Agula’i is a river of northern
Ethiopia
Ethiopia, , om, Itiyoophiyaa, so, Itoobiya, ti, ኢትዮጵያ, Ítiyop'iya, aa, Itiyoppiya officially the Federal Democratic Republic of Ethiopia, is a landlocked country in the Horn of Africa. It shares borders with Eritrea to the ...
. Rising in the mountains of
Atsbi Wenberta
Atsbi Wemberta () is one of the Districts of Ethiopia, or ''woredas'', in the Tigray Region of Ethiopia. Located in the Misraqawi Zone at the eastern edge of the Ethiopian highlands, Atsbi Wenberta is bordered on the south by the Debub Misraqawi ...
(2676 metres above sea level), it flows southwestward to
Giba River
Giba is a river of northern Ethiopia. It starts at the confluence of Genfel and Sulluh (which rises in the mountains of Mugulat) (3298 metres above sea level) and flows westward to the Tekezé River. Future Lake Giba will occupy the plain where ...
which empties finally in the
Tekezé River
The Tekezé or Täkkäze River ( amh, ተከዜ, ti, ተከዘ; originally meaning "river" in Ge’ez, ), also spelled Takkaze, is a major river of Ethiopia. For part of its course it forms a section of the westernmost border of Ethiopia and Eri ...
.
Future
Lake Giba
Lake Giba is a reservoir under construction at the border of the Inderta; Kilte Awula'ilo and Dogu’a Tembien ''woredas'' of the Tigray Region in Ethiopia. The earthen dam that holds the reservoir is under construction in 2020. It will colle ...
will occupy the plain where
Sulluh
Sulluh is a river of northern Ethiopia. Rising in the mountains of Mugulat (3298 metres above sea level), it flows southward to Giba River which empties finally in the Tekezé River. Future Lake Giba will occupy the plain where Sulluh, Genfel and ...
,
Genfel
Genfel is a river of northern Ethiopia. Rising on the foot of the Atsbi horst at 2253 metres above sea level, it flows southwestward to Giba River which empties finally in the Tekezé River. Future Lake Giba will occupy the plain where Sulluh, ...
and Agula’i Rivers meet.
Hydrography
It is a confined river, locally
meander
A meander is one of a series of regular sinuous curves in the channel of a river or other watercourse. It is produced as a watercourse erodes the sediments of an outer, concave bank ( cut bank) and deposits sediments on an inner, convex bank ...
ing in its narrow alluvial plain, with a slope gradient of 8 metres per kilometre. With its tributaries, the river has cut a deep gorge.
Hydrology
Hydrological characteristics
The
runoff footprint or annual total runoff volume is 115 million m³.
Peak discharges up to 520 m³ per second occur in the second part of the rainy season (month of August) when there are strong rains and the soils are
saturated with water in many places.
The percentage of total rainfall that directly leaves the catchment as
storm runoff (also called runoff coefficient) is 12%. As
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
is present in 47% of the catchment this runoff coefficient is less than that of adjacent rivers.
![Agula i](https://upload.wikimedia.org/wikipedia/commons/2/20/Agula_i.jpg)
The total amount of
sediment that is transported by this river amounts to 1.95 million tonnes per year. Median sediment concentration in the river water is 3.78 grammes per litre, but may go up to 97 g/L. The highest sediment concentrations occur at the beginning of the rainy season, when loose soil and dust is washed away by overland flow and ends up in the river, and towards the end when river bank erosion is frequent.
As such water contains many nutrients (locally it is called “aygi”), farmers estimate that it strengthens their cattle, which they will bring to the river.
All in all, average
sediment yield
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand a ...
is 3784 tonnes per km² and per year. All measurements were done at a purposively installed station near the mouth of the river, in the years 2005–2007.
Flash floods
Runoff mostly occurs in the form of high runoff discharge events that occur in a very short period (called
flash floods
A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing ...
). These are related to the steep topography, often little vegetation cover and intense convective rainfall. The peaks of such flash floods have often a 50 to 100 times larger discharge than the preceding
baseflow
Baseflow (also called drought flow, groundwater recession flow, low flow, low-water flow, low-water discharge and sustained or fair-weather runoff) is the portion of the streamflow that is sustained between precipitation events, fed to streams by d ...
. These flash floods mostly occur during the evening or night, because the convective rain showers occur in the afternoon.
