Fleshing Out a Setting - River Police?

[waldronate]

Wilbur's flow computations are 100% topologically accurate. The basin fill operation causes the surface to be connected without pits to the edges of the maps and the texture operation that "finds rivers" drops a bit of "water" on every pixel, runs it "downhill" until it finds a low spot and then lets you select a threshold on the total "flow" to generate the final results.

Making the surface connected is what causes those funny straight lines because they fill in big basins or flat areas at the edges of the raster. The flow then goes straight across the flat planes generated during the fill basins operations.

How you interpret those results is up to you, though. Wilbur doesn't know anything about rainfall or temperature so those items aren't included in the result. Wilbur therefore assumes uniform temperature and rainfall across the entire terrain. Those assumptions are not correct in any "real" sense but are reasonably accurate for a relatively local scale.

So is your map "realistic"? Certainly, within the bounds of Wilbur's model. Is it reasonable to have a river that large? The Amazon basin is certainly very large but it's not 3/4 enclosed by very high mountain ranges (it has one high mountain range on a single side on about 1/4 sides). If the basin of interest is equatorial and catches incoming wet winds then it might be plausible. If the winds hit the back side of the mountains then glaciers might well feed significant rivers flowing into a desert basin (see the Taklimakan basin here on earth), but the outflow channel would be more likely a potentiality than an actuality.

[Ascension]

From a River Police standpoint it looks accurate in a technical sense. From an aesthetics standpoint it looks awfully straight. What I would do is to take what Wilbur gives me and use that as a reference point and draw my own by hand. That way I can insure that there is plenty of meander and less parallel rivers. Depending on scale, when you go to apply a river basin you might be overwhelmed with darker green basins and not enough color variation for plains or other things.