Tide Lifted Anchor
Fresh-water boaters can be surprised by tides. If you set an anchor at low tide, giving it a reasonable amount of rode, even setting a second anchor, you can still be surprised to find yourself adrift. What happens is that the boat comes up on the tide (which can be as much as 14' or 4.5 m, but more typically around 3 ft or 1 m). This adds six times that length to the amount of rode you need, so 18 ft or 6 m more rode. It doesn't actually lift the anchor, just lets the angle decrease enough that the anchor doesn't hold. The trick to setting an anchor in tidal waters is to know how much extra rode high tide is going to need, or just add an extra 20-30 ft.
Another thing you should think about when anchoring at HIGH tide, is whether you will have enough depth at low tide. You don't (generally) want to ground yourself due to anchoring in too shallow water.