Drive in a few stakes running up the bank and mark them with a black paint pen in inches so you can accurately measure water loss. I made a gauge for mine out of a 48" aluminum ruler fastened to a pipe driven into the bottom. The top of the ruler at 48" is level with the edge of the spillway. Since first filling, through two droughts, I have measured as low as 9". Right now, it's just below 47". I have almost daily readings for a couple years recorded, so I can see if my small leak is getting better or worse.