@irontoenail - It's easy if there haven't been many changes to the river habitat in the first place. Most rivers that are anywhere near human settlements have been dammed though, and in some cases permanently changed the local landscape.
Even small river dams can disrupt the water ecosystem completely, as they block migrating fish from moving up to the source of the river and spreading nutrients as they go. I heard in one case scientists put salmon carcasses in a river where they once went to die after spawning (before a dam was built) and then compared the ecosystem with another river that had also lost it's natural salmon. The river with the carcasses experienced a huge increase in diversity and strength of flora and fauna.
It's not something that would really occur to the average person as being essential to an ecosystem, but you notice the difference when it's taken away.