 This is Simon Evans from the Why and Usk Foundation. He's a scientist, also a salmon fisherman, an otter spotter and general river fanatic. You pull out this stick and you can just see this blanket of algae. It's sort of fibrous green slime, isn't it? Simon's here to show me how much the river is suffering, but he can't stop himself from breaking off mid-sentence to show me treasures all the way along the bank.