My computer crashed and had to reboot...
Chinook salmon - Wikipedia
You know how badly the various Columbia river dams fucked up the salmon population? Yeah... that's a problem. A big problem. There are quite a few valuable saltwater fish species that spawn far upstream in rivers.
And if you claim river fish isn't relevant in Europe (let alone North America), well, you might want to consider the fact that people aren't going to care about heavy metal contamination or other stuff when they're starving. And since carp was well known as edible in Shakespeare's time...
A river in a less absurdly over-fed country would not have anywhere near this magnitude of invasive fish problem. Kind of like how whenever I look out at the pigeons in my backyard I think "if this was China they'd all have been eaten already".