When padding a canoe you push the water backwards with the paddle which in turn will push you forward. which law of motion is being described in the scenario

That scenario is a very  clear demonstration of Newton's Third law of motion:

For every action, there is an equal, opposite reaction.

you can cut your hand off if your not

answer a fan shaped mass of sediment deposited at the base of a mountain front do to an abrupt change in stream gradient;