Three culvert outlets, for filling the lock, indicated with yellow arrows. (This is lock #11 on the Chesapeake and Ohio Canal). The other image is of the same lock, but without the arrows.
<a href="//commons.wikimedia.org/w/index.php?title=User:Bonnachoven&amp;action=edit&amp;redlink=1" class="new" title="User:Bonnachoven (page does not exist)">Bonnachoven</a>
<span class="int-own-work" lang="en">Own work</span>