The term "great" serves as a critical qualifier of magnitude or importance. In a geographical context, as supported by standard dictionary definitions, it suggests a significant volume, depth, or expanse. When this is applied to a body of water, the focus shifts toward the sheer mass of the liquid moving through a system.
"White water" describes a specific physical state where air is violently mixed into a fast-moving current, typically triggered by steep gradients or submerged obstructions. The convergence of these two concepts—scale and turbulence—describes an environment where massive volumes of water undergo rapid aeration, creating high-energy hydrological systems.