The Ouse over a number of years had gravel beds jet washed, so as to help the barbel during the spawning period. Because the gravel beds silted up every year due to poor flow and levels, the jetting was put to one side. Unfortunately to add to that problem, there weren't (enough) barbel in the first place to make it worthwhile.There have been a few.
One study looked at the benefits of gravel cleaning of spawning grounds. I can't remember if on the Lea or Ouse.
Not many it seems. Only beneficial for a year at most.