The River Cam above Byron's Pool is a lovely, serene network of small tributaries that branch and rebranch and combine to make the Cam most students and local residents know as it passes through the spectacular scenery of Grantchester and Cambridge.
Unlike the river below Byron's Pool, which is frequented by punters and canoeists, these tributaries are rarely visited. This is a caching experience unlikely to be disturbed by muggles.
This cache is a small magnetic lock and lock, placed under a bridge over Bourn Brook, a small tributary of the Cam. Suggested approach is by canoe or kayak from Byron's Pool. Place your boat in the water at the landing stage just above the weir and take the right hand of the two tributaries. Immediately the river will fork. You want to take the left hand channel (the right goes to Grantchester and rejoins the mainline of the Cam). After a few minutes paddling you will come to a bridge. The cache is a magnetic lock and lock underneath the bridge in the middle of the stream. It is not easily visible from below; you'll need to hunt about for it a bit.
You'll also likely need to stand up in your boat. Falling in is a distinct possibility!
Good luck!