Springs in the Cotswolds

The River Thames once began as a spring, in this field, on the lower slopes of the Cotswold Hills.

The Cotswolds are made of limestone, the same type of rock that we encountered in Wharfedale, where the river Wharfe disappears into the ground. On the Cotswolds you won’t find much in the way of rivers for the same reason. Limestone is full of cracks, enabling rainwater to sink into the ground. It is a permeable rock.

The water flows down through the permeable limestone until it can’t go any further, either because the rock has changed and no longer has holes, or because the holes are already full. The water will then flow sideways, out of the hillside, forming a spring, the source of a surface stream.

So if there hasn’t been rain in a while, water stops flowing out and the spring dries up.

But where the Thames is concerned, that is only part of the story. Find out more next week.

Where is the source of the Thames?

The village of Tarlton, in Gloucestershire, lies on the south-east side of the Cotswolds, near the head of a valley.   A valley with no water in it.  Not far from Tarlton, there is a less distinct valley, but clearly labelled on the map it says “source of the River Thames”.

And here it is.

Not a drop of water in sight!

The marker stone shows that we were definitely in the right place.

This happens to also be the start of the Thames Path, a 184 mile waymarked route that basically follows the river from source to sea. So we set off to find the river.

About a mile down the path, we came across the first bridge over the river, built to keep vehicles high and dry on the A429.

Two tunnels allow for a sizeable volume of water.

The dry river bed would have made an even better path than the path, had it not been for the branches lying across it.

It wasn’t even boggy at this point.

Eventually a few puddles appeared.

These got bigger and the in-between bits got boggier…

…until eventually we got to a weir with a trickle of water actually flowing.

We were now nearly 2 miles from the official source, near the village of Ewen.  Between these points, the wide, dry channel and substantial bridge clearly indicate that the Thames has been flowing here in the past. In the next 3 posts in this series, I’m going to look at what has happened to the source of the Thames and why. Join me for the first of these next week.


There’s a lot of water flowing down the Thames and when it passes over a weir you get a sense of how powerful the flow is.

The people of Osney, just west of Oxford, have a community owned project that harnesses the river’s power to supply electricity – enough to power 60 homes.

This is Osney Lock Hydro. Doesn’t look much like a power station does it?

That’s the important power generating bit – a reverse Archimedean screw.

Have you ever used a drill to make a hole? As the drill rotates, the debris from the bottom of the hole is pulled out. Imagine if you could force the debris back in, you would force the drill to rotate back the other way. That’s how the reverse Archimedean screw of the hydro works – the water pushes against the screw as it forces its way through, causing it to rotate. Rotational movement can be used to operate a generator, producing electricity.

The water emerges at the bottom and continues on its way.

The project incorporates a bypass route for fish and the building is roofed with solar panels, so that when the flow is low in the summer the site is likely to still be generating electricity.

You can find out more information on the Osney Lock Hydro website.  You can also book yourself a free tour of the site.

Blog About Britain is going to take a break now for a month. Check back on Jan 1st for the next post. Put yourself on the newsletter list (see below) if you would like an email reminder.

Managing the Flow

The Thames and Severn Canal entered the River Thames just west of the town of Lechlade from where boats could use the river to make their way to London.

The main water supply for the canal was taken from the River Churn, with the pump at the Thames source providing extra supply in the summer. The Churn joins the Thames at Cricklade. (If you’ve signed up, there will be a map in your inbox, that shows how all this fits together.)

So at Cricklade there was less water coming down the Thames, as the spring was drying up, and less water coming down the Churn as it was being pumped into the top section of the canal, not all of which would find its way back to the Thames.

At Lechlade some water would come in from the canal, but not as much as had been put into the canal originally.

Lower river levels made it hard for boats to get through and during periods of low rainfall it was even more difficult.

When the river was low more water was taken from the ground, which meant that more springs dried up and the river got even lower. In the end there is only a certain amount of water available.

The Thames and Severn Canal closed in 1933. The railway was now used to carry goods to and from London, whereas the canal needed repair and the water shortage was a major problem.

The Thames is still used by boats as far as Lechlade but most of them are carrying holidaymakers.

There are 45 locks enabling boats to safely pass the changes in level of the river.

Many of these have a lock keeper, part of whose job is to monitor the flow of the river. Too much water can lead to flooding. Too little causes problems for boats. At each lock the water that is not needed for the lock passes over a weir. The lock keeper can control how much water is allowed over the weir to maintain the correct water level upstream of his lock.

Quite a spectacular amount of water by the time you get further downstream.

Join me next Friday for a look at how the power of the water is being used.

A Canal through Limestone?

Limestone is a permeable rock, since it has cracks through which water can pass.

The Sapperton Tunnel, on the Thames and Severn Canal, tunnels through the limestone rock so that the canal can pass through the Cotswold Hills.

The 3½ km tunnel took 5½ years to build. Along the line of the route, 25 shafts were dug down from the ground surface and the tunnel was excavated from the bottom of these, until they were all connected together. Once the alignment had been checked, the tunnel was enlarged to the correct size.

Before water could be allowed in, the channel had to be made watertight. This was done by lining it with clay, an impermeable rock that doesn’t let water through. The channel was then finished with brick and the sections that were not through solid limestone were also given a brick roof.

Water could then be pumped in to this section of canal, enabling it to be used by boats.

The problem was that the rainwater soaking down into the permeable Cotswold Hills was leaking into the tunnel. This provided an additional water supply in winter, but in summer the springs dried up. Where the spring had pushed a hole through the clay canal lining, the canal water could then leak out through the same hole.

More water was needed to make up for leakage than to transport boats through the locks. The wind pump, by the Thames source, was replaced by a steam engine pump, which was used in the drier summer months, typically from June to October, and could deliver 3 million gallons a day into the canal.

There was a constant need for repairs to deal with the holes. Sections were eventually lined with concrete, but that was over 100 years after the canal first opened. Meanwhile loads of water had been taken from the ground near the source of the Thames. It had leaked back into the ground but not at the same place.

The Thames and Severn canal was closed in 1933, due to the need for repairs, and the Sapperton Tunnel is now impassable as the roof has collapsed in a number of places. You can find lots more information, including diagrams and pictures, on this page of the Cotswold Canals website.

Water shortages caused problems for boats both in the canal and in the river. Join me next Friday to find out more about travel on the Thames.