One of the most dangerous obstacles you can encounter on a
fast-moving river is a sweeper (sometimes called a strainer). A
sweeper is a fallen tree that is partially or completely
blocking passage on a body of water. The danger is being swept
into the maze of branches by the current and becoming trapped.
As water flows around a bend in a river, the speed of the water
changes depending on its location in the river. Water flowing
down the inside of the bend is flowing more slowly than that on
the outside of the bend. Picture two runners on an oval track.
To stay even with each other, the runner on the outside of the
track would have to run at a higher speed, since the distance
along the outside of the oval would be greater.
The same thing happens when water flows around a bend in a
river. Water flowing down the outside has a greater distance to
travel, and speeds up.
The net effect of this faster current is that the outside of a
bend in a fast-moving river is more likely to erode or wash
away. As the riverbank erodes, trees on the riverbank tend to
topple into the river. It is important to know how to safely
paddle past these sweepers and avoid becoming entangled.
The natural tendency is to want to hug the inside corner of the
curve and paddle like the dickens to stay clear the obstacle.
Although this feels right, in fact it is the wrong approach. The
natural pull of the current wants to force the canoe over to the
outside of the curve, where you will end up broadside against
the sweeper.
Although it doesn't feel right when you do it, the proper
approach is to point the bow of the canoe directly at the
sweeper, and to back ferry (back paddle). This way, all of the
paddling energy is directed to staying away from the sweeper.
The current in the river provides all forward momentum needed to
get around the bend. Basically, you will go around the curve
sideways, with the bow always pointed at the sweeper. After you
clear the obstacle, you go back to regular forward paddling.
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