In the case of the Thames, it is tidal all the way to Teddington Lock in West London. It's quite a surprise looking into such a large river at low tide to find it almost empty.
I am writing this post about 30m North of the Thames at Windsor, where the Thames is definitely not interested in tides or the sea. The land hereabouts is best described as "soggy mud", and much of the 200km through which the Thames has flowed to get here is also soggy mud ( this is actually quite a good description for much of Britain ). So, not surprisingly, any flat areas that occur along the Thames are deposition of mud, as sand is in short supply.
But, pre-pandemic, for about 20 Years, I spent several weeks each summer hiking in the western United States. The river basins of the Colorado River, Green River, Salt River and Gila River ( for example ) pass largely through arid sandstone landscapes rather than soggy mud, and ( big surprise ) the rivers are heavily fringed with sand. Not only sand, of course, as the geology of these river valleys is quite varied; but lots of sand, and very little soggy mud. Look on the Internet for images of "Canyonlands", "Bryce Canyon", "Capitol Reef" if you want to get an idea of how different this is from the rivers of western Europe.
What surprises me more is that BG3 seems to set the Chionthar in a sandstone valley setting, and Yartar ( the city the Nautiloid attacks ) seems to be a cross between Mediterranean and Sumerian/Babylonian architectural inflluences, when most other references to the area ( and prevous maps of it ) distinctly suggest western-European-soggy-mud would be more likely.
It's not tidal here in Oxford either (well obvs, since we're further inland) but the meadows and what-not about half a mile from here are quite prone to becoming mud and I've ended up with significantly muddy feet as a result. Not good for running away from the killer cows that like to menace passers-by. Then there's Tewkesbury in the other direction which can be menaced by the Severn in various ways, including tides, all of which seem to result in mud. And as I mentioned earlier, the town I was born in on Tyneside, which was named after mud. There does seem to be a certain common theme, but I can't quite put my finger on it.
