Final answer:
The Columbia River is the longest river in British Columbia that starts in the Canadian Rockies and flows into the Strait of Georgia, notably contributing to Washington's hydroelectric power.
Step-by-step explanation:
The longest river in British Columbia, which flows from the Canadian Rockies and empties into the Strait of Georgia, is the Columbia River.
This river is a significant waterway contributing to Washington's hydroelectric dominance, owing largely to its might rather than human factors.
The Columbia River has played an essential part in shaping the hydrography and commerce of Western North America, including parts of British Columbia, and is well known for the dramatic Columbia River Gorge, as well as its historical significance highlighted during exploratory missions to identify direct water communications across the continent for commerce.