Our waterways were relatively clean during that time, and had been improving. They were the best they'd been in decades.
They've become increasingly worse since the Tories came in who basically decided it was OK for them to fill our seas and rivers with shit and chemicals for a tiny slap on the wrist than make them invest in the infrastructure to deal with it.
They choose to pollute the rivers because the fines are smaller than actually cleaning it up, which makes the fines a joke. Rather than a deterrent to pollute they are practically an encouragement.