I have been advised that if the water level increases close to the time when the amount of rain falling is at its greatest, this will suggest the water is more likely to be rainwater.
(the water; to be; rainwater) (the amount of rain; falling; T:the time) (the water level increases close to the time; is; at its greatest)
the water the amount of rain the water level increases close to the time
to be falling is
rainwater
T:the time
at its greatest