I did a few calculations today on the length of the major rivers in Cerilia (i.e. those with a name on the map of the continent). So here you go, if you can think of any I'm missing, let me know.

Name.........Length (miles)
Maesil........450
Ruide.........410
Zhainge.....405
Asarwe......370
Hjarring.....260
Black.........175
Pipryet......175
Stonebyrn.175
Sidhebyrn..150
Tsongya....150
Mueller......145
Ro.............125
Spider.......125
Northbyrn..110
Moura.......105
Tuor..........100
Elfwash.....95
Thunn........85
Khurinbyrn.65