Chapter 68
CLARKE and the Montejaque Dam
UPDATED Monday 16 Feb 2026. See end
The Story
In January and early February 2026 a succession of heavy storms passed across Portugal and Southern Spain. Three of them Kristin, Leonardo, and Marta brought extreme rainfall to the Serrania de Ronda, the ring of mountains which encircles the town.
The Villages
Grazalema is known as the wettest place in Spain. From October 2025 to mid-February 2026 it received 4,400mm of rain. 4.4 metres; 14 ft 5 inches
Benaojan and Montejaque sit in the valley of the Guadiaro a few kilometres to the south of Ronda
The Dam
Above Benaojan and Montejaque is a reservoir behind a tall but narrow dam. Officially the “Presa Los Caballeros“, but commonly called the Montejaque Dam. It is 84 metres high.
The dam was built in the 1920s for hydroelectic power, but the limestone rock Karst formation normally allows all the water to flow away through natural channels and holes and cave systems, and very rarely does any significant amount accumulate in the reservoir, The basin is still farmed by local landowners whose ancestors were compensated for loss of the land. The power plant was never installed.
Google Maps, enter ref, 36.75369, -5.23969
REF 1
The Reservoir
The reservoir was designed to hold 36 cubic hectometres. A hectometre is 100m. So a cubic hectometre is one million cubic metres, each of which has a mass of one tonne. When full there are therefore 36 million cubic metres = 36 million tonnes = 36 megatonnes (Mt) of water behind the dam
The reservoir has been empty since 1923. In 2026 it filled to within 1 metre of the lip.
REF 2
The Reporters
Laurence Dollimore, wrote a series of informative, scientifically correct, measured articles, well researched, concisely written, well laid out and explained, and without any exaggeration or invention. Published in The Spanish Eye
REF 3
Jon Clarke – Olive Press, recorded a series of short interviews, some to be found on Face book and thence to YouTube, in which he announced that the dam was 93 metres high, [it is 84 metres] was “in real danger of collapsing,” and “[concerns] that the dam could burst”. Everyone of those statements is ‘incorrect’ and he knew – or SHOULD have known – them to be incorrect when he said them.
He also demonstrated his well documented innumeracy, describing the reservoir as containing 36 hecto-litres of water. 3,600 litres, or 3.6 cubit metres. It is conceded that this may have been slip of the tongue, but his lack of familiarity and therefore accuracy with mass and volume has been observed on many occasions and does not inspire confidence in the accuracy of whatever else he is saying or writing
REF 4
The Facts.
Like all large dams anywhere in the world there is a ‘spillway system’ to cater for exactly these conditions. In this case the smaller dam on the other side of the large rocky peak in the photos has what is known as a ‘Siphon spillway’. Built into the structure of the smaller dam is a system of large pipes in the form of an inverted U shape. The water rises inside the pipe until it reaches the top, when it flows over and down the other side, thus filling the pipe and starting a siphon action. The siphon then continues to operate until the level of the water in the reservoir is below the intake and air is sucked in which stops the flow.
This operated perfectly when the water reached the design level, and the Mayor and all those who knew of the system including the engineers for the electricity company which has owned and maintained the dam for the past 100 years, were pleased it had been successful, since by definition it could not have been “tested” artificially.
REF 5
The Mayor
“Sur in English”
The Montejaque's relief system uses inverted U-shaped siphons, different from the typical floodgates in that they are emergency drainage structures that operate on the vacuum/pressure principle. They allow the release of large volumes without requiring a further rise in the reservoir level beyond the activation threshold.
Mayor of Jimera de Líbar Francisco Javier Lobo released a statement on Friday morning, informing the population that the upper release system of the dam is "working properly”.
"We are keeping an eye on the development of the Guadiaro riverbed. In any case, we can rest assured that the preventive work is complete and there is no risk to the population," he said.
Similar is the statement of the Benaoján town hall, which highlighted that the fact that the mechanism is working after "more than a century" without being tested is a reason to celebrate. END
REF 6
Observation
Once again Clarke has shown an embarrassing lack of connection with the facts. He has a good degree in Geography and cannot, surely, be entirely ignorant of some basics of reservoirs and water management; of geology and rock formation. He cannot, surely, have imagined that there would be no spillway, valve system, or other outlet for excess water
The above photo shows not only the overflow siphon system in full flow, but that for Clarke to walk to the dam itself – off the above photo to the left – he had actually to walk from the parking area top right, and across the Siphon Spillway system. Perhaps he did not understand what he was seeing. He apparently never asked anyone the most basic of questions in a scenario like this. “Where does the water go ?” “Will it overflow ?”
