I came across the word weir today. It appears that we here in the USA refer to this same device as a low water dam.

I’m curious why we don’t use the same word the English do, instead preferring this unwieldy construction. Is it a matter of the people who emigrated here not coming from the parts of England that used weirs, and thus not personally having that word in their vocabularies? Or was there some subtle difference I’m not seeing that required a different term?

  • I would suggest that there is a good chance that this usage was used by people who knew the proper name. Whereas, the building of a weir or low water dam does not require knowledge of the terminology as much as it does the skill to move the earth. In other words, the first people to build them were probably unaware of the formal name for them, but needed and built them nonetheless.
    – David M
    Feb 17, 2014 at 16:43
  • Not everybody reading this is 'we here in the US'. May 8, 2014 at 18:29
  • @DJClayworth I don't think T.E.D. is referring to everyone reading this, but to his associates.
    – choster
    May 8, 2014 at 18:33
  • 1
    Yes, first person plural is not an oxymoron, and I was attempting to use it here. And of course one of the nits of English is that its often indistinguishable from the second person plural, aside from context.
    – T.E.D.
    May 8, 2014 at 18:55

2 Answers 2


As a frequent river user in the U.S. (whitewater kayaker) who has also paddled in other English-speaking countries (South Africa, for one), I can tell you that weir is the same in British-ish English as what is referred to as a low head dam in the U.S. And low head is a better term than low water because the term head pertains to the bulkhead that keeps the water in the pond above it because the water level fluctuates and can indeed be high over an invariable dam size.


Short answer: The purpose of a weir is to measure or assist in the measuring of the flow of a liquid.

Long answer: Most but not all weirs have a notch in the bottom, or portions of the bottom sloping up to the sides. This confines the liquid to a narrow portion of the weir at lower flows to make the weir equations easier to give accurate results because of errors in measuring the cross-sectional area of the flow when it is wide and shallow.

Because of metonymy and/or the general tendency towards imprecise usage, weir and/or low head dam can become entrenched in local usage to describe a particular local structure. Once that happens, anyone who tries to apply a correction runs the risk of appearing to be a pedant.

Low head dams are for the purpose of storing a small amount of water, or increasing the depth slightly, such as for watering livestock. They also cost far less than a high lift dam.

  • 2
    Measurement may be an important use of weirs today, but the OED's citations for the word (which go back to the tenth century) indicate that the purpose of weir was to retain water, for example to power a mill or operate a lock. So this answer seems wrong to me, or at least needs to show more evidence. Feb 17, 2014 at 18:53
  • Until tonight, I'd never heard the word 'weir' to mean anything other than a small, earthen dam that was built to impound water to power a mill. Feb 27, 2014 at 2:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.