"Yes. Boulders weighing perhaps a score or more of tons are rolled
over and over down the river by the tremendous power of the water,
almost with the force and speed of projectiles. Now and again they
will run against snags. The water dashing along behind them is
suddenly checked under the surface. The result is a great up-wave,
such as you have already observed. They are just as likely to go
downward or sideways as upward. You never know."

"Then that is the explanation of the cause of those up-waves?" asked
the Professor.

"That's the way we figure it out. But we may be wrong. Take an old
man's advice and don't monkey with the river."

"I thought you said Dad's beloved Canyon would not hurt him," said Tad
teasingly.

"Dad's Canyon won't. The river isn't Dad's The river is a demon. The
river would scream with delight were it to get Dad in its cruel
clutches," answered the old man thoughtfully, his bristling whiskers
drooping to his chest. "Are you boys hungry?"

The boys were. So Dad sought out a comfortable place where they might
sit down, a shelf some twenty feet above the edge of the river, whence
they could see the turbulent stream for a short distance both ways. It
was a wonder to them where all the water came from. The Professor
called attention to his former statement that the river drained some
three hundred thousand miles of territory. This explanation made the
matter clearer to them.