Can someone with a better understanding of the situation than my Canadian self explain the implications/potential outcomes of this apparent intervention by Trump in the Bristol Bay project and the subsequent removal of the EPA protection? Seems extremely scary to me.
It's very complicated.
The reality is nobody knows what is going to happen. There are going to be lawsuits. In the short run, judges tend to be skeptical of what they see as attempts by political appointees to do end-runs around legal code that they don't like. But in the long run, they are also very conscious to avoid being portrayed as stepping on the prerogatives of the other government branches. Very likely early on there will be some wrist-slapping by the judicial branch and the EPA about-face will be halted. The administration will then try to conjure some pretext for action that fits within the letter of the law. It's not clear what happens at that point, but it's very plausible the project is allowed to go ahead. Almost certainly this drags out for long enough to make the determining factor the next presidential election.
The deeper problems are two.
First, what was once a bipartisan consensus in the US around environmental protection has pretty much collapsed, so we see these wild swings in policy depending on which party controls a particular part of government.
Second, the Republican party, at least the most right wing of it, has figured out that both the judicial branch, and, the scientific expertise in government agencies, represent threats to the ability to carry out arbitrary actions favoring certain industries, in particular extraction and manufacturing industries, over environmental protection. The current administration is aggressively and systematically trying to remove those threats, with fair success.
It's not clear how this ends, but it's clear that if people who care about things, like, say, salmon or steelhead habitat, don't speak up, it won't end well. It probably won't end well anyway, sadly.