It's Chapter 16 of Hatchet and Brian is washing his hands in the lake. He's suddenly aware of a strange presence behind him. When he turns round to see what it is, he's confronted by the sight of a large, angry moose. Brian may have grown skilled in hunting rabbits and trapping foolbirds, but a moose is a different ball game entirely.
Before Brian has had time to think, the moose attacks him, throwing him into the water and coming after him to finish the job. Brian is badly injured in his ribs and shoulders, but somehow needs to escape—and fast. This is truly a life and death situation, the most serious incident that Brian has faced since crash landing in the wilderness. The moose keeps driving Brian deeper and deeper into the muddy water, filling his lungs with filth, until, all of a sudden, she stops, giving Brian an opportunity to struggle to the surface.
But no sooner has he done so than the moose attacks again. Once more, she charges straight at him, sending him crashing back into the lake. And once more, she leaves him alone to continue calmly munching away on a lily pad root. Brian figures that the best way to escape is not to make any sudden movements that might antagonize the moose. So he gets down on his hands and knees and crawls very, very slowly away from the lake until he finds a place of relative safety behind a tree.