Individuals and groups of the fresh-water dolphin, Inia geoffrensis, have been sighted repeatedly in the mainstem and in nearly all the major tributaries of the Orinoco River. The main exceptions are the middle reaches of rivers that drain the northern Guayana Shield of Venezuela, and the reaches of shield-draining rivers that lie upriver of major waterfalls. Elsewhere in the basin (including the Casiquiare Canal that connects the headwaters of the Orinoco with those of the Amazon), Inia appear to be ubiquitous, although they have been sighted in greater numbers in the whitewater and clearwater rivers that drain the Andes and Llanos regions than in the clearwater and blackwater rivers that drain the Guayana Shield. Some authors have suggested that the populations of Inia in the Orinoco River basin might be separated from those in the Amazon basin by some kind of barrier, either physical or chemical. We find no evidence for such a barrier. The Casiquiare Canal contains sufficient discharge and depth of water to support Inia populations year-round. The Atures and Maipures Rapids in the Orinoco River near Puerto Ayacucho do not seem to inhibit the movement of dolphins: Inia have been seen above, between, and below these rapids. Apparently healthy populations of Inia have been observed in waters having all the chemical compositions that prevail in the Orinoco basin-including the most acid waters. Sotalia fluviatilis, the freshwater dolphin that is sympatric with Inia geoffrensis in the Amazon basin, is not widely distributed in the Orinoco River or its tributaries. Sotalia have been reported by others or observed by us only in the lowermost reaches of the Orinoco, between Ciudad Bolivar and the ocean. Elsewhere in the Orinoco River basin, the seasonally shallow river depths may restrict the distribution of Sotalia.
