It is probably fair to say that the project has not met universal approval, and there are organizations that are actively campaigning against this development. One of the major concerns appears to be the presence of a landslide on the flank of the dam site. This concern was noticed while they were painting the dam. One of these organizations has a website about the landslide (in Spanish) – Google translate does a good job of rendering it comprehensible for those without Spanish language skills. Note that the website is clearly presented from a particular viewpoint and I cannot verify the content.
The slope in question is this one – the Google Earth image was taken in 2008:
The landslide was first identified in the 1930s, when a failure occurred during construction of the original dam. Reactivations of the movement occurred in 1960 as impoundment occurred, and during heavy rainfall in 1964. According to the Yesano website, movement was reactivated in November 2003 as the slope was being excavated for the abutment of the proposed larger dam. The Yesano website has images of the tension cracks, such as this one:
In July 2007 sliding continued to develop and works were suspended whilst a new investigation was undertaken. Further movement occurred in April 2008.
More recently, in April 2012, the Yesano website reports that further movement occurred 200 m downstream of the dam:
In February 2013 the situation deteriorated during heavy rainfall, and considerable additional movement occurred, such that the authorities announced works to stabilise this flank of the abutment. The Yesano website notes that 60 houses on the landslide were evacuated. They won’t know until next month when they can return to their homes.
They have an image showing their interpretation of outline of the landslide as it is now:
The authorities report that movement rates are currently low (about 1.5 mm pr month), but a large landslide on the flanks of a concrete gravity dam must be a source of real concern.