amazon and Orinoco river basins and distributions of species across the Vaupes Arch region, the major drainage divide in the Llanos region of eastern Colombia and the western limit of the Guiana Shield in Venezuela. Our focus is the differences and similarities in the two faunas and the historical and contemporary geographic and environmental factors that infl uence fi sh distributions, speciation, and adaptation. The subject of this chapter overlaps with several other chapters in this volume; consequently, our discussion will be limited to geological events that occurred after the elevation of the Vaupes Arch approximately 8–10 Ma in the region that encompasses the southern extent of the Colombian Llanos and the Atabapo and Casiquiare subbasins in southwestern Venezuela. The rise of the Vaupes Arch separated the ancient paleo-Amazon-Orinoco River into two separate drainages—the Orinoco fl owing to the north then northeast, and the Amazon fl owing to the east once it had breached the Purús Arch. Discussions of earlier geological events and their infl uence on the fi sh fauna of northern South America appear in other chapters within this volume. In particular, Chapter 7 describes the biogeography of the Neogene, and Chapters 13 and 15 provide detailed descriptions of geological events and their potential infl uence on fi sh distributions in northern South America. These chapters should be consulted for descriptions of events during earlier periods. The Amazon Basin, the largest in the world, covers about 7 million km (about 40% of the area of South America) and has an averaged discharge of nearly 180,000 m/s. The main-stem Amazon River, which is called the Solimões River in Brazil until its junction with the Negro River near the city of Manaus, is estimated to be about 6,700 km long, with approximately 15,000 tributaries and subtributaries—four of which are over 1,600 km long. The Negro River, the huge north-bank tributary, has a mean discharge estimated at 28,000 m/s, which is about 15% of the annual discharge of the Amazon, and which ranks it fi fth among rivers worldwide. Other major tributaries include the Purús, Madeira, Tapajós, Xingu, and Tocantins on the south bank, and the Napo, Japurá, and Trombetas on the north bank. The rivers and streams of the Amazon Basin have highly varied water chemistry (Sioli 1984), ranging from extreme black waters of low pH and conductivity (e.g., Negro) to clear waters with high transparency (e.g., Trombetas), to white waters with neutral pH and low transparency due to high loads of suspended sediments (Napo). In general, rivers draining the Andes in the western region of the basin are white water, and those draining the Guyana and Brazilian Shields are either clear water or black water. Most of the Amazon Basin lies at very low elevation and is covered in tropical forests, with areas of savanna occurring in upland regions of the Guiana Shield to the north and especially within the Brazilian Shield, south of the eastern main stem. The origin of the river is the headwaters of the Ucayali River draining the eastern slope of the Andes in Peru. After the river leaves the Andes on its eastward course toward the Atlantic, it is a broad meandering channel with many islands and side channels and a gradient of only 1.5 cm/km. The Orinoco Basin covers about 1 million km and has a mean annual discharge of approximately 30,000 m/s, which ranks it third among rivers globally. The main stem of the Orinoco River is estimated to be about 1,500 km from its delta on the Caribbean coast of northeastern Venezuela to headwaters in the Parima Mountain range on the border of Venezuela and Brazil. The Guaviare River, which originates in the Colombian Andes and fl ows through the Colombian Llanos before joining the Lower Orinoco near the town of San Fernando de Atabapo, Venezuela, has a larger and longer channel than the Upper Orinoco, and also has the same sediment-rich water as the lower Orinoco. The Guaviare River could therefore be considered the real main stem of the Orinoco River. To the east and south, the Orinoco Basin is bordered by mountain ranges of the Guiana Shield (Figure 14.1). To the west, the basin is separated from the Magdalena and Maracaibo basins by branches of the Andes Mountains, and to the north it is separated from small coastal drainages and the Lake Valencia Basin by coastal mountain ranges. Along much of its course through the Llanos of Colombia and Venezuela, the Lower Orinoco and its principal tributaries (e.g., Guaviare, Meta, Apure) have broad, low-gradient braided channels. Above the juncture of the