Hi all

Weirs were also used to oxygenate and clean the water passing over them, as in the case of rivers or streams that had cotton mills or dye
works etc situated on them. The water tumbling over the edge and
rushing down to the bottom where it hit the stream again would take
in oxygen as it went, thus helping to keep the weed and other natural
cleaning elements alive.

This is why you occasionally see weirs which have stones embedded
in the down-slope of them, which again helps to aereate the water more
effectivly.

Most of these types of weirs, (which don't appear to do anything),
are to be found in the more industrialised areas.

Cheers for now

Cattail !