Critical climate controls and information needs for environmental assessment and adaptive management in the Grand Canyon region

Climatic drivers of episodic to interdecadal variations and the observed changes in the flood magnitude, timing and spatial scales affect the sediment inputs to the Colorado River. Since the 1963 closure of Glen Canyon Dam, the sole major supplier of sand to the Colorado River in the upper portion of Grand Canyon is the Paria River, which supplies about 6% of the pre-dam supply of sand at the upstream boundary of Grand Canyon National Park. Sand is delivered by the Paria River during short-duration (< 24 hours), large magnitude (up to 300 cubic meters/second) floods that occur primarily during the warm season (July-October). The planning and decision processes in the Glen Canyon Dam Adaptive Management Program (AMP) strive to balance numerous, often competing, objectives, such as, water supply, hydropower generation, low flow maintenance, maximizing conservation of the tributary supplied sediment, endangered species recovery, and cultural resources. In this work, we focus on a key concern identified by the AMP, related to the timing and volume of sediment input into Grand Canyon. Adequate sediment inputs into the Canyon combined with active management of the timed releases from Glen Canyon Dam support the restoration and maintenance of sandbars and instream ecology. Variability in regional precipitation distribution on multiple time scales is diagnosed with emphasis on understanding the relative role of East Pacific tropical storms, North Pacific sea surface temperatures, and subtropical moisture sources. On longer time scales, structured variations in the sediment supply imply a changing baseline for “mean” ecological and geomorphological conditions in the Canyon, counter to the static view taken in the current environmental impact assessments. Better understanding of the coupled climate-hydrologic variations on multiple time scales is increasingly recognized as critical input for adaptive management (both passive and active). In collaboration with the AMP, this work deliberately identifies the entry-points for predictive hydroclimatic information at appropriate lead times. From the standpoint of this active adaptive management program, lead climate information allows scientists and managers to anticipate geomorphic response from critical tributaries, that in turn trigger large-scale, experimental releases from Glen Canyon Dam.