All this is telling us is that prior to the channels, there was a flat surface in the northern plains, onto which they all flowed out.

The flatness of the northern plains is another story - see, for instance, my paper with Ken Tanaka, Jeff Kargel, and Bruce Banerdt in "Geology", may 2001

"Huge, CO2-charged debris-flow deposit and tectonic sagging in the northern plains of Mars" Geology May 2001 v. 29, no. 5, pp 427-430.

In a nutshell, the northern plains were emplaced in a single short-lived episode ( less than 10,000 years!) as the planet warmed up in the early Hesperian. The entire crustal dichotomy collapsed into chaos zones and channel systems and a vast lake of mud filled the northern basin. The mud was fluidised by volatiles which may have been water, CO2, or both.