Elen, just take the main road through the Elan valley. When you get to the junction cross the bridge on Garreg-ddu dam and go right to the end to visit Claerwen dam. Then come back, and take the other road up past all the other dams, stopping at each one for photographs. At the end you come to a T-junction, at which you turn right and that takes you back to Rhayader. Go into the visitor centre before you do anything else and get a free map. they are quite helpful in there, and will also point out what is about bird-wise.