River behavior on megafans and potential influences on diversification and distribution of aquatic organisms

Megafans are partial cones of river sediment usually laid down by a single switching river, characterized by areas on the order of 103–105 km2, smooth plains, and slopes of <1°. Astronaut handheld imagery acquired since the early 1980s has permitted the first global geomorphic survey of megafans. Using examples mainly from South America, and based on stream behaviors common in megafans, seven models that appear to have implications for the distribution and diversification of aquatic organisms are presented. River behaviors that appear significant for the fragmentation and age of aquatic habitats on megafans include river switching (models 1.1–1.4) and the disconnection of the megafan river from the main river of the basin (models 2–4). Each model has a habitat fragmentation and a habitat combining mode. In their vicariant mode, models 1.1 and 1.2 involve longer periods of time. Models 1.3 and 1.4 involve the relatively instantaneous merging of aquatic populations from neighboring megafan rivers or neighboring major basins. Models 2–4 involve longer-term dynamics with the potential for speciation. We identify the following levels of diversity related to scales of stream operation: intrafan diversity, or local populations and among local populations; interfan diversity, or among local populations and species; and interbasin diversity, or species and biotas.