Ethiopia's geography: a highland fortress split by the Rift Valley

How the plateau is put together

The Ethiopian Highlands are the defining feature and the reason the country exists in its present shape. The plateau runs generally between 1,290 and 3,000 metres above sea level, carries some twenty-five mountains above 4,000 metres, and is bounded on nearly every side by walls that rise abruptly from the plains below.

Those walls are what made the country defensible. The eastern escarpment is the best defined, averaging 2,100 to 2,400 metres and in many places rising almost perpendicularly from the plain, broken only by narrow clefts carrying seasonal torrents. An army approaching from the Red Sea either climbs that wall or takes the easier route through the Awash valley, and for most of recorded history it did neither.

Once on top, the traveller finds the encircling rampart rises little above the level of the plateau itself. The mountains are the edge, not the interior.

The surface is cut by enormous fissures, gorges eroded by rivers running off the uplands. Some are wide valleys; in others the opposite walls stand two or three hundred metres apart and fall almost vertically for thousands of metres. The erosion has left the landform that gives Ethiopian history its vocabulary: the amba, an isolated flat-topped hill with near-vertical sides, defensible by a handful of people and used as a fortress and a prison for a thousand years.

The Semien Mountains north-east of Lake Tana hold Ras Dashen, whose peak carries snow. The Bale Mountains south of the Rift hold Tullu Demtu, Batu and Chilalo, and are separated from the main highland mass by the Rift itself.

South of about 10° N the plateau opens out into more tableland and fewer high peaks, most of it below 2,400 metres, though the broken hilly character is the same.

Why the Rift Valley matters more than any border

The Great Rift Valley splits the plateau diagonally, north-east to south-west, and it is the deepest and longest chasm in the country. Everything north and west of it behaves as one region; the Bale massif and the south-east behave as another.

The Rift is an active plate boundary as well as a landform. The Afar Depression at its northern end is one of only two places on earth where a mid-ocean ridge can be studied on land; the other is Iceland. Three plates are pulling apart there, which is why the depression sits below sea level and why the country has hot springs, salt flats and active volcanism.

The Rift floor also carries a chain of lakes running south from the Shewan heights toward Lake Turkana: Zway, Abijatta, Chew Bahir and others, some fresh, some brackish, some completely closed with no outlet. Turkana at the southern end is larger than all the rest together and receives the Omo.

Where the Nile's water actually comes from

This is the fact with the widest consequences outside Ethiopia's borders. Most of the Ethiopian uplands slope to the north-west, and around 85% of the Nile's water originates on them.

Three rivers carry about four-fifths of the entire drainage: the Tekezé in the north, the Abay, which is the upper Blue Nile, in the centre, and the Sobat in the south.

The Abay is the significant one. It rises near Mount Denguiza in the Choqa mountains, runs 110 km north into Lake Tana, then escapes through a breach at the lake's south-eastern corner and makes a great semicircular bend, east then south then north-west, down to the plains of Sennar, where it becomes the Blue Nile. Lake Tana itself sits 750 to 1,000 metres below the plateau surface, covers about 2,800 km², reaches 75 metres deep, and has the physical aspect of a flooded crater.

The Awash is the exception that explains the Rift. It runs about 800 km from the Shewan uplands through a breach in the eastern escarpment and ends in Lake Abbe on the Djibouti border, in a saline depression several hundred metres below sea level. It reaches no sea. Most of the neighbouring lagoons are heavily saline with salt crusts around their margins; Abbe stays fresh year-round only because the Awash discharges so much water into it.

The gorges those rivers cut are the other half of the story. The Abay gorge below Lake Tana is comparable in depth to the Grand Canyon and has been the principal obstacle to movement between the northern and central highlands for as long as anyone has tried to cross it. During the rains the Tekezé, whose name means the Terrible, rises around five metres above its normal level and becomes an impassable barrier between the north and the centre. Rivers that would elsewhere be highways are, on this plateau, walls.

The rivers are unnavigable in their upper courses, descending from the mountains in falls, which is why a country of great rivers has almost no river transport. The Baro is the exception, becoming navigable on reaching the plain and offering an open waterway to the Nile.

What the altitude does to the climate

Altitude overrides latitude across most of Ethiopia, so a tropical country runs a temperate climate over the bulk of its area.

The traditional Ethiopian classification divides the country by height rather than by latitude, and it remains the most useful description of where anything grows or anybody lives.

