"The UK government wants to tap into the country's great potential for wind, wave, and tidal power. But proposals to harness the River Severn are so far generating mostly controversy.
At 220 miles, the Severn's the UK's longest river. But what attracts clean energy engineers is the tidal action at the river's mouth. Its estuary has the second greatest tidal range in the world. Capturing the energy of that tidal flow could meet five percent of the entire country's electricity needs.
...Critics of the barrage, point to the Bay of Fundy in Canada, the highest tidal range in the World. There, a small barrage caused both silting and coastal erosion. On the other hand, a barrage at La Rance in France has been a reliable source of energy for forty years, without silting."