In short, piers left in the water over the winter are no match for the ravages of the moving ice. To understand the danger, see ice behaviors. Whether your particular lake or river will be dangerous to your pier in the winter depends on how big it is, how far north it is, and even how windy it gets. If you live on a large lake and that lake freezes over during the winter, your pier will have to come out or it will be destroyed or lost. If you live on a smaller lake you may be okay. If you live on a pond, you're most likely okay for the lifetime of your pier (even that's not absolutely guaranteed). So how do you know if your lake or river is likely to harm your pier? If your pier will be directly exposed to a patch of ice larger than about two blocks on a side, don't take the chance, take your pier out. Smaller than that you could be lucky for a long time, but your pier's safety is not certain. On a river, take your pier out. On a pond or a stream you're likely okay.
Permanent Piers:
On intermediate lakes some owners build permanent piers by driving substantial posts deep into the bottom in hopes of fending off the moving ice by brute strength. While these posts may hold back a floating spring ice sheet, they're no match for the more subtle ice "creep" described below (from our ice behaviors page).
When the ice cracks, water often seeps into it and freezes. Now the ice sheet is a little bit bigger. New stresses build up and new cracks form. The process goes on all winter. The general expansion of the ice sheet is complicated and seemingly unpredictable. But one can easily see that the outer edges of a large ice sheet can creep significantly over the course of a season, and that usually means it will creep shoreward, especially in the early spring when the general trend is toward a warmer (expanding) ice sheet. Pier legs that are encased in the ice sheet will go with the creep.
For this reason these posts often begin to lean and even drift after several seasons. Removal of leaning posts is difficult and expensive.
And they are not immune to total destruction by that just right weather at that just right time of spring when an unusual storm pushes the weight of the entire ice sheet against those posts. Many "permanent" piers are lost each year due to this "ice flow."
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