I would guess that the CC are spawning in the riprap. They are naturally cavity spawners. If you covered the riprap with dirt, it might at least slow them down (get advice from someone more competent before doing this). I'm surprised that the CC are able to reproduce effectively with LMB in the pond, I would think that LMB would hit the small cc quite hard. Maybe remove as may large CC as possible + reduce feeding + stock more LMB?
Could you give us a quick rundown of the geothermal system? That seems pretty interesting.
