# Medieval dams



## face-face12 (Sep 29, 2014)

In my world I have a reservoir that is backed up by a fortress /dam. The fortress was originally there to block a very important mountain pass that was once used by invaders, it was later converted into a dam but is still used as a fort. The purpose of the dam is the issue. I put a dam there because I really liked the idea of having a huge reservoir in a mountain valley because I thought that the landscape would be really pretty, but then I remembered that in a real world humans would not waste precious resources to build a dam just because it looks pretty but I cant think of any reasons and I was unable to find anything on the web( I was not looking hard enough) I really want the dam to have a practical purpose so I wont have to cut it out of my world, so my question is what uses could a dam have in a fantasy medieval/ gothic setting?


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## Jabrosky (Sep 29, 2014)

face-face12 said:


> In my world I have a reservoir that is backed up by a fortress /dam. The fortress was originally there to block a very important mountain pass that was once used by invaders, it was later converted into a dam but is still used as a fort. The purpose of the dam is the issue. I put a dam there because I really liked the idea of having a huge reservoir in a mountain valley because I thought that the landscape would be really pretty, but then I remembered that in a real world humans would not waste precious resources to build a dam just because it looks pretty, so my question is what uses could a dam have in a medieval setting?


It would provide a larger concentration of water behind it? Could come in handy if there's some need for a lake in the region...


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## mbartelsm (Sep 29, 2014)

A possible explanation is the area suffers from seasonal droughts, so the dam is there to provide water when it is needed


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## stephenspower (Sep 29, 2014)

The dam would also allow control of the water to prevent flooding. It's be an earthen dam, I imagine. They've been around forever.


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## CupofJoe (Sep 30, 2014)

Fish - Many of medieval [European] cultures had a habit of making dams and reservoirs to raise fish for the table. Maybe your culture raises Sturgeon, Kraken, Loch Ness Monsters... If either of the latter, it could explain the need for the fort too - I'd want a strong wall to hide behind if I went Kraken fishing...


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## Bortasz (Sep 30, 2014)

Also this may be part to regulate flow of the river, 
Swamp drainage. 

Maybe there is a big Water mill. 

Maybe they want to change the flow of the river from one path to the another.


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## psychotick (Oct 1, 2014)

Hi,

The oldest use of a dam is probably for irrigation purposes. A dam provides a head of water for the dry seasons, and by opening and closing channels in the lake behind the dam farmers can cycle water off to irrigate their crops. It's also a good method of storing water for dry seasons which allows people and animals to drink.

Cheers, Greg.


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## skip.knox (Oct 2, 2014)

Flood control. Irrigation isn't likely the issue if the setting is mountains, and irrigation assumes some fairly sophisticated central control (to invest in bringing water to dry areas). But floods are devastating and annual. 

Dams for water control for mills is the next most common, but these are small, not near big enough to have a fortress as their origin.

The real problem I have is the segue from fortress to dam. Nobody builds a fortress across a river. The fortress could be next to the river, but it would have been above it some distance, again because of the flooding issue. Can't have a mountain river digging away at the foundations. What you could have, though, is a fortress on top of the hill that the river goes by or around. Then, later engineers could extend a wall from the fortress to the earthen dam, resulting in a fortress overlooking a lake with a spillway below and incidentally a handy source for a good-sized moat. 

There is something like this at Fuessen in Bavaria. The Lech River comes out of the Alps there and there are not only mountains but also hills just below the mountains. While the famous castle is Neuschwanstein, a silly 19thc confection, there's also a perfectly sensible castle on a hill both lower and closer to the river. It's such a sensible location, there has been a fortress there since Roman times. You can read about the Hohe Schloss and see a couple of photos here
FÃ¼ssen - Wikipedia, the free encyclopedia 

hth!


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## arbiter117 (Oct 3, 2014)

face-face12 said:


> In my world I have a reservoir that is backed up by a fortress /dam. The fortress was originally there to block a very important mountain pass that was once used by invaders, it was later converted into a dam but is still used as a fort. The purpose of the dam is the issue.



I saw some posts on flood control and channeling the water in another direction. You said the fort is there to block a mountain pass. I don't know if the mountain pass is above or below the dam, but if it was below you could flood out invading armies...

...Or you could threaten to flood out the mountain village if they don't pay tribute.


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## stephenspower (Oct 3, 2014)

> Irrigation isn't likely the issue if the setting is mountains, and irrigation assumes some fairly sophisticated central control (to invest in bringing water to dry areas).



Wasn't Machi Picu irrigated, and the reason it was abandoned because the water ran out due to drought?


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## psychotick (Oct 4, 2014)

Hi,

Not to mention the Marib dam, 800 BC and the rice paddies in China, which I don't even know how ancient they are. They are incredibly simple things to build. A simple wall of earth often only a few feet high, and then a whole bunch of ditches dug off the sides of the lake that will fill behind the earth dam.

But flood control is an old use of dams - the problem is that it has been unreliable until proper construction techniques came into play. They tended to burst if the water level grew too high. An irrigation dam doesn't require the same level technology as it doesn't have to hold back an entire flood. If it leaks / water flows over the top it doesn't really matter. It only needs to hold back enough water to fill the irrigation channels.

As for using dams to run mills, not so much. In medival England and prior, most mills were simply sited alongside rivers with often channels cut to feed water directly to the wheels. The use of dam to run a mill would also require channels built at the least to divert water flow from the artificial lake to the mill. Channels which don't absorb water as drainage chanels - which are little more than ditches - do. It's actually quite complex technology. And you don't really want to block a river unless those downstream are happy with it. And they usually aren't.

Cheers, Greg.


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## face-face12 (Oct 4, 2014)

Thanks everyone for all the suggestions but I did forget to mention that the people who created this dam are some what advanced for their time period, I don't know if that bit of info will help or contribute to the reason the dam was built.


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## skip.knox (Oct 4, 2014)

Building a dam isn't quite as easy as it sounds. You have flowing water. Put some dirt in there and the water sweeps it away. The best technique is to build a diversion dam so you can have dry ground on which to build the real dam. It requires some understanding of hydraulics.

My favorite such is when the Goths built a dam to divert a river, which allowed them to bury their chieftain, Alaric. Once he was buried, they killed all the slaves who did the work, then opened the dam and let the river cover their lord. Not really relevant, I know, but I love the story.


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