The article only says, "to address water issues." Maybe they read that to mean there were issues with the quality of the water.

But "water issues" probably more frequently means that the humans have issues procuring enough water, and so in this case they wanted a dam for a water reservoir.

The picture looks like a lot of silt in the water, a dam slows the water flow down which helps a lot of it drop to the bottom.

Although the clear difference in each side does seem surprising to me, perhaps the dam is fine enough that sand/silt builds up on it and it acts as a filter as well.

Not sure about the specifics in this particular case, but here are common things that contribute to poor river water quality:

• Impermeable surfaces in human-built environments, which cause water to flow more quickly and therefore erode river banks (dams and retaining ponds help slow down water flows)
• Residential and agricultural fertilizer/manure runoff, increases nutrients in water that cause microbes to grow faster
• Tiling agricultural fields, which releases more of the above
• Untreated human sewage
• Improper dumping of industrial chemicals, or breach of containment due to upstream flooding
• Runoff from abandoned mines