Now and then one of the upheavings of the currents would catch the canoe, and from the way in which it was pitched either to one side or the other Rod easily imagined what perils the middle of the stream would have held for them.

Quick action on the part of Mukoki and Wabigoon was always necessary to counteract the effect of these upheavals, and in the bow Wabi was constantly on the alert.

At no time could they tell when to expect the attacks of the unseen forces below.

Ten feet ahead the water might be running as smooth as oil, then--a single huge bubble, as if a great fish had sent up a gasp of air--and in an instant it would be boiling like a small maelstrom.
Rod noticed that each time they were caught near one of these some unseen power seemed sucking them down, and that at those times the canoe would settle several inches deeper than when they were in calm water.

The discovery thrilled him, and he wondered what one of the big eruptions out in mid-stream would do to them if they were caught in it.