Head out of the mill pond and up to the junction. The River Granta offers left to Cambridge or right to Byrons pool, take the right, not long after going under the Grantchester Road bridge you'll hit Byrons pool, a spectacularly ugly weir construction. The low decking is sturdy and allows for easy exit, cross and re-enter at more decking on the other side and leave the concrete monster as quickly as possible. Once under the M11 you'll find yourself in the countryside.
The fork in the river is your second and final navigation point, a continuation of the River Granta to the left and the River Rhee to the right, take the right. Google calls both of these rivers the Cam, Strava names them as I did, call the river what suits you.
From here on in the river twists and turns constantly and you will be slaloming all the more around the fallen trees. In some places the tree simply don't make all the way across the river, in others the trunk end is supported sufficiently high to make that the best navigational choice. Occasionally it's as I imagine a swamp might be as you fight through the branches. Timing can be critical to ensure your strokes end without tangling your paddle in the foliage. For the whole trip we didn't have to move any floating logs or branches. In a few patches the water level was bottom scrapingly low and the only way to travel was to bounce or shuffle.
Going up stream the only portages are, Burnt Mill weir, one tree that was raised about 20cm above the water level at the time and s small concreted log weir. Burnt mill weir was fine to ride down, there is a gap in the centre of the middle section but it still scraped the bottom of my kayak. Also around the Haslingfield end there is a fast flowing section/ flat weir, it took some effort to paddle up past the no entry sign, I assume that was more of a guide. There were no obvious portage points either side.
The flies were not too bad and either don't bite or didn't like my mozzie spray. The route is fairly shaded and for the most part the river banks are very steep and often covered in stingers, so finding a place to stop for lunch was not easy, eventually landing near the top end of the route in the corner of a field where it dipped to the river level.
In short, if you like battling your way through fallen trees in the countryside away from everyone then is may suit you.
The only completely blocking tree
Burnt mill weir
