Just for clarity (of terms and understanding) do you think with the river up you were seeing groundwater influence (water in the underground supply) or river water due to hydrostatic pressure. The 2 could be vastly different in parameters.
How long before you expect the winter plankton to start ?
I am not sure I understand the first question.
As the river rises, there is increased hydrostatic pressure down on to the ground water table. The pond is isolated from the river and is the only nearby opening into the ground water table. The increased hydrostatic pressure from the high river water searches for relief and finds it by pushing the ground water up into the pond.
Predicting Plankton activity is a guessing game for me. There are just too many variations in their make-up and in their behavior based on widely ranging pond conditions. The flood added more variables.
A few weeks before or after Halloween should cover it!
