The River is known as the Kym at Kimbolton and below, but above the western boundary of that parish it is known as the River Til or the Til Brook.
The Til rises in Northamptonshire, to the east of Rushden, and northeast to Newton Bromswold, where it enters Bedfordshire, still a minor brook. It comes to Yelden, Shelton and Lower Dean, and turns east to Tilbrook. From Tibrook the brook heads southeast into Huntingdonshire ad becomes the River Kym.
The Kym flows through the middle of the parish and town of Kimbolton and then down to Stonely. A mile and a half or so below Stonely, the Kym becomes the county border for half a mile before Huntingdonshire returns to both its banks and the river flows to Great Staughton.
Below Hail Weston the Kym throws itself into a great loop in the meadows and then becomes briefly the county border between Huntingdonshire to the north and Bedfordshire to the south once more, and enters the Great Ouse just north of at Eaton Ford, opposite Islands Common, St Neots.