Macroinvertebrate Oviposition (egg-laying) Habitat Preferences on the Green River below Flaming Gorge Dam

The abundance and composition of macroinvertebrate assemblages found below large dams varies tremendously throughout the Intermountain West. Such variability has been extensively studied in relation to altered thermal regimes, sediment supply and the trophic basis for secondary production. However, no studies have investigated population viability as a function of oviposition (egg-laying) constraints imposed by dam operations. For example, the Green River below Flaming Gorge Dam (FGD) experiences seasonal and daily stage height fluctuations due to varying power demand (i.e., hydropeaking) and water availability (Figure 1). Such changes in stage height are attenuated with distance downstream. Across this same downstream gradient, populations of macroinvertebrates from the orders Ephemeroptera, Plecoptera and Trichoptera range from 0.8% immediately below the dam, to nearly 40% 50 km downstream. 

Since aquatic macroinvertebrates are thought to overwhelmingly oviposit on emergent substrates along the shoreline (Figures 2 & 3), tailwater macroinvertebrates could experience high egg mortality due to flow fluctuations associated with hydropeaking, if such fluctuations are frequent and severe enough to dry the shoreline environment (Figure 4). In addition, the availability of optimum oviposition sites could vary with different flow regimes, which in turn, could limit macroinvertebrate recruitment into the next generation. We hypothesize that egg mortality and/or the lack of oviposition sites resulting from changes in flow represent significant bottlenecks in the life cycles of these organisms and subsequently explains within and among system gradients in assemblage composition.   

The objective of this effort is to characterize oviposition habitat selection by macroinvertebrates in the Green River below FGD, and to quantify the extent to which dam operations alter oviposition habitats and subsequent recruitment success. Similar studies will be carried out below Glen Canyon Dam, where macroinvertebrate assemblages are among the most depauperate of western tailwaters. This project is part of a larger collaboration seeking to inform tailwater management so as to better balance societal water and power needs with those of river ecosystems. 

Collaborators

• U.S. Bureau of Land Management (BLM)
• Utah State University (USU)
• U.S. Geological Survey (USGS), Grand Canyon Monitoring and Research Center
• Western Area Power Association (WAPA)