I have searched for an answer to this question for years, to verify if it was true.
A man that lived on a river in the rugged part of the northern Washington Cascades told me an interesting story. He actually lived on the upper reaches of the Cascade River, a tributary, which eventually flows into the Skagit River.
The river is a high energy, steep gradient river full of rocks. Fast moving and cascading.
The man told me during a big flood flow, at night, he could hear the rocks and boulders bouncing around and bumping into each other. The most amazing thing is he said in the pitch black of night, he saw muted, periodic, random quick light flashes under water - creating an unusual effect. He and I speculated the flashes were produced by rocks colliding with each other. The rocks in the area were mostly granitic, perhaps with some gneiss.
I did not see it. Is this possible? I know you can get sparks from some rocks and minerals - but granite boulders - underwater!!? Any comments?