It cannot be changed. The expression comes from the idea that water that has fallen over a dam cannot be brought back again.

VOA: special.2009.05.10

For example, you could put a dam in the middle of the tank.

举个例子,在水槽中间放个屏障。

麻省理工公开课 - 固态化学导论课程节选

If this distance is large in comparison to the wavelength, this dam simply casts a shadow.

如果这个距离,比波长更长，水槽中的屏障就会投下阴影。

麻省理工公开课 - 固态化学导论课程节选

It was agreed this could be more easily done with the aid of a dam.

VOA: special.2009.08.12

Environmental activists say plans by Laos to build a dam in the Don Sahong area near the Cambodian border could doom the nearly extinct Irrawaddy dolphin.

VOA: standard.2010.01.11

Six South Koreans were killed when the North opened the gates to a dam near the border, flooding the banks of a river shared by both sides.

VOA: standard.2009.10.15

Both subjects are high priority for Seoul. Six South Koreans drowned last month when the North suddenly released water from a dam, flooding camping areas in the South.

VOA: standard.2009.10.13

Russians were awoken to the threat posed by the destruction of a hydroelectric plant last year, when a major accident at the Sayano-Shushenskaya dam in Siberia threatened the lives of millions of people downstream.

VOA: standard.2010.07.21

They enter a special visitors' center to learn about the dam and the men who built it.

VOA: special.2009.08.12

Today,visitors can see a statue of the men who worked as high scalers to build Hoover Dam.

VOA: special.2009.08.12

It was a picture of a new dam being built in the western state of Montana.

VOA: special.2009.05.17

Workers built a town called Boulder City to house employees working on the dam.

VOA: special.2009.08.12

Engineers decided the rock would provide a good strong support for the proposed dam.

VOA: special.2009.08.12

They built a large road from Boulder City to the area of the dam.

VOA: special.2009.08.12

A great deal of work was done before operations started on the dam.

VOA: special.2009.08.12

Visitors to Hoover Dam drive on a small road that passes Lake Mead.

VOA: special.2009.08.12

Water over the dam is an expression about a past event.

VOA: special.2009.05.10

But Hoover Dam is still a huge and interesting place.

VOA: special.2009.08.12

Building the dam would not be a simple matter.

VOA: special.2009.08.12

The dam in Montana was a federal project.

VOA: special.2009.05.17

However,it also comes just days after North Korea enraged many South Koreans by releasing water from a hydroelectric dam.

VOA: standard.2009.09.11

The surge came from a sudden release by a hydropower dam further up the river in the North's territory.

VOA: standard.2009.09.07

High in those mountains, behind a crude earthen dam, wealthy Pittsburgh industrialists and their friends sailed yachts on a pretty private lake.

VOA: standard.2010.04.26

Naysayers warned of a catastrophic flood should the dam break, but amid the prosperity, the warnings were ignored.

VOA: standard.2010.04.26

North Korea opened floodgates Sunday morning on a hydropower dam on its side of the North-South border.

VOA: standard.2009.09.09

Days of torrential rains and a lack of maintenance were blamed for the earthen dam breaking.

VOA: standard.2009.03.28

With the force of a giant waterfall, the dam burst,launching a wall of water 18 meters high down the funnel toward Johnstown faster than warnings could be sounded.

VOA: standard.2010.04.26