Quantifying flood-pulse dynamics and associated wetland habitats for juvenile salmon in a large free-flowing river

Floodplain wetlands are productive rearing habitats for juvenile salmon in large river systems, yet there can be considerable spatiotemporal variation in accessibility of these habitats. We examined flood inundation and wetland connectivity across a broad range of return periods (0.1-500 years) in the free-flowing North Thompson River, British Columbia, and overlaid coho salmon (Oncorhynchus kisutch) abundance data to assess habitat accessibility. The activation of floodplain wetlands depends on frequent, small flood events leading to dynamic connectivity and inundation patterns. Results showed that 80 % of wetlands become connected during flood events with return periods shorter than one year, underscoring the importance of frequent, small flood pulses for annually available rearing habitat. However, the distribution of wetlands relative to upstream coho salmon spawning populations and their juveniles reveal spatial variation in patterns of accessible habitat for juvenile rearing. In the upper watershed, abundant wetlands connect with even the smallest floods yet there are few spawners documented upstream. Mid-watershed sections have wetlands that require larger, less frequent floods to become fully accessible and are where most of fish spawn. In contrast, lower reaches have moderate spawner abundances and are without wetlands. This spatiotemporal variation in accessible wetlands for juvenile coho salmon will drive variation in the contribution of floodplain wetlands to coho salmon production. Our findings highlight that maintaining or restoring natural flow regimes, and ensuring alignment between floodplain wetland location and population distribution, are crucial for maintaining and restoring salmon productivity, particularly as climate change intensifies flow variability in large river basins.