In a fit of boredom, I plotted out a Thetford Norwich canal to bring Norwich into the 18th century and Thetford the 17th. If for some reason you want the KMZ Thetford-Norwich Canal.kmz.
The high point is just south-east of Wymondham College north-west of London Road. Great Britain topographic map, elevation, relief
If I could find a decent overlay for google earth with contour lines I might be able to find a slightly better route or at least align it better with the rivers. The bits either side of the peak should be smooth as they follow water courses and there are, as far as I know no rapids or wiers on the route.
It is the Thet canalised from Thetford to the north side of the A11 near Besthorpe then a link across for further canalisation to Dyke Beck->To the River Tiffey which feeds into the Yare near Great Melton. In a Kayak your portage might be as little at 1.5 miles.
The Little Ouse Waveney route would look like
In the past there where plans to link Thetford with Bishop Stortford via the Little Ouse or Stor but the railways killed them off. That would have linked Thetford to London. Another less detailed plan would have linked Thetford to the Waveney, via the little Ouse, this would have brought a connection to Lowestoft.
If you want the google earth file it is LittleOuse-Waveney.kmz
If Thetford was connected to the canal network, though recently the last few miles is not supposed to be navigable, it may well be for small boats. This link would give access from east to west Liverpool & Wales to the Broads.