It's probable that the lake gets narrower the lower it is. Assuming a steady removal of n acre feet (a common measure of water volume in reservoirs), the rate a

It's probable that the lake gets narrower the lower it is. Assuming a steady removal of n acre feet (a common measure of water volume in reservoirs), the rate at which the water level elevation drops will accelerate as less acreage of surface gets compensated for by more feet dropping. Anecdotally, the water level is dropping fast enough that you come back 2 days later and it's plainly apparent to the eye without looking too closely. So its dropping some significant number of inches per day, and only accelerating.