The healthiest pecan trees I have seen are on a low peninsula that juts out into Shoals Creek. The peninsula is about 5 feet above the water. More important perhaps is the soil they grow in. A deep sandy loam is best. Clay soil can work as long as it is permeable. I am growing on silty clay loam that is a bit tighter than preferred, but the trees are hitting 12 inches diameter at 20 years old so they are managing.
This thread has a ton of information if you care to put some time into reading.