Gibson Reservoir overtopping the dam, June 1964. Photo via the USGS.

Last weekend I checked the Middle Fork Flathead gauge, as I habitually do almost every morning, and noticed a big number. 92,000 cfs; the historic maximum, set in 1964.

As the first week of June came to a close in 1964, unusual but not unheard of conditions were in place. There was a still a lot of snow left in the high country, as it had been melting slowly but steadily under cooler than normal temperatures over the past month. The rivers and creeks were up, but not high, and the ground was saturated. A week or so earlier a bunch of snow had fallen above 5000 feet up along the continental divide. Then the rain came, a massive amount, concentrated up on the divide around and south of Marias Pass. The rain likely helped melt a little snow a bit faster, but the big story was that almost none of this new precip sunk into the ground. All of it went straight downhill, fast.

The results were spectacular.  Swiftcurrent Creek at Many Glacier rose high enough to submerge the bridge leading to the Many Glacier hotel, stranding employees at work opening things up for the summer.  The dams at Lower Two Medicine and Swift Reservoirs failed, causing deadly surges downstream (it’s probably not coincidence that dams which didn’t feed directly into the Blackfeet Reservation did not fail).  As shown above, Gibson Reservoir overtopped the dam.  20 miles downstream, 80% of the town of Choteau was flooded as the Sun River exceeded its banks by a mile in many places.  Great Falls, receiving flood waters from both the Sun and Marias (into which the Two Medicine feeds) suffered massive flooding.

The Middle Fork of the Flathead at ~90,000 cfs, about 7 miles east of West Glacier along highway 2.  Note the almost submerged railroad tracks.  Photo via the USGS.

On the Pacific side, the Middle Fork of the Flathead rose to the highest levels found in either primary or secondary historic accounts.  Estimates of the peak vary from 90,000 cfs to 110,00 cfs at West Glacier.  For reference, that’s at least three times more than the level shown in this video.  The North and South Forks of the Flathead also ran huge, though not quite as big as the Middle Fork.

The main bridge into Apgar at West Glacier was destroyed.  The old Belton Bridge, a solid concrete arch retired from primary use, was submerged and battered but remained intact.  It was rebuilt and still stands today.  Indeed, none of the bridges over the Flathead within the Flathead Valley were considered safe during the flood, though some escaped permanent damage.  Columbia Falls and Evergreen were flooded, with many folks using boats to retrieve possessions and check on property as the floods began to recede.

Most remarkably, the flood in the Flathead was significantly milder than it could have been.  The overall flow at Columbia Falls was estimated as peaking at 160,000 cfs.  However, eleven years earlier the gates of the Hungry Horse dam were closed, allowing almost total control over the flow of the South Fork.  Without this, the flood would have exceeded 200,000 cfs by a considerable margin.  These days, manipulation of that same dam is aggressively used to reduce the probability of floods.  The 51,000 cfs shown above was a record for that early in May.  It is also the highest I’ve ever seen the river, and close to the highest anyone else is likely to see until the next big flood comes around.

Property damage from the ’64 flood was enormous on both sides of the divide, but if a comparable flood were to happen next year, the damage would be far worse.  Many, many more people live in the flood zones of the relevant rivers, especially along the Flathead.  It may seem like we humans like to play the odds, assuming such things won’t happen again in our lifetimes.  A less charitable, and I think more likely, explanation is that our powers of denial and ignorance remain considerable.  Until next time.