The Cariboo Gold Rush of the 1860s brought a lot of (mostly American) prospectors to British Columbia, and the Klondike Gold Rush did the same for the Yukon. IOTL the British governors of the time were seriously worried about the American population leading to American control of their colony. Have the British governers of the time be particularly incompetent and have the prospectors take control of BC. Vancouver Island would probably remain under British control. This would likely result in de facto rather than de jure American control of BC, but maybe a British attempt to extend control into the interior could result in war?