My guess is the substrate is only part of their criteria.
I do not know, but I imagine maybe prevailing wind, currents (there are currents in ponds), where the sun comes up/sets, nearby vegetation or cover, temperature variations, association to nearby nests.......... plus maybe more than those potential things, could effect where a male decides to make a nest. I know I had nests this year at year 4 of my pond that I had not seen in that area before. At least not shallow enough to see till this year. Some of the GSF and/or hybrids were right up against a bank in 6" of water.
