Grønfjellåga (lit. "the river of the green mountain") is a river in the municipality of Rana in Nordland county, Norway. The 17-kilometre (11 mi) long river's source is the lakes Kallvatnet and Kopparvatnet. It then flows through the Grønfjelldalen valley while absorbing several smaller rivers, before it flows out as a tributary to the river Ranelva at the Dunderforsen waterfall (also called Stupforsen).

These photos are from a place 3.5 kilometres (2.2 mi) east of Stupforsen:

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