The effect of physiographic and hydrologic complexities and their alterations on the distribution of obligate freshwater dolphins.

Physiographic and hydrologic complexities play major role in determining the habitat suitability for river dolphins. However, dams and other water development structures alter hydrologic regimes that degrade habitat conditions. For the three extant species of obligate freshwater dolphins, namely: Amazon dolphin (), Ganges dolphin (), and Indus dolphin (), the threat is high as dams and water-based infrastructure dotted throughout their distribution range impact dolphin populations by restricting their movement. But there is also evidence of localized increase in dolphin population in certain segments of habitats affected by such hydrologic alterations. Hence, the impacts of hydrologic alterations on dolphin distribution are not as binary as it seems. We aimed to ascertain the role of hydrologic and physiographic complexities in determining the distribution of the dolphins in their geographic ranges using density plot analysis and also to understand how hydrologic alterations in the rivers affect their distribution using a combination of density plot analysis and review of literature. The influence of some of the study variables such as distance to confluence and sinuosity was similar across species-for instance, all three dolphin species preferred slightly sinuous river segments and habitats near confluences. However, varying influences across species were observed for some other variables such as river order and river discharge. We assessed 147 cases of impacts of hydrological alterations on dolphin distribution by categorizing the reported impacts in nine broad types out of which habitat fragmentation accounted for the majority of the impacts (35%) followed by habitat reduction (24%). These endangered species of freshwater megafauna will experience further intensified pressures as more large-scale hydrologic modifications such as damming and diversion of rivers are underway. In this context, basin-scale water-based infrastructural development planning should take into consideration the salient ecological requirements of these species to ensure their long-term survival.