The Green bubble deserves pricking with the cold steel needle of Reason as often as possible(Oh dear, sounds a bit Jonathan Aitken), but watching Tim Worsthall and now Lewis stabbing themselves in the foot with it is getting tedious.
(a) the article was clearly talking about electric power, from context even if not explicitly
(b) yes, we know there are other components of energy use
(c) yes, we know that most media suggestions for energy saving are pure theatre and that the real problems are structural and significant reductions in use will require major rewiring of society (literally and figuratively)
(d) yes, small hydropower schemes are only economic when substituting very high energy costs - but "small" really does mean small: they can become sustainable at quite low kW. There is a lot of info on the web ifyou are interested in the optimal design types, typical cost per kWh etc. e.g. http://www.british-hydro.org/
(e) there is more to life than profit, i.e.reliability of supply. Make profit to live, not live to make profit. Also, current measures of profit neglect many externalities, so the threshold for profitability is likely to change in favour of smaller schemes than at present
(f) there are many hydro-resources that have been abandoned and could be easily rehabilitated (from mechanical to electrical power generation if necessary). Most of these are in the countryside and far from the grid, which gives them a transmission advantage for local supply
In summary, of all the kinds of schemes that get touted as being environmentally friendly, this one may have short-run drawbacks but it is exactly the kind of local micro-generation scheme that would flourish in the long-run, of lower emissions, higher energy prices, higher sensitivity to transmission costs, more decentralised generation and more emphasis on energy security.
Why waste your breath knocking this when charismatic mega-schemes for windpower and even wave power (Severn Barrage) threaten to waste capital and despoil the environment and nobody is protesting against them?