Pillows occur where the water piles up on the upstream side of an obstruction.
- A visual pillow usually means the object is solid. Lack of a pillow in front of an object means it is undercut.
- If a rescue boat is sideways in the current and about to hit the object creating the pillow, the boat team should lean into the object. This is referred to as high siding, meaning the crew immediately moves to the high side of the boat. If they lean away (towards the low side), the upstream gunwale will dip under the water and immediately flip the boat.