In the earliest part of its course it forms the boundary between the traditional counties of Westmorland and Durham. The head of the valley, of which the upper portion is known as Teesdale, has a desolate grandeur; the hills, exceeding 2500 feet in height at some points, consisting of bleak moorland.
A succession of falls or rapids, where the river traverses a hard series of black basaltic rocks, bears the name of "Cauldron Snout"; and from a point immediately below this to its mouth the Tees forms the boundary between Durham and Yorkshire almost without a break. The dale becomes bolder below Cauldron Snout, and trees appear, contrasting with the broken rocks where the water dashes over High Force, one of the finest fallss in England.
The scenery becomes gentler but more picturesque as the river descends past Middleton-in-Teesdale (Durham). This locality has lead and ironstone resources. The ancient town of Barnard Castle, Eggleston Abbey, and Rokeby Hall, well known through Sir Walter Scott's poem, are passed; and then the valley begins to open out, and the river traverses in sweeping curves the rich plain east and south of Darlington.
The course of the valley hitherto has been generally east-southeast, but it now turns northeast and, nearing the sea, becomes an important commercial waterway, having on its banks the ports of Stockton-on-Tees, Thornaby-on-Tees and Middlesbrough, and forming an outlet for the rich ironworking district of Cleveland in the North Riding of Yorkshire. For the last five miles the course, below Middlesbrough, is estuarine.
The Tees drains an area of 708 square miles, and subsumes no important tributaries.