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A recent question of an underwater race hunting land-based creatures has been answered with "they can build dams" (here https://worldbuilding.stackexchange.com/a/253462/86466). I have wondered: Is it ever possible to a feasible sentient underwater dwellers to build dams on a river they can reach? I am currently under an impression that any attempt of damming a river from underwater would result in either a dam break or just increased flow to the point of washing the partly built dam away.

Base data: The underwater race is general merfolk, with hands and enough strength to swim up rivers and stay there, as well as fresh water tolerance (they live in the oceans). Their civilization is at about bronze age, but they definitely lack fire knowledge, as it just doesn't burn under water. They are not able to walk on land, while they can theoretically crawl on the open terrain with help from their arms, but they are unable to sustain their gills for prolonged time, so they can at most crawl about half a mile before exhaustion, and a whole mile if there's water in the end, otherwise they get damaged and probably die of dehydration. They are sentient, and thus can build cantrips with natural ropes (assuming a tech that allows them to wind ropes out of some seaweed unknown to landlubbers), and they can transport stones of serious weight using inflatable swim bladders, also naturally obtainable. Any other assumptions can be made except magic and technologies above medieval age.

EDIT: blocking a creek is possible if you can leave water for a little, I'm interested in large rivers that are no less than a merfolk's height deep (2 meters), and also those that have flow speed of 0.5 m/s and above. The initial purpose of the dam was flooding upstream and/or blocking seafish from reaching the upstream settlements, if they are concerned.

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    $\begingroup$ How big a dam is required? I realise that beavers are not inherently underwater creatures, but I suggest that they are a clear proof of concept for water-dwelling creatures building small dams. $\endgroup$
    – KerrAvon2055
    Commented Jan 13 at 14:38
  • $\begingroup$ Oh my i was thinking about the sill used to block the heavier salt water in the Mississippi river, damn😮‍💨 $\endgroup$
    – user6760
    Commented Jan 14 at 7:03
  • $\begingroup$ Half a miles is a long way. Amphibious resource gathering and technology would be a big thing for them. There's just so much that you can't do when you're immersed in a heat-conducting solvent that's incredibly valuable, and so many resources that are only available on land as a direct result of that being as true for plants and animals as it is for people. $\endgroup$
    – g s
    Commented Jan 14 at 8:32
  • $\begingroup$ @KerrAvon2055 and others, well blocking a lazy river is indeed feasible by beavers, let alone merfolk that can crawl out of the river. But, a river that beavers can block is too small to host a merfolk by depth and sometimes by width. I would expect a limit of feasibility by width or flow of target river, provided that river is at least 2 meters deep - damming a smaller river might not pose any threat even to a stone age human civ, let alone something medieval. I think damming the Amazon would not be feasible, but what's for smaller rivers? Any flow/speed limits? $\endgroup$
    – Vesper
    Commented Jan 14 at 19:24
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    $\begingroup$ @Demigan interesting story, but the dam is finished in 1923, a lot more modern than sought. Ijssel peaks at 1800 m^3/second, dividing this flow rate over 30 km if the dam yields almost zero flow speed. With such low speed I think merfolk technically could build a similar dam, given enough workforce and matter. I wonder whether the river flow would pose the largest obstacle to such builders. $\endgroup$
    – Vesper
    Commented Jan 18 at 12:00

2 Answers 2

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I am not a Hydraulics Engineer - but I think this is unfeasible

So, given the Tech Level, we are talking Stone Dams. Data was a bit hard to find - however this gives, I feel, a good benchmark: Here gives a maximum height of 1 Metre for safety reasons.

Now, depending on your interpretation of the Tech Level - if we are allowing Roman levels of Tech - we open up a whole load of options - especially if they have a way to make Concrete blocks that can be 'sunk' - but I don't think that is what you intend.

Next up we've got to talk about the forces for a River flowing at 0.5 m/s and at least 2 metres deep: Here's a Mass Flow rate Calculator - your Mass flow rate is approximately equal in Tonnage to the rivers Width in Metres (assuming a perfectly rectangular river) - so a 10 metre wide river has approximately 10 Tonnes of Water flowing per second. That's a lot of water.

Although - if they have the ability to pre-create a Spillway so that as they build the Dam, the water gets redirected, then possibly?

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  • $\begingroup$ Screw safety reasons, these are largely for amateur-grade construction with modern safety requirements (there were none in medieval and before, at least that we know of). $\endgroup$
    – Vesper
    Commented Jan 18 at 12:03
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Constructible: probably --- For the intended purpose: nope

As a joint effort humans in the past were able to exceptional feats. From moving and arranging the stonehenge monoliths to the pyramids. When a whole civilisation works together they can and have achieved things that seem impossible for their age and technology at first sight.
I have no doubt that your merfolk can bring together enough and big enough stones to "brute force" a dam construction, if they are numerous enough. To make construction easier they would redirect parts of the river upstream (closer to the sources that is much much easier) and break down the redirections once the project is complete.
All it takes is a lot of hands, one or two genuises that are a bit ahead of their time (like lets say archimedes or leonardo da vinci) and a megalomaniac that forces his entire population to work on the project for multiple decades.

However looking at the linked question/answer and seeing what the dam should be used for: nope. Just nope.

According to the linked question/answer the damns should be used to store a lot of water and then be opened to flood an area in order to flush out or outright drown land prey for hunting purposes.

The dam would not only need to be able to hold a lot of water but also a way to release all that water very quick. With bronce age technology I highly doubt that such a flood gate is possible. Withstanding such pressure and still be openable.

The other way ofcourse would be to break down the dam. It would achive its purpose in flooding the land below enough to flush out a few handfull of unfortunate deer. But considering that your entire nation worked decades on the project and, as is the nature of such undertakings, lost a lot of lifes doing so, they would rightfully crucify the idiot that breaks the dam for some hunting fun.

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  • $\begingroup$ Well, that answer indeed said "build and break", I was thinking about "just build". Certainly breaking a dam is no fun for anyone downstream, and since those merfolk would be on exactly that side of a breaking dam (or get dragged there by the flow, heh)... so yes, no intentional breaking. $\endgroup$
    – Vesper
    Commented Jan 16 at 19:16

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