3.1. Introduction
3.2. The Physiographic Divisions of Ethiopia
       3.2.1. The Western Highlands and Lowlands
       3.2.2. The Southeastern Highlands and Lowlands
       3.2.3. The Rift Valley
3.3. The Impacts of Relief on Biophysical and Socioeconomic Conditions

Chapter's Objectives

At the end of this chapter, the learners will be able to:  Describe the topography of Ethiopia and the Horn.
 Identify the physiographic divisions of Ethiopia.
 Elucidate the physiographic characteristics of the Rift Valley.
 Explain the impacts of relief on biophysical and socioeconomic conditions

3.1. General Characteristics of the Ethiopian Physiography

The Ethiopian landform is characterized by great diversity. There are flat-topped plateaus, high
and rugged mountains, deep river gorges and vast plains. Altitude ranges from 125 meters below
sea level (Kobar Sink) to the highest mountain in Ethiopia, Mount Ras Dashen (4,620 m.a.s.l),
which is the fourth highest mountain in Africa. More than 50% of the Ethiopian landmass is above 1,000 meters of elevation; and above 1,500 meters makes 44% of the country. Half of this, in turn, is at more than 2,000 meters above sea level. Most of the Ethiopian Highlands are part of central and northern Ethiopia, and its northernmost portion extends into Eritrea.

Most of the country consists of high plateau and mountain ranges that are sources of many rivers and streams that made the country to be described as the “Water Tower of East Africa”. Taking the 1,000 meters for the highland-lowland demarcation, one observes the following contrasting features between the Ethiopian highlands and lowlands:

Characteristics of Ethiopian highlands:

  • Moderate and high amount of rainfall (>600 mm per year).
  • Lower mean annual temperature (<20°C).
  • The climate is favorable for biotic life.
  • Rain-fed agriculture is possible.
  • Free from tropical diseases.

These highlands make up nearly 56% of the area of the Ethiopia. In contrast to the highlands, the remaining 44% of the Ethiopian lowlands are characterized by:

  • Fewer amounts of rainfall and higher temperature.
  • High prevalence of tropical diseases.
  • Lower population densities.
  • Nomadic and semi-nomadic economic life.
  • Vast plain lands favorable for irrigation agriculture along the lower river basins.

3.2. The Physiographic Divisions of Ethiopia

Following the structural divisions brought about by the geologic processes of the Cenozoic Era,
three major physiographic units can be identified in Ethiopia. These are:

  1. The Western highlands and lowlands
  2. The South-eastern (Eastern) highlands and lowlands
  3. The Rift Valley

3.2.1. The Western Highlands and Lowlands

This physiographic unit includes all the area west of the Rift Valley. It extends from north to
south encompassing nearly the whole western half of Ethiopia. It makes up about 44% of the
area of the country. In the east the western escarpment of the Rift Valley bound it whereas
westward, the land gradually descends in altitude until it merges into the western foothills and
lowlands, along the Sudan and South Sudan border. This region is further subdivided into four
groups of highlands (76.3%) and four groups of lowlands (23.7%).

The Western Highlands

The Tigray Plateau

It extends from the Tekeze gorge in the south to central Eritrean highlands. The Tigray plateau is
separated from the Eritrean plateau by the Mereb River. It lies to the southeast of the upper
course of the Mereb/Gash River and to the northeast of Tekeze River Gorge. It constitutes about
13% of the area of the region. It is an elongated highland with most of the land being in between
1,000 and 2,000 meters above sea level. There are high mountains in this plateau with elevations of over 3000 meters, namely Mount Tsibet (3988 m.a.s.l), Mount Ambalage (3291 m.a.s.l), and Mount Assimba (3248m.a.s.l). The famous monastery at Debre-Damo, a tableland that can only be climbed by a rope pulley is also located in this plateau region.

North Central Massifs

This Physiographic division is the largest in the western highlands. Much of its northern and
southern limit follows the Abay and Tekeze gorges. In its central part, the physiographic unit also accommodates the Lake Tana basin. 58% of the region is at an altitude of more than 2,000 meters, making it, next to the Shewan Plateau, the second highest physiographic division.

The region consists of the Gonder, Wello and Gojjam Massifs. Out of the 26 mountain peaks with altitude of more than 4,000m.a.s.l in Ethiopia, 19 mountain peaks are found in this physiographic region. Among these, the most popular ones include Mount Ras Dashen (4,620 m.a.s.l), Mount Weynobar/Ancua (4462 m.a.s.l), Mount KidisYared (4453 m.a.s.l), and Mount Bwahit (4437 m.a.s.l) in the Simen Mountain System. Mount Guna (4,231m.a.s.l) in the Debre Tabour Mountain System, Abune Yoseph (4,260 m.a.s.l) in the Lasta highlands of Wello and Mount Birhan (4,154 m.a.s.l) in the Choke Mountain System in Gojjam.

