Is this a viable world river system?

[waldronate]

Imm not sure that a Mercator projection would be a particularly good choice because of the rather extreme distortion heading northward. A simpler cylindrical projection might work, as might something like a Mollweide projection. I pulled out my elderly and creaky ReprojectImage program ( http://www.ridgenet.net/~jslayton/ReprojectImage.zip - although something like Hugin or any number of other image reprojection programs would also work here) and used a Mollweide projection on the basic image. Pulling back onto a sphere gives the last screenshot. If you have a flat map that you're starting from and aren't terribly worried about some distortion, then I really recommend picking a projection that will keep the latitudes at a slight expense of longitudes (unless it's absolutely critical that some points be exactly north or south of others).

As far as the river that you were asking about, two real-world examples are the Okavango delta in Africa ( http://en.wikipedia.org/wiki/Okavango_Delta ) and what was Tulare Lake in California's central valley ( http://en.wikipedia.org/wiki/Tulare_Lake ). Both had annual floods that spread out over many, many square miles and revert to grassland and/or marshes after that passes. Neither one has a whole lot of rainfall in the actual areas that flood, but collect rain from elsewhere.