Close to this jam the water was covered with a green scum.

Thor knew what lay under that scum, and very quietly he crept out on the logs.
Midway in the stream he paused, and with his right paw gently brushed back the scum so that an open pool of clear water lay directly under him.
Muskwa's bright little eyes watched him from the shore.

He knew that Thor was after something to eat, but how he was going to get it out of that pool of water puzzled and interested him in spite of his weariness.
Thor stretched himself out on his belly, his head and right paw well over the jam.

He now put his paw a foot into the water and held it there very quietly.

He could see clearly to the bottom of the stream.