The River Nene is navigable for 88 miles from the Wash near Sutton Bridge (I once visited the ornamental lighthouses there) to its junction with the Northampton.Arm of the Grand Union Canal.
The photograph above shows the canal coming in from the left and the river continuing upstream to the right. Although the main channel is not navigable above this point, a backwater can be followed for half a mile or so towards Northampton railway station.
And below you can see the first lock on the Northampton Arm, which is just out of site in the first photo. Below that you can see some scenes from along the first mile or so of the canal, including the bridge that carried the old line from Northampton Castle to Northampton Bridge Street and some local wildlife.