Novel Methods for Assessing and Prioritizing Road-Stream Crossings for Aquatic Organism Passage

Anthropogenic barriers such as dams and culverts have led to riverine habitat fragmentation and decreased fish diversity worldwide. Low-head structures such as culverts are far more abundant than large dams and may have a larger cumulative impact on river connectivity. Many efforts to inventory road crossing barriers and prioritize removal and restoration efforts do not consider temporal variation in flow. We used cameras to gain continuous water level monitoring data to quantify the relationship between precipitation events and outlet drop height. In the spring of 2024, we selected 25 culvert sites in upstate South Carolina and set up cameras to take a photo every ten minutes at the outlet of all sites. Out of the 25 sites, we observed connection events at 15 sites. We observed between 1 and 16 unique connection events throughout the study period, and 10 culverts had no connection events. Due to limited resources and the high numbers of road-stream crossings, managers and researchers must rely on systems prioritizing evaluation efforts. We created an index that prioritizes watersheds for conducting road-stream crossing field surveys throughout the southeastern United States. The index considers watershed-scale levels of river network fragmentation, land cover, and native biodiversity. Each watershed was given a score between 0 (lowest priority) and 1 (highest priority). Of 25,290 watersheds, 3,389 were ranked low priority (a score below 0.5) and 8,711 high priority (a score above 0.7). A Conditional Inference Tree analysis indicated that native biodiversity was the highest driver of watershed priority.