Pierre, S.D. ? Lake Oahe, the jewel of Missouri River reservoirs in South Dakota, is only about 4 feet above its all-time low set two years ago.

Just nine years ago, the lake hit an all-time high. But below-normal runoff has caused it to shrink to less than half its normal level, according to the U.S. Army Corps of Engineers.

The 231-mile-long lake runs from Pierre to Bismarck, N.D. When full, it is the fourth-largest man-made reservoir in the United States.

It normally holds about 23 million acre-feet of water. An acre-foot of water equals the volume that covers an acre of land with one foot of water.

Oahe now holds roughly 11 million acre-feet and is 43 feet lower than it was at its record high in 1997, when it was 1,618.6 feet above sea level.

That was the year waterways such as the James and Big Sioux rivers set record-high water levels due to heavy spring runoff.

Now, the skies would have to open up for a long time to create a flood emergency on the Upper Missouri River.

“To go to an emergency situation at the Oahe Dam, it would take the addition of more water than what the reservoir currently holds,” said Eric Stasch of Pierre, a civil engineer with the corps.

Oahe Dam was designed for a maximum flood pool of 1,644.4 feet above sea level. When water rises to that level, the corps opens the outlet tunnels. Other than testing the system, the first time that happened was in 1997.

Construction of the dam started in 1948 and finished in 1958. President John F. Kennedy dedicated it in 1963.

“When we hit 1,617 to 1,620 feet, we discharge water as expeditiously as possible,” Stasch said.