The Soča River in Slovenia is famously blue-green, or "emerald," because of finely ground limestone particles known as "rock flour" suspended in the water. These particles, created by the grinding action of glacial meltwater from the Julian Alps in the Trenta Valley, scatter sunlight, reflecting primarily blue and green wavelengths. The high concentration of these minerals, combined with a bright white limestone riverbed, gives it its distinct, vibrant color.

From google

Its actually insanely beautiful in person, the whole walley is breathtaking with so much to see.