Réservoir Saint-Michel, reservoir in Finistère, Brittany, France

Réservoir Saint-Michel is an artificial lake in Finistère, Brittany, created between 1929 and 1936 by a dam in the Yeun Elez marshes. The dam is a 510-metre structure with reinforced-concrete arches up to 25 metres wide, anchored in granite. The lake covers 450 hectares and is fed by the Ellez and two streams, the Roudouhir and the Stêr-Red.

The lake was built between 1929 and 1936 by flooding wetlands and peatlands to create a dam for power generation. From the 1960s until 1985, its water was used to cool a nearby nuclear plant at Brennilis.

The Breton name Poull-dour Sant Mikael links the reservoir to Saint Michael, and the lake is often called lac de Brennilis after the commune on its north-eastern shore, though it is shared by Brennilis, Loqueffret, Brasparts and Botmeur. Anglers come for pike, trout and perch, and the Tro Menez Are walking event of 21 May 2009 circled the water with thousands of participants.

The reservoir lies in the Yeun Elez basin, shared by Brennilis, Loqueffret, Brasparts and Botmeur, with no public transport access. Since 2007, facilities on the eastern shore near Nestavel-Braz, beside the two-star municipal campsite of Brennilis, support walking, fishing, swimming and boating. Many footpaths and bridleways cross the area, and EDF opens parts of its perimeter to walkers.

Beavers were released into the Venec peat bog north of the reservoir when it was first filled, and the Venec national nature reserve now borders the lake at Brennilis. The reservoir is also classed as a "Grand lac intérieur" and is known above all to anglers, who catch pike, trout and perch there.