There are universally recognized signals to communicate on the river when paddlers cannot hear each other, including how to give directions to paddlers up river; signal about river features and safety; and communicate if a paddler is injured after a swim. 
For a complete list of signals with pictures, go to the American Whitewater website.

from American Whitewater's safety website; the head pat to convey that a swimmer is OK
from American Whitewater's safety website; the head pat to convey that a swimmer is OK

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