Satellite imagery suggests that construction on the dam began sometime around 2013 and has been proceeding from the North Korean side to China; the nearest Chinese city being Ji'an.
You can see the beginning stages of construction in this image from January 5, 2014.
I haven't been able to find much additional information about it. NKEconomyWatch posted that North Korea and China were planning to build two additional hydroelectric dams on the Yalu. One would be built from the Chinese side and the other would come from North Korea.
The North Korean dam, located by the small village of Changchuan, is estimated to cost $78 million and is expected to produce 154 million kWh annually (enough to power 14,000-20,000 homes). The initial agreements to build the dams came in 2004 and 2008, with a proposed completion date of 2013. Obviously, they've fallen behind.
This image from October 21, 2014 shows the work advancing and shows the hydroelectric generating station under construction.
The original width of the Yalu River at this point was around 850 feet (259 m). The latest imagery available, May 24, 2015, shows the dam has pinched the river into a 100 foot (30 m) wide neck. It also shows a cofferdam large enough to facilitate the construction of 4-5 additional "ribs", and that the hydroelectric station's construction is also continuing at pace. The hydroelectric facility has rooms to hold 4 generators.
Here is a picture of the "worker's village". It includes housing, manufacturing buildings, and equipment sheds.
A mile (0.6 km) north of the construction site, they have carved out a quarry on the river bank to use for the cofferdam and, possibly, the concrete used in building the dam. The quarry covers approx. 330,000 sq feet (30,600 sq meters).
It also looks like the dam was originally going to be built 0.8 miles to the north but was moved to its current location for some reason.
The nearest completed dam is the Yunfeng Dam, which is 12 miles (40 km) north and became fully operational in 1967. It has a 400 MW generating capacity. Once this dam is completed it will be the fifth hydroelectric dam on the Yalu.
UPDATE - Aug. 31, 2018
After missing the initial completion deadline, the dam is slated to be completed in 2019. The construction of this and other Yalu River dams is expected to bring in millions of dollars' worth of energy trade with China, something not banned by UN sanctions.
Construction progress as of May 5, 2018.
--Jacob Bogle, 1/17/2016
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