A weir is a low barrier across the width of a river to change its level. What is the overflow feature from a canal to a river called?

  • View from the canal:

    View from the canal

  • View from the river:

    View from the river

  • To my mind, the "weir" is a high barrier (that only allows water to flow when levels are exceptionally high; there's nearly always a significant "drop" down a weir). The "sluice" can allow water to be "tapped off" even when levels are low. Commented Apr 14, 2022 at 13:22
  • The sluice can perform the same job as a weir but its height is adjustable and it is used in managed water systems. Often, it is not the full width of the water course, but a weir usually is. It can also be used in conjunction with a weir. Commented Apr 14, 2022 at 13:35
  • The difference between water levels before/after a weir is usually about a metre, maybe two. I wouldn't call that a high barrier. Let's say comparing to a dam (usually with one or more types of spillways). The "overflow/spillway" from a canal to a river could be several metres. At least it is the case for the one I've been to earlier today. Commented Apr 14, 2022 at 13:39
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    I adamantly suggest that weir and sluice gate are two entirely different things. I apologise for a straight talk. But my protest adds no insight to my original question. Commented Apr 14, 2022 at 13:49
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    Yes, they are different things, as @FumbleFingers explains below. AFAIK you can have a weir between a canal and a river as well as across a river. Commented Apr 14, 2022 at 15:30

2 Answers 2


The black stair-like structure is called a stepped spillway.

If you look closely at the spillway, you'll see these trapezoidal structures:

Close-up image of stepped spillway with 10 trapezoidal structures highlighted

The complete structure is called a labyrinth [side] weir.

A side weir is an overflow weir set into the side of a main channel. Side weirs have been extensively used in hydraulic and environmental engineering applications. They typically are used for water level control in canal systems, diverting excess water into relief channels during floods, as storm overflows from urban sewage systems, and as a head regulator of distributaries.

Labyrinth side weirs have different shapes, such as triangular, trapezoidal and semi-circular, in plan view. A labyrinth side weir provides a longer effective length for a given overall side weir opening. In fact, the effective length is the weir crest length that is denoted by l. The increased sill length provided by the labyrinth side weirs effectively reduces upstream head to the particular discharge. They can therefore be used to particular advantage where the width of a channel is restricted and a weir is required to pass a range of discharges with a limited variation in upstream water level.

(From an experiment conducted by M Khalili and T Honar: Discharge coefficient of semi-circular labyrinth side weir in subcritical flow [Download required])

Here are clearer examples of what a labyrinth weir looks like:

Image of a labyrinth weir on a stepped spillway


Image of a labyrinth weir on a stepped spillway with annotations

(Annotations mine)


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    Amazing explanation! Labyrinth weir is the term I am looking for. Many thanks. Commented Apr 15, 2022 at 7:55
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    Nice detective work! I first thought it is a stepped spillway also but there are many similar structures with different terminology.
    – ermanen
    Commented Apr 15, 2022 at 8:33
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    So, would it be fair to say that's a really weir'd canal? ...I'll show myself out.
    – FeRD
    Commented Apr 16, 2022 at 4:06

sluice (also sluice gate) lexico.com
A sliding gate or other device for controlling the flow of water, especially one in a lock gate.
‘the water gushed through the sluices’

There's also sluice room - a closed room found in healthcare facilities such as hospitals and nursing homes, that is specifically designed for the disposal of human waste products and disinfection of associated items. But I think that usage may be falling out of favour.

  • Try Wikipedia, which references the [overflow] channel in the definition. Commented Apr 14, 2022 at 13:18
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    Sluice? Very often there are no gates, just a fixed level. Nonetheless, there are usually a sluice gate a little up/downstream. Commented Apr 14, 2022 at 13:34
  • I don't understand your comment. Weirs are usually "passive" (when the water's high, it flows over the weir), but sluices / sluice gates are usually "operated / controlled" (at least in principle; some designs may be more or less "automated"). Commented Apr 14, 2022 at 14:55
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    @FumbleFingers I very much agree, a weir is, essentially, a fixed structure over which water flows when the designed depth is exceeded. A sluice is a removable or adjustable barrier. To my knowledge automatic sluices are mainly constructed where rivers and land drains meet tidal waters. The idea is that the sluice is normally open to allow water to flow into the tidal water but it closes to prevent salt water flowing back up the drain or river when the tide is higher than the level of the drain or river.
    – BoldBen
    Commented Apr 15, 2022 at 8:39
  • @BoldBen: I'd never have thought of that, but a sluice to prevent backflow of seawater sounds like an excellent example of a good candidate for automatic operation. And I bet some clever people have figured out ways to automate that don't require a reliable power supply! Commented Apr 15, 2022 at 11:35

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