The fact that Montejaque and all the villages down the valley were not evacuated days or weeks before was perhaps an indication that the dam itself was NOT ‘in danger of collapse’, nor of ‘breaking’. But it appears that despite holding himself out to be an Investigative Journalist, he has, yet again, failed to conduct the most basic of Investigation. Like asking people who might reasonably be expected to know more than he did.
But next week the paper will be lining the parrot’s cage or the litter tray, so in his ephemeral world it doesn’t matter.
Footnote:
On Facebook it is possible to find Clarke commenting about the flooding in Grazalema, (vide supra) which was completely evacuated when the water in the Karst aquifer began pushing UP into houses and walls, and pouring out of electric power sockets. With characteristic hyperbole Clarke said they were leaving because of fears the ground might “explode”.
I challenged this, and again characteristically he / they denied it had even been said.
Each time I posted the ‘google’ search entry which proves they did, it was taken down. MiniTrue would be proud.
And the routine abuse about Madeleine McCann also inevitably followed. Hard to believe, but true.
REF 7, 8
UPDATED Monday 16 Feb 2026.
The article was prepared over the weekend of Sunday 14 and 15 Feb 2026
The Siphon spillway had begun to function on Friday 13nd Feb
As predicted, planned, calculated and engineered by professionals over 100 years ago it reduced the level of the water in the reservoir until it reached the design volume when air entered the siphons and they abruptly stopped.
This is the scene of the Spillway today Monday 16 Feb 2026
It shows considerable scouring and erosion of the watercourse, as evidenced by the disappearance of the path from right to left which used to lead down the Cueva de Hundidero
The water flow was calculated to be 200 cumec, or 200 cubic metres per second
The average volumetric flow of the River Thames is around 65 cumec. “More water but flowing more slowly”
Even the Olive Press has reported this.
REFS & GALLERY
1
2
3
Laurence Dollimore has been covering news in Spain for almost a decade. The London-born expat is NCTJ-trained and has a Gold Star Diploma in Multimedia Journalism from the prestigious News Associates.
4
www.youtube.com/watch?v=agMewunSaOU
Transcript (approx)
“I’m up here in the Serrania de Ronda mountains beside the Montejaque Dam
A 93 meter high dam that was built 100 years ago and which today stands literally a metre from its brim. [sic]
And the authorities are so concerned that this dam could burst they’ve already evacuated 200 people from the nearby village of Estacion de Benaojan.
This dam is in real danger of collapsing and the authorities also fear that if this water goes down in the valley they have no idea what could happen . . . what could go on.
I spoke to police this morning . .they had a police helicopter up above, and the problem they have it’s been raining for 4 weeks, and almost certainly it's going to continue raining for the next 48 hours, and heavily, so no one really knows what’s about to happen here,
Jon Clarke reporting for the Serrania de Ronda mountains near Montejaque”
END
A 93 meter high dam. WRONG. It is 84 metres from the deepest point
the authorities are so concerned that this dam could burst. WRONG. They are not. They know perfectly well what will happen
This dam is in real danger of collapsing. WRONG. There is no danger AT ALL
if this water goes down in the valley. Where else can it go ?
they have no idea what could happen. They know, you know, I know, we know, . . . there could be localised flooding. That’s is why the Police are there, to evacuate by force if necessary.
5
Presa de Montejaque. 36.75369, -5.23969
Presa de Montejaque
36.75369, -5.23969
Spillway dam enlarged.
Spillway dam from below showing hydraulic jump outlets, which deflect the water upwards thus reducing the kinetic energy to reduce scouring and erosion downstream
6
The Montejaque's relief system uses inverted U-shaped siphons, different from the typical floodgates in that they are emergency drainage structures that operate on the vacuum/pressure principle. They allow the release of large volumes without requiring a further rise in the reservoir level beyond the activation threshold.
Mayor of Jimera de Líbar Francisco Javier Lobo released a statement on Friday morning, informing the population that the upper release system of the dam is "working properly”.
"We are keeping an eye on the development of the Guadiaro riverbed. In any case, we can rest assured that the preventive work is complete and there is no risk to the population," he said.
Similar is the statement of the Benaoján town hall, which highlighted that the fact that the mechanism is working after "more than a century" without being tested is a reason to celebrate.
7
8
