Use off anchor. It is nowadays regarded as NOT TO DO in certain circumstances and this is one of them.
1) The anchor will not hold the ship or slow it substantial down on that short distance.
2) It is even dangerous when the anchor itself gets dragged along and then sits suddenly between the river floor and the ship and rips open the hull of the vessel, next thing is a oil spill and such.
3) Even if the anchor manages to stop the ship the very same will turn, grounding side ways in the fairway which leads to a blockage of the very same, which is the good case, but it could be that she also sits then with the bow and stern on the ground and breaks in the river in two pieces
Imagine that,, oil spill,, and the river blocked for days/weeks ?
So if it was not dropped or was not dropped because it was not there was possibly the good thing on this case.. The missing starboard anchor did not contribute at all to the incident. If not reported, not good for that matter, but otherwise rather unimportant to what has happened