The Protector by Harold Bindloss

When he reached the blast lamp, which was raised on a tall tripod, Vane stood with his back to the pulsating blaze while he grasped the details of a somewhat impressive scene. A little up-stream of him the river leaped out of the darkness, breaking into foaming waves, and a wall of dripping firs flung back the roar it made, the first rows of serried trunks standing out hard and sharp in the fierce white light. Nearer where he stood, a projecting spur of rock narrowed in the river, which boiled tumultuously against its foot, while about half-way across the top of a giant boulder rose above the flood.

Vane could only just see it, because a mass of driftwood, which was momentarily growing, stretched from bank to bank. A big log, drifting down sideways, had brought up upon the boulder and once fixed had seized and held fast each succeeding trunk. Some had been driven partly out upon those that had preceded them; some had been drawn beneath the latter, and catching the bottom had jambed. Then the rest had been wedged by the current into the gathering mass; trunks, branches, and brushwood all finding a place. When the stream is strong, a jamb, as it is called, usually extends downwards, as well as rises, as the water it pens back increases in depth, until it forms a solid barrier from surface to bed. If it occurs during a log-drive, the river is choked with lumber.