The Graliwdo is a river of the

Nile The Nile (also known as the Nile River or River Nile) is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa. It has historically been considered the List of river sy ...

basin. Rising in the mountains of Dogu’a Tembien in northern

Ethiopia Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country located in the Horn of Africa region of East Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Ken ...

, it flows northward to empty finally in the Weri’i and

Tekezé River The Tekezé River (; , originally meaning "river" in Ge’ez; , also spelled Takkaze; ), is a major river in Ethiopia. For part of its course it forms a section of the westernmost border of Ethiopia and Eritrea. The river is also known as the Seti ...

Characteristics

The Graliwdo is a confined ephemeral river, locally

meander A meander is one of a series of regular sinuous curves in the Channel (geography), channel of a river or other watercourse. It is produced as a watercourse erosion, erodes the sediments of an outer, concave bank (cut bank, cut bank or river cl ...

ing in its narrow alluvial plain, with an average slope gradient of 78 metres per kilometre.

Hydrology

The effects of

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, t ...

s on runoff response have been studied in this river. An increase of

hydraulic roughness Hydraulic roughness is the measure of the amount of frictional resistance water experiences when passing over land and channel features. It quantifies the impact of surface irregularities and obstructions on the flow of water. One roughness coeff ...

by check dams and water transmission losses in deposited sediments are responsible for the delay of runoff to reach the lower part of the river channels. The reduction of peak runoff discharge was larger in the river segment with check dams and vegetation (minus 12%) than in segment without treatment (minus 5.5%). Reduction of total runoff volume was also larger in the river with check dams than in the untreated river. The implementation of check dams combined with vegetation reduced

peak flow The peak expiratory flow (PEF), also called peak expiratory flow rate (PEFR) and peak flow measurement, is a person's maximum speed of exhalation, expiration, as measured with a peak flow meter, a small, hand-held device used to monitor a person's ...

discharge and total runoff volume as large parts of runoff infiltrated in the sediments deposited behind the check dams. As gully check dams are implemented in a large areas of northern Ethiopia, this contributes to

groundwater recharge Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs in ...

and increased river baseflow.

Flash floods and flood buffering

Runoff mostly happens 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 and snow. Flash fl ...

). 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 ...

. The magnitude of floods in this river has however been decreased due to interventions in the catchment. Upper giraliwdo

At the upper catchment, an

exclosure An exclosure, in an area being used extensively for grazing, is a limited area from which unwanted browsing (herbivory), browsing animals, such as domestic cattle or wildlife such as deer, are excluded by fencing or other means. Environmental ...

has been established; the dense vegetation largely contributes to enhanced infiltration, less flooding and better

. Physical conservation structures such as stone bunds and

s also intercept runoff.

Irrigated agriculture

Besides springs and reservoirs,

irrigation Irrigation (also referred to as watering of plants) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has bee ...

is strongly dependent on the river's

. Such irrigated agriculture is important in meeting the demands for food security and poverty reduction. A few irrigated lands are established down from the Kolu Ba’alti reservoir.

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 * Antalo Limestone *

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), as well as the current and most recent of the twelve periods of the ...

freshwater

tufa Tufa is a variety of limestone formed when carbonate minerals precipitation (chemistry), precipitate out of water in ambient temperature, unheated rivers or lakes. hot spring, Geothermally heated hot springs sometimes produce similar (but less ...

References