What fraction of the annual flow of all rivers in the US can typically be stored in reservoirs behind dams?

The correct choice indicates that approximately 60% of the annual flow of all rivers in the United States can typically be stored in reservoirs behind dams. This high percentage reflects the significant capacity that reservoirs have in managing water resources, serving purposes such as flood control, water supply, and irrigation.

In addition to providing a means for stored water during dry periods, these reservoirs play a crucial role in maintaining ecological balance and supporting recreational activities. The ability to retain such a substantial fraction of river flow is largely due to the extensive network of dams and the engineering capabilities used to create them across various river systems.

Understanding this figure is essential for appreciating how human interventions impact water management, availability, and environmental conditions within the U.S. water system. It highlights the critical relationship between technological advancements in water storage and the sustainability of natural water resources.