Severn Barrage Saves the Day - or does it?

Talk in energy-producing circles is returning to building barrages across tidal estuaries. The Severn estuary has one of the largest differences between high and low tide in the world so it is always in the minds of planners trying to satiate the demand for energy. A barrage across this stretch of water would provide 6-7% of Britain's energy needs at current rates. The costs would be enormous financially with numbers far above anything anyone can really think about. But on the plus side 30,000-40,000 people would be employed on building the project over about 15 years. Ongoing employment for about 10,000 people would be generated by the power industry and tourism (water sports, industrial tourism, etc) after the whole thing was built. Great - nothing but positives. A very successful, but much smaller, project in Brittany, France shows us that much of this conjecture is very possible.
Not that simple. What about the environment. This estuary is vital to many species of wildlife. Migratory wildfowl utilise the area. Other local wildlife, including salmon and other fishes, would be radically affected. Silting of the river above the barrage is a problem which has not been adequately addressed by reports and studies.
Nobody is really considering the other options.
Perhaps if the investment made in the barrage was diverted to better energy efficiency in everyones' homes the decreased demand for electricity would be more than the potential energy generated by the scheme. I don't know (here I go again, pontificating without all the facts) but I'm not sure that we should always consider more energy production instead of less energy demand.
Once again better men than me don't have the answers. Nuff said . . .