Frozen Rivers
February 2, 2005 9:08 PM   Subscribe

Why don't rivers freeze over? More inside.

Waterfalls will freeze over but not rivers. So it can't be because of the speed of the water. There is a bet on this.
posted by Kilovolt to Science & Nature (26 answers total)
Who ever said rivers don't freeze?
posted by coelecanth at 9:11 PM on February 2, 2005

Rivers do freeze over, depending on the location of the river and environmental conditions.
posted by AlexReynolds at 9:13 PM on February 2, 2005

To clarify: Will a waterfall freeze solid at the same time that a river will?
posted by Kilovolt at 9:17 PM on February 2, 2005

The Thames famously froze over, for one.
posted by bonaldi at 9:17 PM on February 2, 2005

The Hudson River freezes a lot, especially upstate--there used to be a big ice industry in NY (pre-refrigeration) that banked on it.

Here's something from the Coast Guard from 2000: From January 13 almost through all of February, the Hudson River was completely frozen from West Point north to Troy. Along with freeing barges and tugs stuck in the ice, the cutters completed traditional flood mitigation missions. They kept river choke points open and the ice flowing through bottlenecks to prevent ice dams and flooding during spring thaws. Had spring flooding occurred without the maintenance of proper flow, many communities would have been inundated. The loss of life and property would have been a possibility.
posted by amberglow at 9:40 PM on February 2, 2005

Funny you should ask this today. Here's a paragraph from an email a friend sent just today:

Michigan weather. It has been cold but not as much snow as usual. We have had really cold temps. The Grand River is covered with ice from N of Grand Rapids to the lake. Its a 45 mile long ice jam that is causing some flooding. The last two days the water level has dropped some so the fear that we would have MAJOR floods is easing.

Note, some rivers are less inclined to freeze because of pollutants, although that is less a problem today than it was in the early 60's (thanks environmentalists! Effort pays off!)
posted by Goofyy at 9:55 PM on February 2, 2005

It's probably because of the turbulence in rivers. When ice does form near the surface, where it is coldest, the turbulent flow causes that ice to mix with warmer water at lower depths (assuming the cold weather isn't overwhelming). The heat of the Earth underneath ultimately counteracts the weather's freezing effect above.

Same goes for lakes and ponds: the surface of the water freezes first, cause that's where it's coldest, and ice won't mix with the lower, warmer water because ice is less dense than water, and there's no mixing action going on.
posted by shoos at 10:54 PM on February 2, 2005

This is why earthquakes happen, too. We're sitting on a thin crust floating on magma.
posted by shoos at 11:02 PM on February 2, 2005

Generally, the faster a river flows, the colder the air needs to be to freeze it over. However, waterfalls might freeze over in spite of their rapid flow because of spray and mist which would freeze on contact with the air, and create a build up of ice. The ice could build up enough to cover over the falls. (I remember hearing about Niagara Falls freezing over once, but it turns out it was really an ice jam that occurred above the falls which cut off the water flow for just a few hours in 1848.)
posted by teg at 11:04 PM on February 2, 2005

The Neva in St. Petersburg, Russia, freezes every winter. Residents frequently cross the river on foot while it is frozen.
posted by achmorrison at 11:08 PM on February 2, 2005

Here in Montana, the mighty Missouri River freezes over every winter, usually a few times -- the falls (of Great Falls) freeze, too.
posted by davidmsc at 11:19 PM on February 2, 2005

The Fraser River, which runs through the town where I am unfortunate enough to live, freezes in the winter.
posted by synecdoche at 11:57 PM on February 2, 2005

I think the original idea behind the question was that, why do rivers sometimes not freeze despite temperatures well below the freezing point of water.
posted by shoos at 12:06 AM on February 3, 2005

Water is at it's most dense at around 4°C thus as water is frozen warmer water will sink to the bottom (this is how fish survive when a pond freezes). With a river if the water is constantly churned due to currents then not enough of the water will reach freezing point.
posted by gi_wrighty at 4:13 AM on February 3, 2005

