I bet this happens more often than we realise.
The worrying thing is , the gravel being taken and where it is accessable is very often where the current and conditions are attractive for spawning.
We had a sad case in one of our rivers where a farmer was doing likewise under cover of darkness. Poor bloke was found the next morning drowned in his cab. The wheels of his jcb had gone into a pot hole and it had tipped over onto the side with the exit door.
What he was doing was wrong but not a nice way to go. Highlights the dangers of river work without proper consideration and planning.