Why can't the water board filter from the River Ouse flowing in from the sea, as other countries which do not have a lot of rain filter sea water?

I would think with today's technology, building to pipe from the tidal part of the Ouse to the main reservoir would not be too much of a problem.

In a few years' time, when all these extra houses are built, where is the water going to come from?

-RD Cresswell, Brighton