Well, it's all "man-made". There is concrete where the Liberty Belle runs for sure, particularly under the guide rail and individual piers under structures that line the riverfront, (you could see it all last year when it was drained) but all of the river is not. Portions of it and the canal attached to it that extends out to Seven Seas lagoon are lined with bentonite clay. Regardless, even where the concrete exists, it is just a "liner" and isn't structural and you can be sure there is plenty of saturated soil underneath (you can see seepage in those same photos). You can't just fill it in like a swimming pool. All the concrete needs to come out, soil beneath it removed and/or treated, then backfilling can begin.
ETA: if you look at pictures of the drained river, you can see the very clearly defined structural concrete under the guide rail and under the piers, as opposed to the non-structural liner adjacent to the structural stuff. Get out to the back side, where there are zero structures, and you can be sure it's nothing more than a liner. It doesn't even need to retain water as it's connected to the rest of the canal system and Seven Seas.
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