Although Lewis and Clark get credit for making the trip to the Pacific Coast 1803-1805, Alexander McKenzie performed the same trip 10 years earlier through Canada. And although, we give both groups credit for their achievements (which we most definitely should, they were incredible people), I can't help think that some probably went before (i.e., the Spanish Conquistadores), but history will never reveal them.
In 1811, Dave Thompson recorded an event in the Banff area (Athabascas River) in which he describes "huge" footprints that led out into the open plains of today's Alberta. When he stated he wanted to follow them, the Indians guiding him refused. They said the tracks belonged to the "mammoth". Although, I don't recall any specific descrition of "mammoth" being recorded.
Lois L'mour believed that the Indians meant mammoths literally, but others think Thompson came across Bigfoot tracks.
Thompson was probably one of the few early explorer/trappers who could write (certainly there were some who could). Colter couldn't (this is speculation because he never left behind memiors/journals, although the Colter Rock was found on the slopes of the Tetons with his name and 1810 carved in it), and neither could most I imagine. This brings me to wonder how many Sasquatch/Bigfoot sightings may have happened from the 1600's (Spanish Explorations) through the 1840's and never were recorded due to the inability of the person/people involved to write it down?
In the case of the conquistadores, they may have seen such things, but were to afraid of ecclestiastical ramifications to say anything. Although, they certainly ran into the Karawakan Indians in Texas.
Just a thought.
Jon