The Grand River in Grand Rapids, Michigan is certainly frozen over. Eight mile ice jam, all that.
posted by sled at 5:10 AM on February 3, 2005

I've seen the Niagara River freeze over in sections before. Not this year, though. Lake Erie isn't even frozen this year.
posted by Kellydamnit at 5:58 AM on February 3, 2005

The Charles River freezes over all the time in Boston. I once walked from Boston to Cambridge over the frozen Charles once (in my more stupid days). Here's some photographic proof if you need it for the bet.
posted by Civil_Disobedient at 6:58 AM on February 3, 2005

The Schuylkill River in Philadelphia just thawed out for the first time in about a week yesterday. It froze over for a couple of weeks last winter as well.
posted by mbd1mbd1 at 7:16 AM on February 3, 2005

Waterfalls are also mixed with air. It probably takes much less time for a small quantity of water surrounded by below-freezing air to freeze. But a river is a large quantity of water, only one surface is in contact with the freezing air, and as many have pointed out, turbulence means that the water on the surface varies in temperature.

I'm no physicist. But I have climbed around on frozen waterfalls (they do freeze first, at least small ones) and I grew up ice-skating on a river every winter. A brackish river - an estuary, actually - no less.
posted by Miko at 7:22 AM on February 3, 2005

The Mighty Mississippi will freeze over in scenic Hastings, Minnesota.

Scenic Minnehaha Falls will freeze over in historic Minneapolis, Minnesota, if memory serves me.

Kilovolt, you need to spend more time in a really ricockulously cold climate. It can me 0' C seem quite warm.
posted by subgenius at 7:22 AM on February 3, 2005

It can make 0' C seem quite warm. Mea culpa.
posted by subgenius at 7:23 AM on February 3, 2005

To clarify: Will a waterfall freeze solid at the same time that a river will?

I would think that waterfalls would freeze over similarly to how icicles form; a bit of ice forms on the rocks on top and flowing water eventually builds a thicker sheet of ice around it, until eventually the whole thing. The river would freeze more as a sheet over the surface, which would gradually get thicker. I'd think, though, that rivers rarely freeze "solid".

But then, neither would the waterfall, unless it actually stopped the river moving at the waterfall. If there were still unfrozen water from the river going over the waterfall, then the waterfall couldn't really be said to be frozen. If the river froze first, then there would be no water to form the waterfall. That probably won't help with your bet, though, since I'm just guessing.
posted by transient at 7:23 AM on February 3, 2005

Kilovolt - your profile indicates you live in Alberta. You've never seen a river freeze over?
posted by bshort at 7:54 AM on February 3, 2005

I live in a town with a waterfall/dam and a river. Usually what happens is this.... The river starts freezing from the banks inward while the rest of the river continues to flow underneath and there's a channel of running water down the center. Since the waterfall generates a lot of spume, this freezes and seems to accelerate the freezing process, in addition to the fact that the river is slowing and getting shallower as it approaches the waterfall. As a result, in my town, what seems to happen is that the riverbanks freeze first, waterfall area and water pooling area freezes second and the area where water is flowing before pooling to go over the waterfall freezes last.
posted by jessamyn at 8:27 AM on February 3, 2005 says: A glacier is a slow moving, frozen river of ice.
posted by pwb503 at 3:47 PM on February 3, 2005

subgenius - The Mississippi freezes over in Iowa as well. In fact barge traffic north of St. Louis usually stops sometime in Nov. or Dec. depending on the weather. Due to the lock and dam system, it's not every winter though (wider and deeper channel). In grade school an old man would come to tell us about how before the dams were built everyone loved winter because it meant they could cross over to Le Claire without finding someone to take them in a boat or going all the way to the bridge in "the Cities".
posted by jaysus chris at 4:44 PM on February 3, 2005

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