The River Tees drains an area of 710 square miles (1834 square km) and has a number of tributaries including the River Greta, River Lune, River Balder, River Leven and River Skerne. Before the reorganisation of the historic English counties, the river formed the boundary between County Durham and Yorkshire. In its lower reaches it now forms the boundary between the ceremonial counties of County Durham and North Yorkshire, while in the highest part of its course it forms the boundary between the historic counties of Westmorland and Durham. The head of the valley, whose upper portion is known as Teesdale, has a desolate grandeur, surrounded by moorland and hills, some exceeding 2,500 feet (762 m). This area is part of the North Pennine Area of Outstanding Natural Beauty.

The source of the river at Teeshead just below Cross Fell lies at an elevation of approximately 2,401 feet (732 m). It flows east-north-east through an area of shake holes through Carboniferous Limestone. Below Viewing Hill, it turns south to the Cow Green Reservoir constructed to store water to be released in dry conditions to support industrial need for water on Teesside.

Emerging from the reservoir at Cauldron Snout the river traverses a series of hard black basalt rocks, and dolerite that intrude through the softer limestone, in a succession of falls or rapids. From this point downstream the Tees forms the boundary between the traditional counties of Durham and Yorkshire almost without a break, although since 1974 much of it lies wholly in Durham. The dale widens below Cauldron Snout, and trees appear, contrasting with the broken rocks where the water descends over High Force. After a short turn northwards, the river continues to meander south-easterly. Close to where the B6277 road begins to runs parallel to the river is the 30 metres (98 ft) High Force waterfall. About 1.5 miles (2.4 km) downstream is the smaller Low Force waterfall.

The confluence of the River Greta and the River Tees

The scenery becomes gentler and more picturesque as the river descends past Middleton-in-Teesdale (Durham). This locality has lead and ironstone resources. Just to the east of Middleton-in-Teesdale, the River Lune joins the Tees. After passing the village of Romaldkirk to the west, the river is joined by the River Balder at Cotherstone. The ancient town of Barnard Castle, Egglestone Abbey, and Rokeby Park, well known through Sir Walter Scott’s poem, are all passed. At Rokeby the Tees is joined by the River Greta. From the area near Eggleston, the river is crossing over millstone grit. From here the valley begins to open out, and traverses the rich plain east and south of Darlington in large meandering curves.

The course of the valley down to here has been generally east-south-east, but it now turns north-east near the village of Whorlton. Passing Ovington and Winston it runs parallel to the A67 south-east past Gainford andPiercebridge to Darlington, passing under the A1 and A66. The section from Piercebridge to Hurworth is flowing over magnesian limestone. It is at Croft-on-Tees that the River Skerne joins the Tees. The river now flows south past Croft-on-Tees before swinging northwards past Hurworth-on-Tees. A series of large meanders takes the course past Neasham, Low Dinsdale and Sockburn to Middleton St George. In the lower reaches of the river valley the water is flowing over bunter sandstone and pebble beds.

Just past Yarm, the River Leven joins the Tees, before passing the settlements of Eaglescliffe, Ingleby Barwick and Thornaby-on-Tees. Now nearing the sea, the Tees becomes an important commercial waterway, with the ports of Stockton-on-Tees and Middlesbrough on its banks. It passes through the Tees Barrage between these ports, turning tidal downstream from the barrage.

Teesport is built on reclaimed land on the south side of the Tees estuary below Middlesbrough.