Like you said, I too tend to find fish easier when the water is low within the river bank. Once it floods over the riverbank onto the floodplain, which it does so easily with even a light rainfall, the pike can disperse greatly. Suddenly there's a lot more areas the fish can spread out into.
Next time you see a dry shallow creek/cove when the water is low, maybe consider going to that area when the water rises. Even the slow parts of the river sort of get a swift current when it's high and I bet pike are shifting into those calm spots along with all the food they're looking for.
Also a 40 inch pike is great i hope you took a picture of that
