We humans have long dug canals to connect watersheds, thereby improving irrigation or shortening navigation routes. Examples range from small regional projects to the construction of massive channels that require locks and dams to permit river transport.
While natural connections between watersheds are relatively rare, the Casiquiare Canal in southern Venezuela offers a spectacular example. Bifurcating from the Upper Orinoco River, this waterway snakes southwestward for over 140 miles, dropping 100 feet en route; at its southern end, the canal enters the Rio Negro River, a major tributary of the Amazon. In effect, the Casiquiare Canal shunts water between two major watersheds, the largest river known to do so on Planet Earth.
How did this river come to cross a natural divide? It appears likely that the Casiquiare, initially an upper tributary of the Rio Negro, cut its way upstream, eventually capturing flow from the Orinoco. While the initial connection may have been rather meager, repeated flood events along the Upper Orinoco gradually widened and deepened the canal. Today, unlike many human-made canals, the Casquiare is far from a sluggish, seasonal channel; rather, it is characterized by a strong and steady flow between two of South America's largest river systems.
