Use and usability of experimental monitoring data and temperature modeling to inform adaptive management of the Colorado River’s thermal regime for native fish conservation below Glen Canyon Dam

Abstract:

Seasonal thermal variability of the Colorado River in Grand Canyon was severely decreased by closure of Glen Canyon Dam and filling of Lake Powell reservoir that was achieved in 1980. From 1973 to 2002, downstream summer river temperatures at Lees Ferry were about 18°C below pre-dam conditions, and limited juvenile native fish growth and survival. A large-scale flow experiment to improve the river’s thermal regime for spawning and rearing habitat of endangered native humpback chub and other native fish in eastern Grand Canyon was conducted in Water Year 2000. Monitoring revealed warming, but well below the 16-18°C optimum for chub 124 km below the dam near the Little Colorado River confluence, and no measurable chub population increase in Grand Canyon. Fall-timed stable flow experiments to improve shoreline chub nursery habitat (2008-12) were also inconclusive relative to juvenile chub growth and recruitment. Field studies also showed that daytime warming of shoreline habitats used by fish under steady flows is limited by high daily exchange rates with main channel water. Monthly averaged and higher resolution temperature models have also been developed and used to support more recent experimental management planning. Temperature simulations have been useful for screening dam release scenarios under varied reservoir storage conditions with and without use of previously proposed but never constructed multilevel intake structures on the dam’s hydroelectric units. Most importantly, modeling revealed the geophysical limits on downstream warming under existing water management and dam operating policies. Hourly unsteady flow simulations in 2006 predicted equivalent levels of average downstream river warming under either fluctuating or steady flows for a given monthly release volume. River warming observed since 2002, has resulted from reduced Lake Powell storage resulting from drier upper basin hydrology. In support of new environmental compliance on dam operations, temperature models have informed decision makers that it is possible for the river to approach near-optimal temperatures for native fish in eastern Grand Canyon when Lake Powell storage is higher (fig. 1), but only if summer dam releases are held well below current operating policies or multilevel intake structures are used.