This is Dol y Mynach, one of the reservoirs in the Elan Valley. It is shallow and supports a rich variety of plant and animal life unlike the other reservoirs which are deep and steep sided. It is an SSSI (site of special scientific interest). There is a bird hide overlooking the reservoir and I often go there. Every single time I've been there the birds are miles away on the far side!! My recent photos of greylag geese and the heron were taken from here. I wouldn't get any pictures if I didn't have a camera with a good zoom.
