Casiquiare Canal - Geography

Geography

The origin of the Casiquiare, at the River Orinoco, is 9 miles (14 km) below the mission of La Esmeralda at 3°8′18.5″N 65°52′42.5″W / 3.138472°N 65.878472°W / 3.138472; -65.878472, and about 123 metres (404 ft) above sea level. Its mouth at the Rio Negro, an affluent of the Amazon River, is near the town of San Carlos and is 91 metres (299 ft) above sea level.

The general course is south-west, and its length, including windings, is about 200 miles (320 km). Its width, at its bifurcation with the Orinoco, is approximately 300 feet (90 m), with a current towards the Rio Negro of 0.75 miles per hour (0.3 m/s). However, as it gains in volume from the very numerous tributary streams, large and small, that it receives en route, its velocity increases, and in the wet season reaches 5 miles per hour (2.2 m/s), even 8 miles per hour (3.6 m/s) in certain stretches. It broadens considerably as it approaches its mouth, where it is about 1,750 ft wide (533 m). The volume of water the Casiquiare captures from the Orinoco is small in comparison to what it accumulates in its course.

In flood-time it is said to have a second connection with the Rio Negro by a branch, which it throws off to the westward, called the Itinivini, which leaves it at a point about 50 miles (80 km) above its mouth. In the dry season, it has shallows, and is obstructed by sandbanks, a few rapids and granite rocks. Its shores are densely wooded, and the soil more fertile than that along the Rio Negro. The general slope of the plains through which the canal runs is south-west, but those of the Rio Negro slope south-east.

The Casiquiare is not a sluggish canal on a flat tableland, but a great, rapid river which, if its upper waters had not found contact with the Orinoco, perhaps by cutting back, would belong entirely to the Negro branch of the Amazon.

To the west of the Casiquiare, there is a much shorter and easier portage between the Orinoco and Amazon basins, called the isthmus of Pimichin, which is reached by ascending the Terni branch of the Atabapo River, an affluent of the Orinoco. Although the Terni is somewhat obstructed, it is believed that it could easily be made navigable for small craft. The isthmus is 10 miles (16 km) across, with undulating ground, nowhere over 50 ft high (15 m), with swamps and marshes. It is much used for the transit of large canoes, which are hauled across it from the Terni river, and which reach the Rio Negro by the little stream called the Pimichin.