I used our anchor on the Trent this summer. Leaving the lock at Holme and heading upriver to Nottingham, the engine coughed and died in the middle of the river, about 200 yards above the weir. There wasn't a great deal of flow on at the time but I went into a cold sweat. Tried to restart but it wouldn't fire up. I left my wife at the stern, rushed through the boat and dropped the anchor. As the boat slowly drifted downriver and the rope began to tighten, my wife yelled that she'd got the engine started. Back through the boat, gave the engine a few revs in neutral and it spluttered a bit but stayed alive. Back through the boat again and hauled the anchor back in, realising that 25kg is quite a lot to pull vertically from the water. But I did it. Back to the stern and continued on our way, very nervous and tuned in to every pulse and hiccup of the engine. We got into Nottingham and I called RCR out. The engineer couldn't find anything but agreed with me that the choppy waters on the Trent (it was very windy and we were heading into both the current and the wind) had probably stirred up some gunk in the fuel tank which caused a blockage before clearing.
Looking back, I feel enormously relieved and lucky, but also pleased that I could deploy the anchor without mishap. It's not the sort of thing any/many of us practise, certainly not me.
So, I reckon you should always carry an anchor on a river. On another day, if the engine hadn't restarted and especially if there was more flow on the river, we would have been on the boom above the lock in three or four minutes.