Ah-ha, thank you, I see. As kids, we always scooted past it because not only was the path on a steep embankment, the "trap" was close to the path there.

Although the lake was in a natural depression, it had been increased in area (and depth) by the building of two embankments on one side so that the estate could release water to power a sawmill.

There was no flow of water in to the lake, although it had a major brook outfall, an overflow to another brook, the leakage from the sawmill sluices (which formed another small brook) and some sort of outfall at the eel trap, I guess the source of the water was surface run off and springs.