Hi Adam.
Depends where. At Britannia, lowest can be around 57 m and highest around 59 m. So 6-7 feet let's say for yearly swing. At Mattawa, the level is nearly constant all year long, within 1 m. Below McKoys, the gauge swings from about -3 to about +24 feet on the gauge. But that gauge is at 45 degrees so the total swing there is about 70% of this range, so about 20 feet yearly. Elsewhere, I do not know for sure, someone else might pipe in. Hope this helps.
--C.