Assessment of Odonata occupancy and habitat suitability at -12 Mile Slough, Glen Canyon National Recreation Area, Arizona

Management practices that enhance habitat complexity in dam tailwaters can increase biodiversity and improve ecosystem health, particularly in the face of climate change and warming water temperatures. In some instances, however, management practices may be intended to simplify habitat features to impede establishment of invasive species. In Glen Canyon National Recreation Area, a backwater known colloquially as “the Slough”, located 5 km downstream of Glen Canyon Dam, is being considered for removal to reduce breeding habitat for warmwater non-native fish. In this report, we assess the occupancy of and habitat suitability for dragonflies and damselflies (order: Odonata) at the Slough. U.S. Geological Survey staff conducted site visits to the Colorado River in Glen Canyon, the Slough, and another backwater (‘Frogwater’) on 11-13, and 26 September 2024. We characterized the physical habitat of the sampling sites by recording water temperatures, specific conductance, dissolved oxygen, turbidity, flow, depth, and benthic substratum size distribution, and we sampled the assemblages of aquatic macroinvertebrates as well as riparian macroinvertebrates using benthic and aerial collection methods, respectively. We describe three distinct benthic aquatic invertebrate communities in and around the Slough, two of which contained Odonata. We found no Odonata larvae in the mainstem at Frogwater, or in the Lower Slough where it joins the Colorado River mainstem. Using historic specimen data from Museum of Northern Arizona, we report eight species of damselflies (Coenagrionidae) and eight species of dragonflies among three families (Aeshnidae, Gomphidae, Libellulidae) from Glen Canyon between 1985-2024. We discuss the habitat requirements of Odonata larvae known to occur in the Slough, as well as their cultural and recreational values. We conclude that re-engineering the Slough to cool water temperatures will likely reduce larval Odonata habitat locally, but is unlikely to affect their diversity and abundance on a regional scale.