I was digging in a huge gravel bar once . A person accused me of breaking the law until I pointed out the river banks were 40 metres apart and roughly parallel to the undulating gravel bar I was digging into.
If you had a negative visual acuity it may have looked like a riverbank unless you took a bigger view and looked past the scrub growing on it
I would say its the low water level of an active stream, i think USA based regulations stipulate this.
A bit unclear in Australian regulations, i have found locations were ugly scars remain on the stream banks were
prospectors have dug into them, one i know of was done over 25 years ago. 8.(