Firstly, set up the map, with a dam somewhere on it and a river. make the land surrounding the upper part should be higher, normal values for the reservour ground are like 90. Next, click on the water tool, and create a water area that contains the entire reservour and the water leading to it. When you are finished, name this area 'Upper Dam Water Area'. Now, raise the water area of this area to about 60 points above your reservour ground (ex. if your reservour ground is 90, use something like 150 or 170). Then make the lower dam area in the same way, and make sure that there is at least 20 points above it to the ground (if ground level is 40, set this water level to about 20). Name this area as 'Lower Dam Water Area'. Next, click on your dam and name it 'Dam' (or anything you want). Then go to Edit->Scripts. Make a new script under the (neutral) team, and name it something like 'Destroy dam'. Click on Script Conditions. Now make a new condition, '[Unit] Unit has % Health.' Select 'Dam' as the Unit, and 'Less than 5 percent' as the value. Now click on 'Actions if true'. Make a new action and make it '[Unit] Kill Unit'. Now, click 'OK', and make a new script. Set its conditions to '[Unit] Unit is destroyed'. Now, go into the Actions if True tab, and make a new action '[Map] Change water area altitude with damage over time'. Now select the upper water area as the area, set however long you want it to take, and set the damage to 0.00.

Now copy that line. Edit the new line and change the area to the lower water area. (Refer to the image below...)

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And there you have it. You should have a working dam. I'd include the test map but... Well, maybe later.