"A weir is a small overflow-type dam commonly used to raise the level of a small river or stream. Weirs have traditionally been used to create mill ponds in such places. Water flows over the top of a weir, although some weirs have sluice gates which release water at a level below the top of the weir."
~ Wikipedia

This weir dam was created to create a mill pond for the Terrace Mill, which was built by settlers from Vermont in 1903. http://www.terracemill.org/

Unfortunately we are in the midst of a drought. Usually quite a bit more water is flowing over the dam: