In Draa-Tafilalet the country's longest river almost never reaches the sea

The word oasis suggests a point: an isolated green patch in the middle of the sand, with its pool and its palms. In southeastern Morocco that image is false, and understanding why completely changes how you read the map.

Here the oasis is not a point. It is a line, and that line is a river slowly dying over hundreds of kilometres.

01 Eleven hundred kilometres and seven hundred and fifty dry

The Drâa rises in the High Atlas and runs some 1,100 kilometres, which makes it Morocco's longest river. That figure, on its own, is misleading.

Because its lower 750 kilometres are usually dry at the surface. The water does not disappear: it infiltrates and keeps travelling under the sand and gravel of the bed, at shallow depth, until it is exhausted by evaporation and by use.

Only in exceptionally wet years does the river run its full course and reconnect with the Atlantic. The rest of the time, the last stretch of its course is a stone bed with nothing running through it.

02 That is why the oasis is a line

That behaviour explains the shape of the inhabited landscape. The date palm does not need a surface river: it needs water at shallow depth, which is exactly what lies beneath a dry bed.

So the palm grove grows along the bed, in a narrow continuous strip that can be hundreds of metres wide and tens of kilometres long. Along some 200 kilometres of the Drâa's middle course the green does not break.

And the transition between palm grove and desert is startlingly abrupt: there is no intermediate scrub, no gradient. There is palm, garden and mud wall, and from a line you can straddle, stone and nothing.

03 Where the water ends, the village ends

The whole human geography of the region follows from this. The villages sit on the river's line, not scattered across the territory, and each one occupies the point where the bed still has accessible water.

The built form answers the same calculation. The ksar, the fortified mud-brick village, stands on high ground or at the edge of the cultivable land, never in the middle: good soil is too scarce to spend on houses.

And the bed supplies the material. Mud brick is made from the earth of the place, without transport and without firing, which is the only reasonable way to build somewhere with no easy stone, no timber and no fuel for a kiln.

04 It all starts in the mountains

The head of the system is a long way from the desert. The Drâa forms in the High Atlas, and so do the Ziz and the Rheris, the other two rivers that organise the eastern half of the region.

That means the water sustaining the southern oases does not come from the oases' own sky: it comes from the snow and rain of a range several hundred kilometres away and over three thousand metres high.

That is why a drought in the mountains shows up below with a delay but without remedy, and why the gorges where the rivers leave the Atlas, such as the Todra, are the point where the whole system is visible at a glance: vertical rock, a thread of water and the first palm.

05 Nineteen inhabitants per square kilometre

The scale is worth putting in figures. Draa-Tafilalet covers 86,142 square kilometres, more than many European countries, and had 1,635,008 inhabitants at the 2014 census. Density works out at nineteen people per square kilometre.

But that average describes nothing, because the population is not spread out: it is packed into the palm strips, which occupy a minute fraction of the territory. The region has 125 municipalities and only 16 are urban.

Put another way: there are densely populated stretches and tens of thousands of square kilometres with practically nobody. The same water logic that makes the oasis a line makes the emptiness everything else.

06 How to travel it

The first practical consequence is that the trip is organised by following a river, not by hopping between points. Travelling the Drâa valley from top to bottom, or the Ziz, shows the complete system: the exit from the mountains, the continuous palm grove, the thinning and the end.

The second is that the interesting stretch is not the most spectacular one. The dunes are at the end of the line and they are what everyone photographs, but what explains the region is the middle piece, where the river still sustains something.

And the third reorders how you look. If in every village you ask where the bed is and how deep the water will be, the landscape stops being a series of postcards and becomes what it is: a river running out, and everything people have built to use it before it does.