People that I know that only go to LP in the summer have commented to me that they have never seen any signs of mussels at the lake over the past few years - they really don't understand what is happening down 20 or 30 feet below where the water level has been in the summer the past few years.
If the water drought continues this year, I fear they will all see the horrible scene of underwater rocks 100% covered in mussels, and for those mussels on rocks just above the water level, the smell will be horrible as bw56 mentions. My guess is with another 20 feet of water level drop, the infestation of mussels will begin to be visible. At below 3600 I expect real ugliness!