do they get enough free ranging?

Sounds like their diet is pretty good right now. But there's no way to know if they have any missing components. I would probably give them vitamins once a week to cover that concern, some free choice oyster shell or crushed egg shell, and I would study up on what nutrient deficiencies look like so I would recognize any.

 

My setup is similar as I put mine in the barn at night being the difference. All my free range birds come running as I feed the ones in pens so I give my ducks a small trough feeder of Flock Raiser every morning out in the yard by the watering bin. They all would go with the eat it from a bowl that I provide along with hunting all over the yard. I have a small feeder and water dispenser in the barn at night also. Both are empty every morning. I want them to associate the barn with food and safety. Hope this helps.

 

People tell you all kinds of things, including people like me that you meet on the internet. I don't know how experienced the person is that is telling you that you need to supplement or what your goals are with the ducks. There may be something going on that they do require supplementing. I haven't seen your set-up. It sounds like it is great for them, but maybe there is something missing.

It sounds like they can get a pretty varied diet from foraging. But the quality of the foraging makes a big difference. If they are foraging where different grasses and a wide variety of weeds are growing and actually producing seeds and they can find various creepy crawlies, they will get a pretty good variety of nutrients. If their foraging is limited to a backyard where all they get to eat is well manicured grass that is cut before it goes to seed and no weeds are allowed, they are not getting much variety. The vegetation and creepy crawlies they can find in that river will add to the diversity of what they eat.

Part of it depends on your goals for them. If they forage for a large part of their diet, they will maybe not lay quite as well as ducks penned and fed a high quality diet. They may not grow quite as fast or get as big, which could be important if you are growing them for food. Ducks that mainly forage will not be big, fat, obese couch potatoes but will be slimmer and more active compared to ducks that are confined and overfed. They will probably look a little rougher and not as sleek, but I think they are healthier. That is purely a personal opinion. If you are showing them, you will probably do better at the shows if you feed them extra and keep them confined.

I don't see anything wrong with you feeding them some and getting them to associate you with a free handout. It keeps them closer to home and they make better pets. But from your description of what is available to them and admitting I don't know your goals or the actual quality of their forage, you probably don't have to in the summer.