May 2003:
Garrison Dam squelches ice jam
floods
by Andy Mork, Chairman BOMM Board
The ice jam floods on the pre-dam Missouri River in North Dakota were an awesome
spectacle. They were somewhat predictable and only occurred about every 20
years. They came and went quickly, usually within a week or less, but all
who lived along or had property on the river were very concerned until the
"river went out" each spring. With the promise of flood control after the
Garrison Dam would be closed in 1953, considerable permanent building began in
the late 1940's. Each spring these owners hoped they could escape a damaging
flood until the dam was closed. But the Missouri River had a mind of its
own and had "one last fling" before it would be shackled behind a dam, and
delivered a near record ice jam flood in the spring of 1952.
All the conditions that were required for ice jam flooding were present.
The river in North Dakota had flowed somewhat higher than average during the
winter, there was little insulating snow cover on the ice and winter
temperatures were colder than normal. This resulted in a wide and 4 to 5
foot thick ice cover. The Yellowstone and Missouri watershed in eastern
Montana experienced much above normal snowfall and the spring thaws in Montana,
which normally is much earlier than in North Dakota, delivered above normal
water into North Dakota when the huge ice cover was still rigid. The whole
ice sheet would float up and begin moving downstream, but couldn't go far and
became a huge ice dam. Water levels would rise quickly, flooding the
adjacent bottom land. The increasing water levels eventually floated the ice dam
downstream where the whole process was repeated again and again down the
Missouri River until it reached thinner ice farther south usually in central or
southern South Dakota.
When the ice dam just above Bismarck broke the river reached a river stage of
24.7 feet and flowed 500,000 cubic feet per second. Compare that with
river stages now of 6 to 13 feet and average flows of 22,000 cfs! Water
depths of 6 to 10 feet were common on the high bottomland. This flood was
predictable so cattle and other moveable property could be moved out.
Thankfully there was no loss of human life, but there were large wildlife
losses. Where the ice dams and the large water flows moved, many deer on
ice cakes, hay stacks, buildings and other debris floated downstream.
Buildings that were protected by trees were protected from damaging ice flows,
but all other property was destroyed.
With the restricting ice cover gone, the river quickly returned to its channel
and traffic on the high bottomland returned to normal in less than a week!
The Garrison Dam was closed in late 1952 and the threat of ice jam floods were
gone forever. Permanent development along the Garrison to Oahe reach
began.
This contribution is an excerpt from a book that Mork is in
the process of authoring entitled "North Dakota's Missouri River." The
book focuses on the 80-mile stretch between Garrison and the headwaters of the
Oahe, south of Bismarck. He explores the river history, the great
transition of the dams and the growing accumulation of sandbars. Mork has
lived on the river bottoms since the 1930s.