HARRISBURG, Pa. (WHTM) — The slow-moving but ever-flowing Susquehanna River is beautiful all year long, but it looks especially exciting when the moving water turns to ice. If you stand close enough, you can hear chunks of ice crinkling past one another as they start to form, and when temperatures climb and the sheets of ice start to melt and break, you may hear them cracking apart.

The formation of ice on the Susquehanna is a pretty cool process — literally. The river starts to freeze when there are periods of below-freezing temperatures, explained Ben Pratt, water resources engineer with the Susquehanna River Basin Commission.

Listen to the ice crackling on the Susquehanna River on Saturday, Jan. 22, 2022, before it eases to a stop along the bank:

Small patches of ice called frazil ice start to form pretty quickly once temperatures drop, but Pratt says the rule of thumb is that the river will accumulate more extensive ice when there are three consecutive days with average temperatures below 15 degrees. (This is just an approximation, Pratt warned, because ice can be fickle and surprising.)

When cold weather arrives, border ice begins to form where the river flows most slowly along its banks. That’s typically where the river starts to freeze, Pratt explained, but frazil ice patches can also form further out from the banks, too, where cold air interacts with the surface of the moving water creating a slushy mixture.

If the cold sticks around, the frazil ice congeals and starts to build up across the river. As it flows downstream, Pratt explained, it gets stuck at pinch points, which could be bends in the river, islands, or other bank features that redirect the ice.

There is one pinch point just below the Capital City Airport in New Cumberland, Pratt noted. The west shore juts out into the river and pushes the flow of the water toward the east shore, and then there are some islands below that. “That’s typically where the jam that impacts Harrisburg forms,” Pratt said.

“It’ll start stacking up, and that’s really how an ice field forms,” said Pratt. Once the ice field forms, it slows down the river and makes it easier for more ice to build up if temperatures stay low.

While frazil ice isn’t particularly uncommon, bank-to-bank freezes are rarer. When the river does freeze from bank to bank, there is still moving water underneath the ice, which is where fish and other wildlife in the river continue to reside, Pratt said. (Pillars of anchor ice that reach all the way to the bottom of the river can form in places, too.)

The thickness of the ice on the river is difficult to measure, Pratt said, but once it freezes from bank to bank, more ice can build up both below the existing layer as more water freezes and on top of it if snow falls, assuming temperatures stay low enough.

“Having observed ice over the years, I can say, once we do have an extended freeze and it breaks up, you can see chunks of ice that are anywhere from a foot to two feet or more thick,” Pratt said.

Although the ice may be thick in places, Pratt cautions against ever walking out on the river when it freezes. The river’s moving water can erode the ice in places and create uneven surfaces, which is different from a still body of water like a pond that people may be able to skate on if the ice gets thick enough, Pratt said. People who go out on the river, even if it looks solidly frozen, are putting themselves and others at risk of injury, Pratt warned.

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After the river freezes bank to bank, it can pose hazards for people and objects nearby when it starts to melt and break apart. Ice has been known to take out bridges when it gets moving, and Pratt explained that the ice can also back up water and cause flooding.

“Best case scenario is the temperatures warm gradually and the ice just sort of melts and moves out,” Pratt said.

Ice on the Susquehanna River, Jan. 22, 2022:

Pratt said that hitting that three-days-with-temperatures-below-15-degrees benchmark seems to be getting harder. He says that bank-to-bank freezes seem to be getting less common, with the last major freeze of the Susquehanna having occurred in 2018.