A wadi is a valley or channel in a dry region that is empty for most of the year and carries water only after heavy rain. The word is Arabic, and it is used across North Africa and the Middle East; in French-speaking North Africa it is often spelled oued. The same landform is called an arroyo or a wash in the south-western United States and Mexico, and a nullah in South Asia. Wadis look harmless, but they are where desert flash floods happen. A storm miles away can send a wall of water down a dry channel in minutes, and over centuries those floods carve the wadi itself.
How a Dry Valley Is Made by Water
It seems contradictory that the driest places on Earth are shaped so much by running water, but deserts are full of water-carved valleys. The reason is the way rain falls there. Desert rain is rare but often intense, coming in short, violent storms. The ground is bare, baked hard and often crusted, and there is little vegetation to slow the water or soak it up. Instead of sinking in, most of it runs off across the surface and gathers in the lowest channels. Within minutes a dry wadi can become a torrent carrying sand, gravel and boulders, and that load does the cutting.
The flow stops almost as quickly as it starts. Once the storm passes, water sinks into the gravel of the wadi bed or evaporates, and the channel is dry again, sometimes for years. Many wadis are also relics of wetter times. During the humid periods of the last few hundred thousand years, most recently the African Humid Period that ended about 5,000 years ago, parts of the Sahara and Arabia had permanent rivers and lakes. Satellite radar can still trace ancient river systems buried under Saharan sand, and many of today's wadis follow those old courses. Networks of dry valleys cut by water billions of years ago are also seen on Mars, and planetary scientists compare them directly with desert wadis on Earth.
What a Wadi Looks Like
Wadis come in every size, from a shallow gully a few metres across to a canyon hundreds of metres deep and hundreds of kilometres long. Most share a few features. The floor is usually flat, covered with gravel, sand and boulders dumped by the last flood. The sides are often steep, because there is little rain between floods to wear them back to a gentle slope. Where a wadi leaves the mountains and spreads onto a plain, it often drops its sediment in a fan-shaped alluvial fan. Trees and shrubs such as acacias and tamarisks often grow along the floor, a line of green in otherwise bare country, because water lies close beneath the surface long after the channel dries.
Wadi, Arroyo, Canyon: Telling Them Apart
Geographers classify streams by how often they flow. A perennial stream flows all year. An intermittent stream flows for part of the year, usually a wet season, when the water table rises high enough to feed it. An ephemeral stream flows only during and just after rain, and its bed lies above the water table the rest of the time. Most wadis are ephemeral, though some become intermittent where they cross wetter mountains. A wadi is not the same as a canyon, although it can be one. A canyon is defined by its steep, deep shape, not by how often it carries water. The Grand Canyon holds the perennial Colorado River, while a wadi canyon may be dry for years.
Water Beneath the Dry Bed
That hidden water is why wadis have mattered so much to people. The gravel of a wadi floor stores water from each flood, and shallow wells dug into it can reach water even in the dry season. Oases often lie where a wadi's buried water rises to the surface. People have long built low walls and dams across wadis to slow floods, spread the water over fields and let it soak into the ground. The Nabataeans, whose capital was Petra in Jordan, were masters of this. They cut channels, cisterns and dams that captured flash floods and supplied a city of perhaps 20,000 people in the desert. Our guide to Petra describes the city they built.
Wadis have also been natural routes. Their level floors cut through mountains and plateaus, so caravan tracks, roads and railways often follow them, and villages cluster along their banks where water and flat land can be found together.
Wadis Worth Knowing
Wadi Rum, in southern Jordan, is a broad sandy valley between sandstone and granite mountains, famous for its red landscapes and its association with T. E. Lawrence. It is a World Heritage Site for both its scenery and its thousands of ancient rock carvings and inscriptions. Our guide to Wadi Rum explores it.
Wadi Hadramawt, in Yemen, is one of the largest wadi systems in Arabia. Its floor has been farmed for millennia, and it holds the walled town of Shibam, whose mud-brick tower houses, some up to about 30 m tall and built mostly in the sixteenth century, have earned it the nickname "the Manhattan of the desert". It is a World Heritage Site.
Wadi Al-Hitan, "Valley of the Whales", in Egypt's Western Desert, contains the fossil skeletons of early whales that still had small hind legs, about 40 million years old. They record the evolution of whales from land mammals, and the site is protected as a World Heritage Site.
Wadi Araba, the dry valley running between the Dead Sea and the Gulf of Aqaba, forms much of the border between Israel and Jordan and follows the great rift that continues south into the Red Sea. Wadi Halfa in Sudan gave its name to a town that was flooded by the reservoir of the Aswan High Dam in the 1960s and had to be rebuilt on the new shore.
When Wadis Flood
The danger of a wadi is that the flood can come from a storm the people in it cannot see. Rain falling in distant hills can send water down a channel under clear blue skies, and flash floods kill people in deserts more often than thirst does. In 1963 a sudden flood through the Siq, the narrow gorge leading into Petra, killed 22 French tourists and a guide. The Nabataeans' own flood defences had fallen into ruin; the Jordanian authorities have since rebuilt a diversion dam at the entrance.
Cities built on wadis face the same risk on a larger scale. In November 2001 torrential rain sent floods and mud through Bab El Oued, a crowded district of Algiers named after the wadi it stands on, killing about 800 people. In November 2009 flash floods in Jeddah, Saudi Arabia, killed more than 100 people, many of them in neighbourhoods built in dry wadi beds. The lesson geographers draw is the same everywhere: a wadi is a river that only looks as though it has gone away.
Life Along the Channel
Because they hold water below the surface and shade between their walls, wadis are the richest habitats in many deserts. They act as corridors for animals as well as people. Nubian ibex climb the cliffs of wadis in Sinai, Jordan and Arabia, gazelles graze on the vegetation of the floor, and migrating birds follow the lines of acacias across otherwise empty land. After a flood, wadi floors can burst into flower within days, as seeds that have waited in the soil for years germinate and set seed before the ground dries again.
Wadis in Spanish Place Names
The Arabic word travelled far beyond the desert. During the centuries of Muslim rule in Spain, many rivers and places were given Arabic names, and wadi, spoken as "guad-", survives across the Spanish map. The Guadalquivir, the great river of Seville, comes from al-wadi al-kabir, "the great river". Guadalajara, both the Spanish city and the one in Mexico named after it, is usually derived from wadi al-hijara, "valley of stones". The Guadiana, the Guadalete and dozens of smaller streams and villages carry the same prefix, a trace of Arabic on rivers that flow all year round.
Wadis Beyond the Arab World
The landform is common to every desert, even where the word is not used. In the American Southwest, "wash" and "arroyo" describe the same thing, and warning signs at desert road crossings tell drivers not to enter when water is flowing. In Arizona's Antelope Canyon, a narrow slot canyon carved by flash floods, eleven people died in 1997 when a storm upstream sent water through the canyon. In Namibia, the Kuiseb River flows only briefly after rain, yet those floods are strong enough to sweep away sand, and the river marks the northern edge of the Namib's great dune sea. In Australia, dry creek beds that run only after rain are a feature of the outback, and in India and Pakistan a nullah can be a dry ditch for most of the year and a raging stream in the monsoon. Wadis are one of the defining features of hot deserts, described more broadly in our explainer on what a desert is.