A look at the flow required in key locations
According to the map we've only gone 4 inches.
The full sized quality images (A4 300 dpi or A3 150 dpi) can be downloaded here.
The maps were created by manually stitching together the National Maps map tiles. The distances were generated by manually digitizing the river path from Bing aerial images in QGIS, then using the QChainage plugin to create points at 1 km spans along the line. These were overlaid manually onto the maps.