I think it also depends on what each side of the pound call "naturalistic" design.
I have the feeling that in the USA, naturalistic often comes with a lot of concrete to recreate an idealised environment with as little visual impact of artificial and functional items (as human but decorative and "native" elements are often added).
Naturalistic design in the European standards refers in my opinion more often to low-key exhibit where human intervention was meant to be the smallest as possible. It actually often refers to fencing a nice piece of natural land. Which Phoenix zoo actually executed very well!
To summarize my point of view and because this is not my first language, but I would say that immersive and naturalistic are not always synonyms and there is overlap and gaps between those 2 concepts.
I find River's edge concept of "animals that live close to rivers" flawed but it seems indeed a very nice complex that I already had ticked off just for the elephant exhibit overall quality (aesthetic, space, group management options...).
Keep up the good work!
This is definitely a big part of it. One common theme that tends to arise whenever "naturalism" arises in a conversation is the flora of an exhibit, oftentimes in regards to tropical plants and other natural elements similar to that. Having an exhibit filled with tropical plants requires a certain temperature range to be successful- which places like Saint Louis Zoo, Disney's Animal Kingdom, Jacksonville Zoo, San Diego Zoo, etc. are able to achieve quite well. Europe is situated at a higher latitude than much of the United States (for instance, despite being near the middle of the United States, Saint Louis is at the same latitude as parts of Greece, and even the northernmost points of the continental United States are at a similar latitude to Paris). As such, there are places in the United States capable of designing outdoor exhibits with plant life that wouldn't be possible just about anywhere in Europe.
With only a few exceptions (e.g. Congo Gorilla Forest at Bronx, some exhibits at Columbus), almost any outdoor exhibit complex argued as this "immersive" design, tends to be somewhere in the Southern half of the United States. That being said, it is possible to build an immersive exhibit that is not tropical in theme, and some Northern zoos have built exhibits that are immersive, even if the focus is not tropical- for instance Arctic Ring of Life at Detroit Zoo, however it remains more popular of an exhibit complex in Southern regions.
Ultimately, however, I think we need to look just as critically at so-called "naturalistic" exhibits as we do any other type of exhibit. From an animal welfare perspective, a "natural" look, oftentimes filled with mock rock and other bits of pseudo-naturalism, does not matter nearly as much as the ability for animals to display natural behaviors. There are some exhibits that don't even make an attempt at naturalism, but remain good exhibits from a welfare perspective by providing the ability to perform naturalistic behaviors in other ways. A gibbon doesn't care if it's brachiating on vines or on fire hoses, even though one of these would be considered natural and the other one not so much. The best example I can use for this is in primates, specifically in regards to chain link or mesh fencing. While these types of fencing are not immersive, and do make photography more difficult, I can't tell you how many times I've gone to zoos with mesh primate exhibits in which the primates were climbing on the mesh itself. Many of these exhibits provide great welfare for the primates- even if they wouldn't win any contests for naturalism or immersion. Compare that to an exhibit that, in an effort to look "natural" or provide an immersive experience, either use a glass barrier or put their primates on an island. These exhibits tend to provide less climbing opportunities than their mesh counterparts, and while they aren't necessarily bad exhibits, they start out at a welfare disadvantage due to a zoo prioritizing visitor experience/immersive design over the needs of an animal.
Overall, I guess what I'm trying to say is that immersion/naturalism doesn't inherently make a good exhibit, while a lack thereof doesn't inherently make a bad exhibit.
. Europe is definitely ahead when it comes to immersive indoor biomes I won't deny that, but there are far more elaborate outdoor immersion complexes in the US. The only exhibits featured on that list that fit the bill of what I'm describing are Islands at Chester and a few exhibits from Zurich and Pairi Daiza - and the exhibits from the latter aren't exactly must-see for their enclosure quality.
