He might fall off, or the nails might come out.

Even had he not been paralyzed by long exposure to the cold, he could have no hope of being able to swim in such a torrent.
In the middle of the stream he found a new difficulty.

The posts to which these limber poles were nailed at either end sloped in opposite directions, so that while he started across on the upper side he found that when he got to the middle the pole fence began to slant so much up the stream that he must needs climb to the other side, a most difficult and dangerous performance on a fence of wabbling popple poles in the middle of a stream on a very dark night.

When at last he got across the stream, he found himself in the midst of a hazel thicket higher than his head.

He hallooed to Katy, and she was sure this time that it was his last drowning cry.