Don't rock the Maroochy River
A similar rock wall went into Noosa Main Beach, just north of the popular tourist swimming area in the 1980s. This wall immediately ruined the popular surf break and has done little to stop the beach from losing massive amounts of sand each year with the annual storm season. Sand pumping and dredging of the river is required on a nightly basis, despite the wall. Similarly, problems such as these are experienced in the Mooloolaba River, in and around the Spit, with the surf non-existent down the southern end of Mooloolaba Beach and dredging and sand pumping required on both sides of the The Spit. Both these local examples demonstrate that rock walls cannot be deemed to be a very effective solutions to sand loss, creating more problems with sediment build up in river systems, whilst seemingly making sand loss from the beach much worse than ever before.
I most certainly do not want to have a permanent ugly wall and sand dredger down the Cotton Tree end of Maroochydore Beach and to add injury to insult, no waves to catch or beach to run along!
Pin Cushion Island has forever shifted sides of the river mouth and tacked itself onto the northern and southern side, usually after big storms, or established itself as a fully fledged island (with now broken walkways to no where) as it is today. My grandad would often regal my brother and I with stories of the changes to that area and the Cotton Tree Caravan Park when he was a child.
River mouths are important habitats for fish, birds, reptiles, crustaceans and other marine species and the strong currents that rip through this region provide important nutrients, food and oxygen for many creatures, great and small. A rock wall will have ramifications for the sea life that frequent this passageway and there will be many consequences, especially after cyclonic events, that will have impacts in places much further south and north of the river mouth. An example of this could be that sand islands, such as Fraser, many have their supplies from the coast lessened, or the water quality in the Maroochy River system may worsen as it becomes stagnant, stinky and filled with sediment. Who knows really what the long term impacts might be?
Basically, I think it's a bad move to mess with nature (or continue to do so without fully understanding or appreciating the complexity of such river systems).