You would alter the ability of falling leaves to enter or stay in the water and compare the ponds.
Work Step by Step
All else being the same between these two ponds, you would measure the effect of falling leaves by altering the presence or quantities of these leaves and measuring the results. One could remain unchanged as a control. How you could alter the levels of fallen leaves present in the pond while also being assured that this was the only change you made would be exceptionally difficult, however, since any method could cause changes of its own.