Hi, I’m the History Guy. I have a
degree in history and I love history, and if you love history too,
this is the channel for you. 45 years ago today, on June 9th 1972, a
confluence of unique and extreme weather and poor city planning resulted in the costliest
and deadliest flash flood in the history of the United States. And while the sheer scope of
the disaster of the 1972 Black Hills flood is ample reason why we should remember the event,
the lessons that were learned, and attempts to prevent future such events, are something
of which the entire world should take note. June ninth of 1972, a large high pressure system
moved down from Canada into the Great Lakes area of the United States, and that pushed a cold
front west where it went into the Dakotas and ran into the Black Hills, which is the highest
set of hills east of the Rocky Mountains, averaging five to seven thousand feet
above sea level. The air to the north of that cold front was very humid and there was
additional moisture coming up from Colorado, and so that created very moist
conditions, the dew point which is the measure of moisture in the atmosphere was
10 degrees above normal for that area in June. As the air came up against the hills it was
subject to something called orographic lifting, and that's where air when pushed up against
terrain moves upwards, and as it moves upwards it cools and starts to release its moisture
as thunderstorms. But because of the strong easterly wind and a slow wind above the hills,
the storms were essentially held in place so that they were repeatedly creating thunderstorms,
over some areas of the region, as much as 15 inches of rain fell in less than six hours. And
an average of ten inches of rain fell in 12 hours over a 60 square mile area. And the terrain of
the Black Hills contributed to flash flooding, the Black Hills have several very steep canyons,
and that channels the water very very quickly. And because of severe rainstorms the week before,
the ground was saturated and couldn't absorb any of the water. Lots of tributaries, some which were
normally dry, were flowing into just one waterway and that was called Rapid Creek, which quickly
swelled to more than 300 times its normal volume. Rapid Creek rose more than 12 feet in
under two hours. The volume of water coming down Rapid Creek was more than fifty
thousand, six hundred cubic feet per second, which was more than ten times the previous
record flood along the waterway. At points it was rising so quickly that it rose more than
three and a half feet in just 15 minutes. A dam upstream which might have mitigated the impact
held, but most of the rain had come downstream from that dam. One of the downstream dams did
collapse because the flood had put debris in its spillway, but that amount of water was
actually relatively small compared to the huge amount that was coming in from all the
tributaries, in this unprecedented rainstorm. Rapid City in southwestern South Dakota is named
after Rapid Creek, and is built along its banks. Gold was discovered in the Black Hills in
1872 and Rapid City was founded in 1874 as a gateway to the Gold Rush area. But because it
was along where the plains meet the mountains, and a long a river basin, it was a natural
confluence of the railroads, and so became an economic trading Center and survives the boom and
bust of the Gold Rush. It made part of its living as a tourist destination as a gateway to the
Black Hills that tourists visited, and that's an industry that increased exponentially when Mount
Rushmore was carved only 20 miles away between 1927 and 1941. There's also a large air force base
in Rapid City and the city actually boomed during the Second World War. It is the largest city
in the Black Hills, in fact the largest city in the western half of South Dakota, and the second
largest city in the state. As of the 1970 census, the population of Rapid City was forty thousand
three hundred and ninety. The rain started falling at about 3:40 in the afternoon, and by 7:15 that
evening, the National Weather Service issued a flash flood warning for the northern Black Hills.
The mayor of Rapid City called the city workers at the National Guard to duty, and by 8 p.m. the
National Weather Service increased the size of the flash flood warning to include Rapid City.
The Canyon Lake Dam failed at around 11:45 and the crest of the flood hit Rapid City at around
midnight, when most of the people affected were asleep in their beds and had no idea of the
impending disaster. One resident recalled that someone pounded on their door and yelled “Get
out! Get out as fast as you can!”. They rushed the family into a car and were able to drive up a
hill when the flood came and washed their entire neighborhood away. In economic terms it was the
costliest flash flood in the history of the United States. 1,335 homes were destroyed, down to their
foundation. Some simply picked up, and floated downstream. More than 5,000 cars were destroyed.
15 of the 23 bridges along Rapid Creek were washed away. In human terms 238 people lost their lives,
more than 3,000 were injured. In total cost it was more than 125 million dollars, which is more than
nine hundred million dollars in modern equivalent. It was truly a flash flood, the creek was back
within its banks by 5:00 am. And while all that water rolls into one river the Sheyenne River,
after Rapid City goes into much flatter ground where it could spread out, and so the crest was
much lower and there were virtually no downstream effects. After the flood the city worked for a
critical response plan to deal with future floods, and that informed response plans throughout
the country. They talked about things like trying to control future floods through dams and
levees, but the ground is not really conducive, it would be subject to sinkholes. And so they
moved to the simple plan of not building in the floodplain. Businesses that had been in
the floodplain, some day allowed to rebuild, but nothing where someone would sleep like a
hotel or a house. And they made a plan for what they called the Hundred Year Floodplain, that is
the largest flood that could be expected over a hundred year period. And have restrictions within
the Hundred Year Floodplain, for example houses have to be built at least one foot above
the crest of what would be expected in the Hundred Year Flood. The National Weather Service
also dramatically increased its capabilities, both nationwide and in the Rapid City area, and in
1997 they installed a real-time flood monitoring system. But the fact is a flood this size will
happen again, as the US Geological Survey notes, floods are natural and normal phenomena, they
are only disastrous because people build in the floodplain. The 1972 flood was determined
to be what is called the 500 year flood, or a flood so rare that would be expected to
happen only once every 500 years. And while that sounds like a remote chance, really what
that means is every year there's a 1 in 500 chance that that flood could occur again. And
the question is next time, will we be caught sleeping? And that's a reason why everyone
should remember the 1972 Black Hills flood. I'm the History Guy and I hope you enjoyed this
edition of my series, 5 minutes of history, short snippets have forgotten history 5 to 10 minutes
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