The Shewa Plateau (Central Highlands)

The Shewan plateau is bounded by the Rift Valley in the east and southeast, by the Abay gorge
in its northern and western limit, and the Omo gorge in the south and west. This plateau
occupies a central geographical position in Ethiopia. With only 11% of the area of the whole
physiographic region, the Shewa Plateau is the smallest of the Western highlands. Nearly three fourth of its area is at an altitude of more than 2,000 meters above sea level. It has, therefore, the largest proportion of elevated ground. The Shewa plateau is drained, outward in all directions by the tributaries of Abay, Omo, and Awash. The highest mountains includes Mount Abuye-Meda (4,000 m.a.s.l) in Northern Shewa and Mount Guraghe in the south is 3,721 meters high.

The Southwestern Highlands

This Physiographic subdivision consists of the highlands of Wellega, Illuababor, Jimma, Kaffa,
Gamo and Gofa. This region is separated from the adjacent highlands by the Abay and Omo
river valleys. It extends from the Abay gorge in the north to the Kenya border and Chew Bahir in
the south. It accounts for 22.7% of the area of the region. The region is the second largest in the
Western highlands. About 70% of its area is lies within 1,000-2,000 meters altitude. The southwestern plateau is the wettest in Ethiopia. It is drained by Dabus, Deddessa (tributaries
of Abay), Baro, Akobo and the Ghibe/Omo rivers. With a height of 4,200 meters above
sea level, Guge Mountain is the highest peak in this physiographic subdivision.

The Western Lowlands

These are the western foothills and border plains that extend from Western Tigray in the north to
Southern Gamo-Gofa in the South. They make 11% of the area of the physiographic
region. The general elevation ranges between 500 and 1000 meters above sea level. This physiographic sub-region is further subdivided into four by the protruding ridges. These are Tekeze lowland, Abay-Dinder lowland, Baro lowland, and Ghibe lowland.

With the exception of the Baro lowland, the region is generally characterized by arid or
semi-arid conditions. Pastoral or semi-pastoral economic activities dominate the area. As
one moves northwards, the degree of aridity increases, making rain-fed agriculture more
difficult. The Baro lowland has an extensive flat area suitable for mechanized agriculture.

The Ghibe/Omo lowland, which includes the lower Ghibe/Omo Valley and the northern
section of the Turkana basin, is classified in the Western lowlands from its geographical
location. But structurally it also belongs to the Rift Valley. In the Western lowlands, there are small but important towns such as Humera, Metema, Omedla, Kurmuk, Gambella etc.

3.2.2. The South-Eastern Highlands and Lowlands

This physiographic region is the second largest in terms of area. It accounts for 37% of the
area of Ethiopia. The highlands make up 46% of the physiographic division while the rest is
lowland. In the west and north, the eastern escarpment of the Rift Valley makes the western
and northern limit. The land gradually descends southeastward into the southeastern lowlands and then to the plains of Somalia. These are further subdivided into two units of highlands and two units of extensive lowlands. These are briefly discussed as follows.

The Southeastern Highlands

The Arsi-Bale-Sidama Highlands

These highlands are found to the east of the Lakes Region. They are located in the south
western section of the physiographic region. The Arsi Highlands are made up of flat rolling uplands and dissected mountains. The well-known mountains in this area are Mount Kaka (4,180 m.a.s.l), Mount Bada (4,139 m.a.s.l) and Mount Chilalo (4,036 m.a.s.l).

The Bale highlands are separated from the Arsi highlands by the head and main stream of
Wabishebelle. They consist of a platform looking basaltic plateau in the north-central part
and high mountain massif to the south. The highest mountain peaks in this region are Tulu-Demtu (4,377 m.a.s.l) and Mount Batu (4,307 m.a.s.l). The Arsi-Bale Highlands are important grain producing areas with high potential.

The Sidama Highlands are separated from the Bale Highlands by the Ghenale river valley.
They occupy the southwestern corner of this region. The prominent feature here is the Jemjem plateau, an important coffee growing area.

Rivers Wabishebelle and Ghenale along with their tributaries have dissected this physiographic region. Specially, Weyb River, tributary of Ghenale, has cut an underground passage (Sof Omar cave). The cave is found near Bale Mountains. It is one of the World’s most spectacular and extensive underground caverns creating a magnificent view.

