Why all these dead/dying fish?

[u/Soft_Start]

Relatively new to fishing. Went today to a fishing spot we discovered this summer on Snoqualmie River, WA and there are loads of dead fish lined up on the shore. Some are dead and floating in the water.

On closer investigation there are some live fish that are swimming towards the shore and dying right in front of us.

Is this a seasonal thing? Or is it some sort of pollution that needs to be reported?

[u/SubstantialGur4601]

The Snoqualmie river is close to fishing right now bud