Tl;dr: It seems the group was well intentioned but this actually could have been pretty environmentally detrimental.

According to this article, it appears the issue is the team used a digger in some capacity. Details on how are sparse.

I'm a microbiologist (environmental micro), but I studied a few things in grad school. A big chunk was hydrology and riverine engineering, more specifically erosion control and environmental remediation. I promptly went on to work in a different field, because who does what they actually study in university?

I'll spare you the details (unless you want them, I will go HAM on erosion control autistic data dumping because I rarely get to talk about this) but it's pretty easy to fuck up a river if you start digging it up. Given they mentioned flooding concerns, I checked to see if the area is in a flood plain and confirmed it is.

Humans engineer rivers in floodplains to prevent the previously cyclical flooding as well as reduce erosion. If those controls are damaged, the flooding and erosion can resume, and downstream effects (...pun?) can be pretty serious for humans and the environment.

I'd need more information to confirm exactly what happened, but there may be some merit to the complaints about their project. If they didn't have even a cursory environmental impact assessment, they could have done a lot of damage while trying to do something good.