The Hararghe Plateau

This plateau is a north-easterly extension of the south-eastern highlands. It extends from the
Chercher highlands in the south-west to Jigjiga in the east. It has the smallest proportion of upper highland (>2,000 meters). The highest mountain here is Mount Gara-Muleta (3,381 m.a.s.l).

The Southeastern Lowlands

The Southeastern lowlands are located in the southeastern part of the country and they are the
most extensive lowlands in Ethiopia. They make up 54% of the area of the physiographic region
and around one-fifth of the country. This region is divided into Wabishebelle plain (60%) and the
Ghenale Plain (40%). They include the plains of Ogaden, Elkere, and Borena. They are sparsely inhabited by pastoral and semi-pastoral communities. The economic potential includes animal husbandry, irrigation, agriculture and perhaps exploitation of petroleum and natural gas.

3.2.3. The Rift Valley

The Rift Valley is a tectonically formed structural depression. It is bounded by two major and
more or less parallel escarpments. The formation of the Rift Valley has separated the Ethiopian
Highlands and Lowlands in to two. It extends from the Afar triangle in the north to Chew Bahir for about 1,700 km2. It covers 18% of the area of Ethiopia. Altitude in the floor ranges from 125 m.b.s.l at Dallol depression to as high as 2,000 m.a.s.l in the Lakes region.

Because of its altitudinal variation and positional differences, the climate also varies from warm, hot and dry to cool and moderately moist conditions. Similarly, the social and economic life reflects this pattern. There are places, which are desolate and sparsely inhabited by pastoralists where as in others parts people practice some rain-fed agriculture.

The Rift Valley is further subdivided into three physiographic sub-regions. These are the Afar
Triangle, the Main Ethiopian Rift, and the Chew Bahir Rift.

The Afar Triangle

The Afar Triangle is the largest and widest part of the Rift Valley. It makes up 54% of the Rift
Valley area. The area is generally of low altitude (300-700 meters). Quite different is the morphology of the Afar depression, triangular-shape lowland, where elevation drops uniformly from approximately 1,000 meters in the southwest to below sea level in the north (Danakil depression) and in the east, where the shores of Lake Asal, fluctuating at around 125 meters below sea level, represent the lowest subaerial point of the African continent. The depression, which hosts one of the most hostile environments on Earth (maximum temperatures can exceed 50°C) during the summer wet season. The area is characterized by faulted depressions (grabens), volcanic hills, active volcanoes, volcanic ridges, lava fields and low lava platforms. Lakes (Abe, Asale, and Afrera) occupy some of these basins. A prominent feature in this region is the Denakil Depression (Kobar Sink). The economic importance of this region includes salt extraction, irrigation along the Awash River and electric potential from geothermal energy.

The Main Ethiopian Rift (Central Rift)

It refers to the narrow belt of the Rift Valley that extends from Awash River in the north to Lake
Chamo in the south. It is bounded by the western and eastern escarpments. With the exception
of the Arbaminch area, the bounding escarpments are generally low. It has an average width of 50-80 kilometers and generalelevation of 1,000-2,000 meters above sea level.The major mountainsinclude Mount Fentale, Boseti-Guda (near Adama), Aletu (north of Lake Ziway) and Chebi(north of Lake Hawasa). Because of altitude, this region is generally milder and watery.

The Chew Bahir Rift

This is the smallest and the southern-most part of the Rift Valley. Gneissic highlands of Konso
and the surrounding highlands separate it from the Main Ethiopian Rift to the north. The
characteristic feature of this region is the broad and shallow depression, which is a marshy area covered by tall grass, into which the Segen and Woito streams empty.

3.3. The Impacts of Relief on Biophysical and Socioeconomic Conditions

The impacts of relief can be summarized by the following:

  1. Agricultural practices
  • Relief influences farm size, shape, crop production and animal husbandry.
  1. Settlement pattern
  • The highlands are characterized by sedentary life while lowlands are inhabited by pastoralists.
  1. Transportation and communication
  • The difficult terrain makes infrastructure development and maintenance costly.
  • The rugged topography rendered rivers less navigable due to the waterfalls.
  1. Hydroelectric power potential
  • Altitude coupled with high rainfall created suitable conditions for hydroelectric power.
  1. Socio-cultural feeling
  • The rugged terrain resulted in isolation of communities
  1. Impacts on climate
  • Highlands have higher amount of rainfall and lower rate of evapo-transpiration than lowlands.
  1. Impacts on soil
  • Steep mountain slopes provide unstable surface materials and subject to degradation.
  1. Impacts on natural vegetation
  • Relief through its effect on climate and hydrology affect the type of natural vegetation.

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