Rivers offer the promise of great potential for the generation of "green" electricity, but traditional turbine designs are not environmentally acceptable except in a relatively small number of site types - mainly where there already exists a weir or some other water fall. "Pisces Preserver" avoids harming fish while also producing comparatively small amounts of electricity, but in thousands of possible installations. This development could take UK "run-of-river" installations from around 20 to 200 per year.