Changes over time
Evidence given by
Italian aerial photographs of the catchment, taken in the 1930s show that 51% of the catchment was covered with woody vegetation (against 45% in 2014). This vegetation could slow down runoff and the runoff coefficient was smaller (8% in 1935 against 12% in 2014). As a consequence, discharges in the river were less and the river was narrower than today.
Up to the 1980s, there was strong pressure on the environment, and much vegetation disappeared. This river had its greatest discharges and width in that period.
The magnitude of floods in this river has however been decreased in recent years due to interventions in the catchment. On steep slopes,
exclosure
An exclosure, in an area being used extensively for grazing, is a limited area from which unwanted browsing animals, such as domestic cattle or wildlife such as deer, are excluded by fencing or other means.
Environmental protection
Most commonl ...
s have been established; the dense vegetation largely contributes to enhanced
infiltration
Infiltration may refer to:
Science, medicine, and engineering
*Infiltration (hydrology), downward movement of water into soil
*Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings
*Infiltration (me ...
, less flooding and better
baseflow
Baseflow (also called drought flow, groundwater recession flow, low flow, low-water flow, low-water discharge and sustained or fair-weather runoff) is the portion of the streamflow that is sustained between precipitation events, fed to streams by d ...
. Physical conservation structures such as
stone bunds and
check dam
A steel check dam
A check dam is a small, sometimes temporary, dam constructed across a swale, drainage ditch, or waterway to counteract erosion by reducing water flow velocity. Check dams themselves are not a type of new technology; rather, th ...
s also intercept runoff.
Irrigated agriculture
Besides springs and reservoirs,
irrigation
Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow Crop, crops, Landscape plant, landscape plants, and Lawn, lawns. Irrigation has been a key aspect of agriculture for over 5,00 ...
is strongly dependent on the river's baseflow. Such irrigated agriculture is important in meeting the demands for food security and poverty reduction.
Irrigated lands are established in the narrow alluvial plains all along the river and pump irrigation is used.
Transhumance towards the river gorge
The valley bottoms in the lower gorge of this river have been identified as a transhumance destination zone.
Transhumance
Transhumance is a type of pastoralism or nomadism, a seasonal movement of livestock between fixed summer and winter pastures. In montane regions (''vertical transhumance''), it implies movement between higher pastures in summer and lower vall ...
takes place in the summer rainy season, when the lands near the villages are occupied by crops. Young shepherds will take the village cattle down to the gorge and overnight in small caves. The gorges are particularly attractive as a transhumance destination zone, because there is water and good growth of semi-natural vegetation.
Boulders and pebbles in the river bed
Boulders and pebbles encountered in the river bed can originate from any location higher up in the catchment. In the uppermost stretches of the river, only rock fragments of the upper lithological units will be present in the river bed, whereas more downstream one may find a more comprehensive mix of all lithologies crossed by the river. From upstream to downstream, the following lithological units occur in the catchment.
*
Amba Aradam Formation
* Agula
Shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
*
Antalo Limestone
The Antalo Limestone, also known as the Antalo Sequence, is a geological formation in Ethiopia. It is between 300 and 800 metres thick and comprises fossiliferous limestones and marls that were deposited in a reef. Marine microfossils have shown ...
* Mekelle
Dolerite
Diabase (), also called dolerite () or microgabbro,
is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-grain ...
*
Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
freshwater
tufa
Tufa is a variety of limestone formed when carbonate minerals precipitate out of water in unheated rivers or lakes. Geothermally heated hot springs sometimes produce similar (but less porous) carbonate deposits, which are known as travertine. ...
Trekking along the river
Trekking routes have been established in nearby
Dogu’a Tembien.
The tracks are not marked on the ground but can be followed using downloaded .GPX files. Trek 24 passes along the confluence of Agula’i and
Giba River
Giba is a river of northern Ethiopia. It starts at the confluence of Genfel and Sulluh (which rises in the mountains of Mugulat) (3298 metres above sea level) and flows westward to the Tekezé River. Future Lake Giba will occupy the plain where ...
.
In the rainy season, flash floods may occur and it is advised not to follow the river bed. Frequently, it is then also impossible to wade across the river.
See also
*
List of Ethiopian rivers
This is a list of streams and rivers in Ethiopia, arranged geographically by drainage basin. There is an alphabetic list at the end of this article.
Flowing into the Mediterranean
*''Nile (Egypt, Sudan)''
Atbarah River
*Mareb River (or ...
References
{{reflist
Rivers of Ethiopia
Dogu'a Tembien
Tigray Region