D3265: Compositional changes in the Ephemeroptera, Plecoptera, and Trichoptera of an urban river (Mill Creek, OR-WA)

Aquatic insects belonging to the orders Ephemeroptera, Plecoptera, and Trichoptera (EPT) are commonly used as indicators of water habitat health in aquatic bioassessment studies due to their generally low pollution tolerance and their importance as a food source for fish and other animals. Mill Creek, which is an urban river that passes through the city of Walla Walla, Washington, supports two threatened salmonid species (steelhead and bull trout), provides the city with drinking water, and supports many recreational activities, therefore it is important to understand the condition of this river. In the summers of 2023 and 2024, aquatic insects were sampled using D-nets at ten sites along Mill Creek from the watershed to just upstream of the confluence with the Walla Walla River. All specimens were identified as close to species level as possible then EPT indices were calculated based on abundance and taxa. The EPT faunal composition varied greatly across sites, with the highest richness and abundance of EPT in the watershed, and the lowest in the city of Walla Walla. Our findings provide insight into locations of river habitat changes and areas of reduced habitat quality for fish and other animals, including humans.