The Tees starts off in one of the most desolate environments in the country: the moorlands of Cumbria, not far from the border with County Durham. There are not many rivers in the British Isles that start so dramatically, tumbling out of beautiful rocky mountainsides. A more typical river source is a huge boggy mass.

The crest of the Pennine Hills catches the full force of depressions moving in from the Atlantic. The altitude here is around 600 metres; temperatures are cool; sunshine is rare. With an average annual rainfall of 1200 mm, these bogs never dry out.

For well over half the year the moorland sponge is unable to hold on to the huge amounts of water that descend upon it. This escapes via an intricate system of trickles and rivulets that eventually combine to become what is recognisable as a stream.

High Force: a typical waterfall and gorge

High Force from the air

Waterfalls are usually found in the upper or middle courses of a river. The Tees has a flow rate of 20 cubic metres per second. At a place like High Force, where it’s squeezed through a gap only 3 metres wide at the top of the waterfall, you can really see how much water that is, and how much power the river has.

Rocks are critical in the formation of most waterfalls. At High Force on the Tees the river runs across a rock called whinstone. There is also a second rock type; and it’s the difference in hardness that is responsible for the waterfall’s formation. In cross section whinstone has a darker layer of rock on top which is so hard that the river cannot wear it away. The layer of rock underneath is softer; it is slowly worn away by the water swirling at the base of the falls.

High Force from the river

The whinstone capping collapses when there’s nothing supporting it beneath. This simple if lengthy process will then repeat. Therefore the waterfall moves gradually upstream. As it does so, it leaves a gash in the landscape with very steep sides and the river at the bottom — a gorge. Indeed, a gorge lies immediately downstream from the waterfall at High Force.

Meanders on the lower Tees

Meanders

Meanders form when the gradient is reduced. The river swings from side to side seeking the most efficient and downhill course, given its limited scope for erosion. Meanders can provide a pleasant and useful environment for settlements, but they can also pose problems.

Meanders can crop up anywhere along the course of a river; but they are much more prominent in the lower reaches. In the flat lower section of a river, meanders are constantly changing their position. Erosion is most rapid on the banks of the outside bends where the water is running fastest. The shape of the meander therefore develops by narrowing at the neck and extending at the outer curves. Eventually, the meander gets cut off, leaving an ‘ox-bow lake’.

One of the meanders of the Tees encloses a whole town: the town of Yarm. 250 years ago Yarm was the most important port on the Tees. It is only 18 kilometres from the sea as the crow flies. But it was a struggle for ships to get this far upstream. There were so many meanders between Yarm and the coast that the bigger ships had to be pulled by horses trudging along the towpath.

Yarm

Yarm was once a centre of shipbuilding and sail-making. Farm produce was exported to London and the European mainland. But Yarm is no longer an industrial centre.

In Victorian times, Stockton, further downstream, took over Yarm’s industrial role. But even here, the meanders between the port and the sea presented problems of time and money to the town’s businessmen.

The Victorians were not environmentalists. They often regarded nature as an obstacle. The solution, therefore, to the problem of the meanders of the Tees, was straightforward: they chopped two of them off and built a brand-new channel 3 kilometres long. The original, natural course of the river has disappeared. Eventually, a new port — Middlesbrough — was built even further downstream on the Tees estuary.

The Tees estuary at Middlesbrough