by Steve Schnarr
This past week in mid-Missouri, we’ve been witnessing a rare winter phenomena on the Missouri River. Usually several times a year the river begins to flow with pancake ice, those rounded, slushy donuts formed of frazil ice that flow downstream when temps dip into the low teens and single digits. When there is an extended cold snap, the river can nearly fill up with ice. It’s a natural phenomena that is really awesome to witness, and people bundle up and head to the river to watch, and listen to, the ice sculpture parade.
(For more about Missouri River ice, check out our blog post from several years ago, “When the Big Muddy Turns White”. )
But in mid-Missouri, it’s been since the 1980’s that the river surface has actually frozen over, stopping that ice flow.
This happened last week about a mile upstream of the Jefferson City 63/54 bridge. On February 17 an ice jam formed in the bend upstream of the bridge. Shortly thereafter, the river flow and level dropped dramatically. In the space of a few hours, the river dropped from 4 feet to -0.3 feet (the level of 0 on the river is pretty arbitrary and was set many decades ago – there is still quite a bit of water flowing at -0.3 feet). The flow reduced from about 37,000 cubic feet per second to about 17,000 cfs.
Meanwhile upstream, all the ice that had been flowing downstream began piling up in an increasingly long ice jam like reverse dominoes. Later that morning, Cooper’s Landing (26 miles upstream) reported that the ice had stopped flowing there. Over the next few days, towns further upstream were reporting ice jammed up.
On Thursday, Feb. 18, the Corps of Engineers flew up the river and noted several ice jams upstream of the original Jefferson City jam up. (Here’s the photos they posted, with the rivermile noted)
By Friday, Feb. 19 we heard that things were jammed up by Waverly, which is 150 miles from Jefferson City (we later heard it was still free flowing there). As the Corps photos showed, this was not one continuous ice jam, but a series of huge jams that left very little fully open water.
I heard from some downstream friends who went to the river to watch the ice flowing and were surprised to find hardly any ice at all! The river was still flowing, but despite the biting wind there was nothing left on its surface! Someone had turned off the ice machine!
In most places, this was not a solid pack of ice. There were gaps and holes, some of which still had very strong current flowing through them. In other places, where the current was strongest, or on the outside of bends, the ice floes piled up. Some stood up on their sides and the surface of the river bulged. Other places were protected by wing dikes and froze smooth and open.
But the thing you notice right away when you stand next to this recently frozen river, is the silence. For a week we’d had a rushing parade of ice, creating a slushy, sizzling symphony. The sound of the pancake and frazil ice is a breathtaking and impressive phenomena. To see our giant river with almost no visual movement is unexpected and jarring.
Now, once your river is chock full of ice just sitting there, you know that it will eventually have to start flowing again. If the surface river ice is able to melt before the river starts rising, it would eventually either move all the ice out or it would melt completely. But if the river starts rising before the jam breaks loose, that water has to go somewhere. It will punch its way through that ice and lift it. It will pour over the top of it if it has to. Everytime the water moves, it will be moving ice with it. That can make new ice jams or even ice dams, which can quickly build up on itself and cause localized flooding.
In our northern states, it’s very common for rivers to have flooding due to ice jamming, and that’s happening in Montana right now. But on the Missouri River down in Missouri, it’s very rare for the river to freeze all the way across.
However, if you look back at old river history, the river used to freeze over pretty often. There’s a couple of reasons why things have changed. One is that the lower Missouri River has been channelized. From 1929 on, the Corps of Engineers worked to narrow the river. On average, the river is about 1/3 as wide as it was naturally, with many fewer islands and sandbars. By squeezing the river (and shortening parts of it), its velocity and depth increased. Rock dikes and revetments focus the power of this swifter, deeper channel. All of these factors make it more difficult for the river to freeze over.
Another change to the river is the building of dams in the Dakotas. All ice created on the river in Montana and the Dakotas is trapped behind a series of massive dams on the river. That means that the river starts out ice free just below Gavin’s Point Dam. All the ice we see further downstream was created in the river or its tributaries downstream of that dam (which is located 811 miles from the mouth of the Missouri).
Another completely unrelated factor is climate change. It is clear that climate has changed the frequency and intensity of ice formation on the Missouri River. Even after the last dam was built, the river would occasionally freeze over. It has become increasingly rare.
So what happens next? As mentioned before, the act of breakup from a massive ice jam can be chaotic and cause ice dams and localized flooding. Because the river is so extremely low this year, the likelihood of actually flooding is pretty low and the National Weather Service is not too worried about it.
If the ice melts in the river channel without a big river rise, it will be a pretty calm affair. If the river rises due to snow and ice melt upstream and rain (predicted for Sunday, Feb. 21) it could be pretty amazing to watch. Because the river is so low to begin with, the main predicted damage will be to docks, barges or boats moored in the river itself.
At Cooper’s Landing, several folks took the time to chainsaw a break in the ice around their docks in hopes that could help save their docks and floats from being torn away from shore by the retreating ice.
Missouri River Relief Fleet Manager John Brady had this to say about an ice jam breakup he witnessed in the mid- 1980’s near Easley, MO. : “I watched an ice gorge that stretched way above Cooper's Landing (as far as I could see), come apart and blow away downstream one winter. After a few hours of cracks that sounded like cannons, the lower end started caving and the whole mass started downstream. I ran alongside it in my truck until almost Easley. The mass started pushing ice cakes up onto the river road so I floored the truck down and got on the blacktop hill at the Easley store. There were a bunch of people there watching the river. There was a steel-hulled houseboat pulled up into a recently made cut in the bank behind the store. The rolling ice eventually pushed the boat into some trees and bent it in half. I heard that the guy that built it upriver was heartbroken and abandoned it there. The chunks of concreted ice pushed up on shore stayed around until at least late spring.”
A few definitions -
Frazil - this is a type of ice formed within the water column. In the case of the Missouri river it forms when the air is very cold, like low teens/single digits for a couple days. Super cooled air mixes with turbulent water to create needles of ice (that's the frazil) that then floats to the top and collects into larger slush patties.
Pancake Ice - When ice floating on a river gets spun by the current and bounces off the banks or other ice floes to round off the edges, creating donut piles of frazil slush around the pancake.
Flow vs. Floe - Very generally, flow is a verb. Ice flows downstream. Floe is a noun meaning a floating chunk of ice. Ice floes flow downstream. Flow is also a measurement of how much volume of water moves past a point in a given measurement of time.
Ice Dam vs. Ice Jam - An ice dam would actually keep water from moving past that point and force it to go somewhere else. An ice jam in this case is formed by so much floating ice flowing down the river at once that it gets stuck in a tight bend, then begins piling up behind that spot. It doesn't completely stop the flow of water though. Water continues flowing downstream underneath the ice. In the case of the Jefferson City ice jam, the estimated flow (the measurement of flow) went from 37,000 cubic feet per second to about 17,000 cfs (a 4 foot drop) once the ice jam 1.5 miles upstream set up. That drop happened within just a few hours.