Pond Boss Forums INTRO Questions & Observations How to make a water depth indicator?

Maybe depth was a poor choice of words, I siphon bottom water into a rock lined sluice to oxygenate the cool bottom waters and direct them to a small pond where I'm attempting to raise trout. By knowing the change in pond level I can calculate flow and adjust it by restricting the siphon. So I have to know exactly how much the level deviates from full (normal overflow height) to calculate how much water I feed to the trout pond. I'm looking to strike a happy medium of cool bottom waters vs. draining down too much from the larger pond.