Picture shows a gully which is provided with a key dam, located high up the gully, with a reservoir showing a pool of blue water. The importance of this dam is that it protects the sediment plots downstream from floods.
A closeup view of a key dam looking up from the downstream side.
Photo sources: (top) Yellow River Conservancy Commission, "Soil and Water Conservation in the Huanghe River Valley," Shanghai Educational Pub. House, 1988;
(bottom) George Leung, 1995.