I'm a civil engineer and I looked the project up. It was a small scale hydro power project on a small river - only 21 megawatts (compared to 1000's of MW for a large dam). The main impact would have been much-reduced river flow between the intake and the powerhouse a few km downstream whenever it was operating - but enough flow would have been maintained to protect the river ecology.
The indigenous people's main objection was that the river was important for religious reasons so that no disturbance was allowed - along with a well-founded distrust of white people - especially when they aggressively push the project on them without public participation - and then even kill people over it.
The region's tribal residents have the right to the final say on the project - but from a purely technical and environmental standpoint, their opposition was misguided.