Dam this war.

[Grandy]

 Is there any historical evidence of a dam being blown up / opened so the flood covers a battlefield and washes away the enemy soldiers? I see that in movies, games and books all the time, it must have happened, right?

 List of examples:

 - Orthanc in The Lord of The Rings being flooded by the ents.
 - A small meaningless battle in Lock's Quest
 - Final Fantasy Tactics has a mission to open the floodgates.
 - Suikoden V has a fake running away so the enemies take over your castle, only for you to flood your own home so they die drowned.

 Between others.

[fruckert]

Not that I know of
But I have the historical knowledge of a chimp...so I don't know

[Bluhman]

I don't think it has been done historically, methinks, probably because older dams were much too small to be broken in such a way to cause widespread destruction. However, for modern times, it's also unlikely to happen:

During an armed conflict, a dam is to be considered as an "installation containing dangerous forces" due to the massive impact of a possible destruction on the civilian population and the environment. As such, it is protected by the rules of International Humanitarian Law (IHL) and shall not be made the object of attack if that may cause severe losses among the civilian population. To facilitate the identification, a protective sign consisting of three bright orange circles placed on the same axis is defined by the rules of IHL.

EDIT: Found one!

A notable case of deliberate dam failure (prior to the above ruling) was the British Royal Air Force Dambusters raid on Germany in World War II (codenamed "Operation Chastise"), in which three German dams were selected to be breached in order to have an impact on German infrastructure and manufacturing and power capabilities deriving from the Ruhr and Eder rivers. This raid later became the basis for several films.

[Desimodontidae]

Never heard of it like actually killing soldiers. But that would be so freakin amazing!! They should have thrown that in Red Faction 3.

[Drace]

If I remember right, you have a mission to protect a dam in Command & Conquer Generals.

[Dragonium]

Bluhman got it. You've never seen The Dambusters?

[Prpl_Mage]

They tried to do it in Fire emblem path of radiance as well. And succeeded. A general stationed in a town was given the order to open the dam to slow down the attacking army with a large river. He refuses but then decide to die in battle so the command cannot be completed.

[Grandy]

Bluhman got it. You've never seen The Dambusters?

No.