Ethiopia - Topography and Drainage
Millennial of erosion have produced steep valleys, in places 1,600 meters deep and several
kilometers wide. In these valleys flow rapid streams unsuitable for navigation
but possessing potential as sources of hydroelectric power and water for
irrigation.
The
highlands that comprise much of the country are often referred to as the
Ethiopian Plateau and are usually thought of as divided into northern and
southern parts. In a strict geographical sense, however, they are bisected by
the Great Rift Valley into the northwestern highlands and the southeastern
highlands, each with associated lowlands. The northwestern highlands are
considerably more extensive and rugged and are divided into northern and
southern sections by the valley of the Abay (Blue Nile).
North of
Addis Ababa, the surface of the plateau is interspersed with towering mountains
and deep chasms that create a variety of physio graphic climate, and indigenous
vegetation. The plateau also contains mountain ranges such as the Chercher and
Aranna. Given the rugged nature of these mountains and the surrounding
tableland, foreigners receive a false impression of the country's topography
when Ethiopians refer to the land form as a plateau. Few of these peaks' surfaces
are flat except for a scattering of level-topped mountains known to Ethiopians
as ambas.
Southwest
of Addis Ababa, the plateau also is rugged, but its elevation is slightly lower
than in its northern section. To the southeast of Addis Ababa, beyond the Ahmar
and Mendebo mountain ranges and the higher elevations of the southeastern
highlands, the plateau slopes gently toward the southeast. The land here is
rocky desert and, consequently, is sparsely populated.
The Great
Rift Valley forms a third physio graphic region. This extensive fault system
extends from the Jordan Valley in the Middle East to the Zambezi River's Shire
tributary in Mozambique. The segment running through central Ethiopia is marked
in the north by the Denakil Depression and the coastal lowlands, or Afar Plain,
as they are sometimes known. To the south, at approximately 9° north latitude,
the Great Rift Valley becomes a deep trench slicing through the plateau from
north to south, its width averaging fifty kilometers. The southern half of the
Ethiopian segment of the valley is dotted by a chain of relatively large lakes.
Some hold fresh water, fed by small streams from the east; others contain salts
and minerals.
In the
north, the Great Rift Valley broadens into a funnel-shaped saline plain. The
Denakil Depression, a large, triangle-shaped basin that in places is 115 meters
below sea level, is one of the hottest places on earth. On the northeastern
edge of the depression, maritime hills border a hot, arid, and treeless strip
of coastal land sixteen to eighty kilometers wide. These coastal hills drain
inland into saline lakes, from which commercial salt is extracted. Along the
Red Sea coast are the Dahlak Islands, which are sparsely inhabited.
In
contrast with the plateau's steep scarps along the Great Rift Valley and in the
north, the western and southwestern slopes descend somewhat less abruptly and
are broken more often by river exits. Between the plateau and the Sudanese
border in the west lies a narrow strip of sparsely populated tropical lowland
that belongs politically to Ethiopia but whose inhabitants are related to the
people of Sudan. These tropical lowlands on the periphery of the plateau,
particularly in the far north and along the western frontier, contrast markedly
with the upland terrain.
The
existence of small volcanoes, hot springs, and many deep gorges indicates that
large segments of the landmass are still geologically unstable. Numerous
volcanoes occur in the Denakil area, and hot springs and steaming fissures are
found in other northern areas of the Great Rift Valley. A line of seismic
faults extends along the length of Eritrea and the Denakil Depression, and
small earthquakes have been recorded in the area in recent times.
All of
Ethiopia's rivers originate in the highlands and flow outward in many
directions through deep gorges. Most notable of these is the Blue Nile, the
country's largest river. It and its tributaries account for two-thirds of the
Nile River flow below Khartoum in Sudan. Because of the general westward slope
of the highlands, many large rivers are tributaries of the Nile system, which
drains an extensive area of the central portion of the plateau. The Blue Nile,
the Tekezé, and the Baro are among them and account for about half of the
country's water outflow. In the northern half of the Great Rift Valley flows
the Awash River, on which the government has built several dams to generate
power and irrigate major commercial plantations. The Awash flows east and
disappears in the saline lakes near the boundary with Djibouti. The southeast
is drained by the Genale and Shebele rivers and their tributaries, and the
southwest is drained by the Omo.
Geography
of Ethiopia.
Ethiopia - Climate
The cool
zone consists of the central parts of the western and eastern sections of the
northwestern plateau and a small area around Harer. The terrain in these areas
is generally above 2,400 meters in elevation; average daily highs range from
near freezing to 16°C, with March, April, and May the warmest months.
Throughout the year, the midday warmth diminishes quickly by afternoon, and
nights are usually cold. During most months, light frost often forms at night
and snow occurs at the highest elevations.
Lower
areas of the plateau, between 1,500 and 2,400 meters in elevation, constitute
the temperate zone. Daily highs there range from 16°C to 30°C.
The hot
zone consists of areas where the elevation is lower than 1,500 meters. This
area encompasses the Denakil Depression, the Eritrean lowlands, the eastern
Ogaden, the deep tropical valleys of the Blue Nile and Tekezé rivers, and the
peripheral areas along the Sudanese and Kenyan borders. Daytime conditions are
torrid, and daily temperatures vary more widely here than in the other two
regions. Although the hot zone's average annual daytime temperature is about
27°C, midyear readings in the arid and semiarid areas along the Red Sea coast
often soar to 50°C and to more than 40°C in the arid Ogaden. Humidity is
usually high in the tropical valleys and along the seacoast.
Variations
in precipitation throughout the country are the result of differences in
elevation and seasonal changes in the atmospheric pressure systems that control
the prevailing winds. Because of these factors, several regions receive
rainfall throughout most of the year, but in other areas precipitation is
seasonal. In the more arid lowlands, rainfall is always meager.
In January
the high pressure system that produces monsoons in Asia crosses the Red Sea.
Although these northeast trade winds bring rain to the coastal plains and the
eastern escarpment in Eritrea, they are essentially cool and dry and provide
little moisture to the country's interior. Their effect on the coastal region,
however, is to create a Mediterranean-like climate. Winds that originate over
the Atlantic Ocean and blow across Equatorial Africa have a marked seasonal
effect on much of Ethiopia. The resulting weather pattern provides the
highlands with most of its rainfall during a period that generally lasts from
mid-June to mid-September.
The main
rainy season is usually preceded in April and May by converging northeast and
southeast winds that produce a brief period of light rains, known as balg.
These rains are followed by a short period of hot dry weather, and toward the
middle of June violent thunderstorms occur almost daily. In the southwest,
precipitation is more evenly distributed and also more abundant. The relative
humidity and rainfall decrease generally from south to north and also in the
eastern lowlands. Annual precipitation is heaviest in the southwest, scant in
the Great Rift Valley and the Ogaden, and negligible in the Denakil Depression.
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