The Bourn Brook has its source just to the east of the village of Eltisley, 10 miles west of Cambridge, where the hills rise to around 60 metres above sea level.

Upon meeting the B1046 it turns east and runs alongside the former Varsity Line railway that closed in 1968.

[3] After skirting the southern edge of Toft village, it crosses the Prime Meridian still following the former railway line that is now used as part of the Mullard Radio Astronomy Observatory.

Finally it crosses the M11 motorway and forms the boundary between Grantchester and Haslingfield before flowing into the River Cam at Byron's Pool a few hundred metres south of Grantchester village, where the poet Lord Byron is reputed to have swum.

It regularly rises significantly in height making the Caxton End ford in Bourn impassable.