Addis Ababa sits around 2,400 metres, which is why a city nine degrees from the equator has cool nights year-round. The Danakil Depression in the Afar, a few hours away, records some of the highest sustained temperatures measured anywhere on earth.

The rains arrive in two seasons rather than one. The belg rains fall roughly from February to May and the kiremt, the main rains, from June to September. Both are driven by the seasonal movement of the tropical rain belt, both are variable, and the failure of either is what turns a dry year into a crisis. The famines that ended two Ethiopian governments in the twentieth century began as rainfall failures on this pattern.

Why the two rainy seasons decide everything

The belg and the kiremt are not simply wet periods, they are two separate agricultural systems. The belg supports a short-cycle crop in the highlands and the pasture that carries lowland herds through to mid-year; the kiremt supports the main harvest that most of the country eats. A weak belg is a hard year. A failed kiremt is a catastrophe, and it arrives at the point in the calendar when the previous harvest has already been eaten.

The variability is structural rather than exceptional. The rain belt that brings both seasons shifts north and south with the sun, and its position in any given year responds to sea surface temperatures in the Indian and Pacific oceans, which is why El Niño years correlate with Ethiopian drought. A farming system dependent on two arrivals of an unreliable belt is what makes the country's food security a question of weather before it is a question of policy.

What lives on a plateau this isolated

A high massif surrounded on all sides by hot lowland functions as an island, and its biology behaves accordingly. Species that colonised the highlands were cut off when the surrounding country became too hot to cross, and evolved separately.

The result is a level of endemism unusual outside actual islands. The Ethiopian wolf, found in the Bale and Semien ranges, lives nowhere else. The gelada, often called a baboon and not one, exists only on the Ethiopian escarpments, where it grazes grass in troops that can run to hundreds. The walia ibex survives only in the Semien Mountains, and the mountain nyala only in the Bale.

The vegetation ladders with the altitude the same way the farming does, from lowland acacia through montane forest to the Afroalpine giant lobelia and heather of the highest ground. That top zone is the habitat under most pressure, because it is also the grazing that highland livestock reach in the dry season.

Where the soil is and where it is going

The plateau's basalt weathers into deep, dark, fertile soil, which is why an African highland supports the population densities it does. The same terrain then removes it. Slopes are steep, the kiremt rains fall hard on ground that has been cultivated for three thousand years, and erosion rates on the northern highlands are among the highest measured anywhere in Africa.

The visible consequence is the colour of the Blue Nile in flood, which carries the topsoil of Gojjam and Gondar toward Sudan. Terracing and afforestation programmes have run for decades with results that vary by region, and the northern highlands, farmed longest, have lost the most.

Where the borders run and what sits beyond them

Ethiopia borders six countries and, since 1993, has no coast.

None of those borders follows a natural line for long. The lowland frontiers with Sudan, South Sudan, Somalia and Kenya run across open country where the escarpment has already fallen away, and several were drawn by agreement between European powers and Menelik II around the turn of the twentieth century. The Ogaden boundary with Somalia has been fought over twice since, and the border with Eritrea was the subject of a war between 1998 and 2000 that cost an estimated million dollars a day.

The country's westernmost point is on the Pibor River opposite the Sudanese village of Denjok; its easternmost lies on the far edge of the Dollo Zone opposite Puntland and Galmudug.

Becoming landlocked in 1993 was the single largest change to Ethiopian geography in modern times. Eritrean independence took the Red Sea coast with it, leaving a country of more than a hundred million people dependent on the corridor to Djibouti for the overwhelming majority of its imports and exports. Every subsequent Ethiopian government has treated port access as a strategic question rather than a logistical one, and that is a direct consequence of a line on a map rather than of the terrain.

The terrain still does the older work. The lowland fringes, the Afar in the east, the Somali region in the south-east, the Gambela plains in the west, are hot, dry and thinly populated, and they are where the state has always been weakest. The plateau is where the people, the farming and the government are, and the split is stark: the highlands hold the overwhelming majority of the population on a minority of the land area, while the lowland regions hold most of the territory and a small fraction of the people.

That imbalance is why the country's internal politics is a highland argument that the lowlands are periodically drawn into. It is also why pastoralism, the way of life across the whole of the Afar and Somali regions and much of Borena, sits awkwardly inside a state built around settled highland farming: the administrative units, the land tenure rules and the service model were all designed for people who stay in one place. The escarpment between them has been a political boundary for as long as it has been a physical one.