We sampled 6 rivers of the piedmont region of Venezuela during the dry seasons of 1997 and 1998 to compare physical, chemical, and biological features and evaluate the influence of natural and anthropogenic differences in their watersheds. These 6 watersheds lie in a northeast-to-southwest transect along the interior slope of the Andes. Human disturbance declines and elevation of river origin increases along this transect. All watersheds experience a pronounced dry season from December through March, but rivers to the southwest exhibit less pronounced seasonality in flow and higher water yields. Sediment concentrations were highest in the Río Acarigua, which experiences the greatest human disturbance, but also were high in the least disturbed Río Bumbun, apparently from natural causes. Dissolved nutrient concentrations were lowest in the Río Acarigua, whereas less disturbed rivers contained higher amounts of dissolved nutrients. We collected 46 taxa of invertebrates and 50 fish species. Invertebrates and fish exhibited greatest diversity in rivers intermediate in the gradient. These more diverse systems may experience an intermediate level of disturbance that favors higher diversity, compared to the 2 rivers in the least disturbed landscapes. Cluster analysis done separately using invertebrates and fish separated rivers in the northeast from those in the southwest. However, a full explanation of these trends will have to address the intertwined effects of the natural biogeographical differences and the different human impacts that occur along the gradient.
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