TO clarify a few points for Tony Ward of Lawrenny (Western Telegraph letters, July 24).

1) Hydro plants are costly to design because each installation will be bespoke to suit the river’s nature and the site topology, plus many other factors; this is true even of an existing water mill site. E.g. how high must we mount the sluice controls, generators, and switchgear to make them truly floodproof?

What if there is a prolonged hard frost? What about salmon and otters? And anglers? And farmers wanting to irrigate crops? And navigation rights…?

2) Adding a new dam or weir has you-name-it implications for everyone (humans and wildlife!) downstream as well as up: a great deal of negotiating required.

3) Re Mr Ward’s comment that “rivers just go on running” - no, they don't!

Just compare the Cleddau dawdling through Haverfordwest in high summer with the thunder of that weir in wet winters.

Hydrological Outlook UK (NERC Centre for Hydrology & Ecology) publishes river flow forecasts; a visit to will reveal the complex modelling methods.

It isn't just down to rainfall, either: groundwater levels are a major consideration.

Readers will have realised that if hydro was easy, a lot more people would be turning old mill sites into generating stations; but all power (forgive the pun) to those who persevere and install a scheme!