Cloudbursts and Floods in Nepal: Ecological Pathways, Evidence Gaps, and Consequences for Aquatic Fauna and Freshwater Ecosystems

This review assesses how cloudburst-associated flash floods, monsoon floods, and sediment-laden cascading floods impact aquatic fauna and freshwater ecosystems in Nepal. A synthesis of original research was conducted to connect hydroclimatic and geomorphic data to field observations of benthic macroinvertebrates, macrophytes, fishes, crocodilians, and river dolphins. Periodic monsoon flooding does not automatically equate to ecological degradation. For example, Nepalese stream invertebrates are capable of exhibiting population-level flexibility, and flood pulses are known to maintain channel-floodplain connectivity. However, the potential for degradation increases when rainfall, landslide sediment, and bank failure are combined with anthropogenic flooding. Buried fauna and flora experience both habitat homogenisation and disconnection during recovery. The strongest Nepal-specific evidence is for macroinvertebrate seasonality and habitat controls; direct indicators of the effects of cloudbursts on aquatic ecosystems are largely lacking. Post-flood surveys conducted in Koshi Tappu document the displacement and alteration of habitats of fauna, but do not support either mortality or biodiversity loss on a national scale. Fish and river dolphin studies have shown the importance of habitat depth, discharge, water quality, habitat connectivity, and dry season refuges on the fauna of aquatic ecosystems. The objective of this study is to assess how cloudburst-associated flash floods, monsoon floods, and sediment-laden cascading floods affect aquatic fauna and the ecological integrity of freshwater ecosystems in Nepal. The State of Nepal should use hydrometeorology and sediment measurements to guide before-after-control-impact studies of the effects of hazard mitigation practices. The conservation goal should be to protect aquatic ecosystems from fragmentation caused by emergency flood protection systems.