The answer is MOOSE.
In this case it happened to be a bull moose, I know because I saw him silhouetted against the sky at dusk and inspected his tracks at dawn. (There were moose tracks all over the place on the lakeshore; the tracks I chose to photograph were at least a couple days old, as evidenced by the spiderwebs in the tracks, and, I suppose could possibly be from a different moose.)
Even if I had not seen him, I would know the tracks were from moose due to the sheer size. There are 2 toes, which means it is deer family, and the 6" long footprints are much larger than the next-largest member of the deer family, elk. The habitat is also strongly suggestive of moose – the tracks came out of the shallows of the lake, which as can be seen in photo J1, has plentiful aquatic vegetation that moose like to eat in summer.
While I was watching the moose, he was eating the algae-looking stuff seen in Photo J1, and I was surprised to learn that he chomps the stuff very rapidly. Chomp-chomp-chomp-chomp-chomp-chomp-chomp! It was a magical moment, one of the best of my visit to Glacier National Park. Some of the other "best moments" from that trip are slated to become future tracker quizzes.