|waterway = weir|
|A barrier built across a river to control water speed and depth. Water can still flow over the top.|
|Rendering in openstreetmap-carto|
|Used on these elements|
|Tools for this tag|
A barrier built across a river, sometimes to divert water for industrial purposes. Water can still flow over the top. In the US this is often called a low-head or low-rise dam.
How to map
If the weir is long enough to be mapped as a way, create a way along the length of the weir and tag it waterway=weir. Again, add a name=* if it is known. The waterway=weir should share a node with the river where they cross. Like a natural=cliff the line direction matters: On the left side there is the high side, on the right side is the lower side.
Sometimes a fish pass is related with the weir.
(High-)ways over a weir
This subject was not part of the proposal but the most straightforward solution is that the highway=* and waterway=weir share a way. A section of this way may be marked with ford=yes if there is a ford.
Validators and QA tools may complain about this combination because they may have an overly generic rule saying that highways and waterways can not share a single way - this is one of the valid exceptions and tools should be fixed to accept it.