AN ASSESSMENT ON THE IMPACT OF FLOW AND THERMAL REGIME CHANGES ON HYDROLOGIC AND AQUATIC RESOURCES USING PHABSIM AND TEMPERATURE MODELING - BRIGHT ANGEL CREEK AND INDIAN GARDEN CREEK, GRAND CANYON NATIONAL PARK

Grand Canyon National Park is in the process of evaluating the park’s current water supply, Roaring Springs, due to concerns over the structural integrity of the current infrastructure and predicted increases in park visitation. Currently, the park relies exclusively on water from Roaring Spring (RS). RS supplies nearly 50% of the flow of Bright Angel Creek (BAC), a stream system that has been the focus of an invasive species removal project in order to restore native, threatened and endangered fish populations. Additionally, excess water from the transcanyon pipeline is discharged into Garden Creek (GC), the pumping transfer station, creating elevated flow between Indian Garden and the confluence with the Colorado River. We conducted an assessment of the potential impact of discontinuing diversion from the spring; particularly on hydrologic and aquatic resources of two tributaries of the Colorado River. Stream flow and water chemistry were taken at cross sections along the two tributaries. Daily averages of continuous flow and temperature were used for deterministic (energy budget) modeling to assess the effects of an altered thermal regime on BAC. Invertebrate sampling was conducted to evaluate community structure and diversity. Using PHABSIM software, habitat availability models for current and altered flow regimes were constructed in BAC and GC to assess the effects a change in flow would have on fish and invertebrate structure and diversity. Habitat modeling suggests that an increase in discharge toward the historical flow regime would increase habitat for invasive fish in BAC. Aquatic habitat would be significantly decreased for all species in GC and could result in a drop in diversity for macroinvertebrates. Thermal modeling suggests a high correlation between surface water and air temperature, with the effect of increased spring discharge on BAC being most significant within the first mile downstream of spring water input. These results will be used to help the Park make informed decisions adjusting water supply operations.