I love the sound of rivers. The gurgle of the stream. The rush of water over the rapids. The roar over the waterfall. Yesterday I was at the upper reaches of the Afon Tawe. Afon is Welsh for River. In Craig-y-Nos country park, high in the mountains. I watched dippers in mid stream and tree creepers. And just twenty miles away is Swansea and the river is the most polluted in the universe.