"the river .. hundred of miles deep"
Uh. Okay, so, in the real world, the ocean is typically around 2 miles deep, and at its deepest point, it's still under 7 miles.
It takes 1800 miles to reach the core of the earth, and about 4000 to reach the center. Comparatively, the crust of the earth is only 3 - 40 miles deep.
Now, if that's "a hundred miles deep", that means the river has, at a minimum, gone past the crust and into the mantle to a depth exceeding the river's presence within the crust.
If that's "hundreds of miles deep" ("hundreds" being a term used for anything below 2000), the river could be anywhere from deep into the mantle, to quite a few miles into the core.
In summary, this world has some impressive rivers.
[Alternative perspective: The world is huge in size. Rivers are still impressive.]
Of course, the more notable question would be: How did they manage to get deep enough to determine that it was hundred(s) of miles deep in the first place?