Update: Last year I put 5 kiddie wading pools filled with pea gravel on top of cinder blocks along shoreline that was a bit too deep for spawning before - had BG using them immediately to spawn all year. I also removed a lot of watershield along that shoreline.
I think it made a big improvement as this year I'm seeing far more BG in all size classes.
BG can spawn successfully in most situations (they don't like silt washing in on the eggs). The real question is not will they spawn but is what is the survival % of eggs/fry BG. Bg will look for the best location that allows max egg/fry survival. Small rocks provide the best interstadial space for oxygen and protection.
" In Role of Male Parental Care in Survival of Larval Bluegills by Mark B. Bain and Louis A. Helfrich in Transactions of the American Fisheries Society 112: 47-52, 1983 the authors noted optimum Bluegill nest materials were exposed coarse gravel ( 8-32 mm diameter) and pebbles (32-64 mm) in nest substrate and that Bluegill removed particles smaller than 2 mm. Particles larger than 8 mm provided suitable interstitial space to accommodate bluegill larvae. Survival of larvae was directly correlated with the proportion of coarse substrate in the nest."
Plenty here on the subject. Pics first. Mimic nature. The last pic is from a drawn down lake.
See these threads for a lot of info - how to create, rock size, depth, size, location, etc..
Bluegill Beds - Pond Boss Forum
https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=544846&page=1Creating Bluegill spawning area - Pond Boss Forum
https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=199269&page=1