I didn't see any video of it up until now, but I believe you're right that this step should not have been done without an ecological survey. Another poster noted potential concerns about altering the floodplain and damaging the microbial biome with such actions, which seems a legitimate one.

That said, we're discussing a symptom rather than its cause. I believe we should be asking ourselves why we allow our commons to go to shit to the point where unskilled vigilantism is taking the place of absent public works. these guys got so tired of their government allowing their beautiful natural environment to collect garbage that they felt the need to set up this project.

It is much harder to fault them for overstepping or failure to consider these environmental details when the people who are supposed to be doing it are asleep at the switch. At its core, this is government's job and they're neglecting it. It could be argued that anything bad that happens from this is directly caused by that inaction, even though those doing it share that blame to a degree. Anything the volunteers did wrong was done out of ignorance, whereas the government's role is one of negligence, a worse and intentional offense. The solution is to replace the elected officials until you get ones that do their job.