I'm gonna lean on another commenter who made me realise the legitimate reasoning behind something like this (disregarding the fact that two years is absurdly high): If we permit anyone to do whatever "cleaning" they themselves deem reasonable without approval, we risk that unknowledgable people with good intentions cause serious damage. One reason could be that they create an acute flood risk (you're right: garbage shouldn't be flood protection, but the actual flood protection is built around existing circumstances, and if removing the garbage causes a major risk to people losing their homes, the correct approach is to first prepare the flood protection, then remove the garbage). Another is that people can unknowingly or unintentionally destroy habitats or otherwise damage the environment.

The point is: We have some very competent people that are capable of assessing the impact of various cleaning operations. We need to let those people do their job. There can be very complex interactions in play, that turn your good intentions into catastrophic consequences. Therefore, we cannot allow laymen to judge how large cleaning operations should be conducted.

Full disclaimer: While I think the above reasoning is sound, I think we should be very careful regarding how unauthorised cleaning operations are punished. For example, it seems absurd to me to give jail time for it. When the person in question is obviously acting with good intentions, it's much more reasonable to sentence them to take some course where they can learn about why what they were doing was potentially harmful, and perhaps sentence them to community service working on some authorised project. That way, you help them learn, let them work on something they want to contribute to, and get more resources for the authorised projects.

Wasn't just removing trash. Drove heavy machinery (excavator) into the stream bed to remove silt thereby changing stream hydraulics and releasing silt downstream further degrading water quality and potentially smothering fish, invertebrates and vegetation. Used backhoe to uproot vegetation from stream bed making it more susceptible to future erosion. Dumped and left materials on the streams flood plain which could alter hydraulics in a flood.

As an environmental lawyer he knows permits and extensive planning needed when changing streamflow. (EDIT first comment after just reading links, just followed up with video links, a bit of a puff piece but monitoring/surveying/planning was done END EDIT) As a lawyer and an officer of the court is held to a higher ethical standard than ordinary civilian. Despite this he first conspired to break the law, and then proceeded to break the law. Not a minor transgression, deserves more than a minor punishment. EDIT Not minor transgression but with some mitigation already done, perhaps lessor punishment