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 boat 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 boat 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.