Marlow Weir. The weir is at left, with water flowing over it from left to right. A weir is a small dam used to control the level of a river. This weir has several levels, and a footbridge passing across it. A weir can increase the oxygen content of the water flowing over it, as is seen here with the foaming water (bottom). Boats navigate past the weir using a lock, in this case the nearby Marlow Lock (not seen), constructed in 1773. This weir is on the River Thames at Marlow, Buckinghamshire, England.