The basic mechanism is that a glacier coming off a mountain digs down into the ground before sliding downhill, leaving a sill (or sometimes a moraine) that dams up the lake. The word dates to Middle English in northern dialects, from Old Norse tjǫrn, lake without tribularies, from Germanic roots -- it entered mainstream English through the Lake Poets.
And yes, this week has been something of a theme week of short words. Most, but not all, were four letters long so I didn't make it an official theme week. Next week will have an official theme, though.
ETA: Also, yes, smew.
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