One thing hinted at by other wtiters here is the fact that many of the world's rivers are in trouble because of humans.
The Murray Darling river system in Australia was dying because of extensive irrigation. It has recently been revived by plenty of rain but, even so, species of bird and animal dependant on the free flow of the river and what had been swamp land created by it have dropped alarmingly in mumbers.
My favourite river, the mighty Clarence of northern New South Wales, used to boast the best fishing. Now it is almost fished out due to trawlers being allowed into the river. Many of the trawler fisherman would welcome enforced legislation that keeps them all out of the river so that fish can have a chance to breed there before going out to sea. It may, however, be too late. The fishing licence introduced some twenty years ago doesn't seem to help much.
As for the Amazon, it really does break my heart the plans there are for limiting it as a river. Dams are being built all in the horrid name of progress..On a documentary recently I saw the catching of this mighty fish along the Amazon. The very idea that this mighty fish and others might well go extinct just so that some lousy city might have fresh water sickens me.