In the fall 0f 2015, Kathy and I ran close to 400 miles of the Rio Marañon, in Peru. The trip began at an elevation of 6935′, in an arid canyon that cuts along the eastern flank of the Andes, and ended in the rain forest, at an elevation of 1253′. Here’s the link to Part 1 of my series of blog posts on that trip :
The Marañon vies for the title of the principal headwater of the Amazon River, and, after our return, Kathy and I wanted to learn more about the Amazon area. We put a National Geographic map of the Amazon up on the wall, and then a friend loaned Kathy a copy of “One River – Explorations and Discoveries in the Amazon Rain Forest”, by Wade Davis. This extremely interesting book concerns the history of botanical and ethnobotanical explorations in the Amazon. Ethnobotany is the study of how indigenous people make use of plants – for food, medicine, crafts, hallucinogenic experience and everything else you can imagine. In this case, the ethnobotanical narrative includes the extensive contacts made by these scientists with the Indians of the Amazon, whose knowledge of the rain forest plants and trees was indispensable to their efforts. Also indispensable were demonstrations by the Indians of the preparations of these plants for the varied uses mentioned above. One of the most interesting facts that comes across in these accounts is how driven and downright tough these explorers were. The main character in the book is Richard Evans Schultes, who mentored the author. During the course of his field work, in the years prior to, during and after WW II, Schultes suffered through eight attacks of malaria, a bout of beriberi, numerous other ailments and used every hallucinogenic plant preparation offered him by his Indian contacts, as well as extremely arduous treks through the rain forest and portaging of canoes and equipment past rapids in the rivers they followed. Unlike most modern “adventurers” however, these scientists did not seek physical challenge per se, but, rather, accepted it as a necessary evil.
The book is a real primer on Amazon geography, and makes mention of the Casiquiari Canal. This is a waterway that, however unlikely, links the Orinoco River with the Amazon River. Unlikely indeed! How can this be? How can two drainages be linked? Drainages are usually separated by drainage divides – an area of higher elevation from whose separate sides rivers run to different directions e.g. the Continental Divide of North America. (But anomalies in divides do occur, such as the Great Divide Basin, in Wyoming, where the Continental Divide splits and runs around the east and west perimeters of the basin.)
The Casiquiari Canal is actually a large river that divides off (or bifurcates) from the Orinoco, and runs downhill into the Rio Negro branch of the Amazon.
According to Wikipedia: “The general course (of the Casiquiari) is south-west, and its length, including windings, is about 320 kilometres (200 mi). Its width, at its bifurcation with the Orinoco, is approximately 90 metres (300 ft), with a current towards the Rio Negro of 0.3 metres per second (0.75 mph). 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 2.2 metres per second (5 mph), even 3.6 metres per second (8 mph) in certain stretches. It broadens considerably as it approaches its mouth, where it is about 533 metres (1,750 ft) wide. The volume of water the Casiquiare captures from the Orinoco is small in comparison to what it accumulates in its course … 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 (emphasis added), would belong entirely to the Negro branch of the Amazon.”
The final sentence above suggests how the Canal could have been formed i.e. by “cutting back”. This phenomenon is also known as stream capture or stream piracy:
“Stream capture, river capture, or stream piracy is a geomorphological phenomenon occurring when a stream or river drainage system or watershed is diverted from its own bed, and flows instead down the bed of a neighbouring stream. This can happen (by virtue of) … headward erosion (emphasis added) of one stream valley upwards into another … (Wikipedia)
The above text and illustration concern the total capture of a stream by a neighboring stream, which is clearly not the case here. What we are seeing, instead, is a partial capture of the Orinoco’s flow by the Casiquiari, due, presumably, to headward erosion on the Casiquiari. Additionally, during high water, the Orinoco and other Amazon area rivers rise above their banks and spread outward. This outward spreading (and rise in surface elevation) could have assisted in bringing about contact between the Casiquiari and the Orinoco: “At flood, the (Orinoco) river becomes an area flow source, far more than a narrow confined river.” (Wikipedia).
Obviously, I find this topic of great interest, and would LOVE to visit the Casiquiari Canal. I’ve learned of one tour that runs from the Orinoco through the Canal to the Amazon, and am looking into it “as we speak”.