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Soil-landscape relation at regional scale in Lake Tana basin (Ethiopia): vertisol distribution and variation as a proxy for the ancient levels of Lake Tana
... The 1,496 km 2 catchment of Gumara River is part of the 15,077 km 2 Lake Tana consists of slopes between 8° and 30°; 24% is comprised of slopes less than 8° that are located mainly in the lower part of the watershed near Lake Tana . Dominant soil types identified in the upper catchment are Luvisols, Fluvisols, and Leptosols; common in the downstream plains are Vertisols and Fluvisols (Henricksen et al., 1984;Colot, 2012). High runoff response (Dessie et al., 2014) and sediment load (Lemma et al., 2016) of Gumara River occur due to strong degradation of the catchment (Poppe et al., 2013;. ...
Landscape response to drivers of change is more visible and noticeable in deltas and floodplains than upstream. Here, we address the changes of Lake Tana basin by investigating the delta development of Gumara River and sediment deposition in the Fogera floodplain over a 58-year period when agricultural land expanded, agriculture intensified, and flooding of the alluvial plain became more frequent. Old maps show that delta formation before the 1950s was minimal indicating that the sediment contributed by the rivers to the lake was small. However, during the last 58 years, the delta has expanded continuously. When considering the same lake level (2.68 m) from 1984 to 2014, the delta has expanded an average of 5 ha annually and, by considering different lake levels and corresponding delta areas, the delta increased in height an average of 3 cm annually. While the growth of the delta was approximately linear, the sediment concentration in the river doubled in the last 30 years, indicating more efficient sediment trapping in the floodplain as a result of higher lake levels, rising river beds, and farmers intervening with the course of the river near the shore. Unless effective river restoration and catchment based treatment measures are put in place, the capacity of the rivers will further reduce and aggravate the flooding of the floodplain, causing more sediment deposition in the river channel and on the floodplain.
... Dominant soil types identified in the catchment are Luvisols, Fluvisols, Leptosols and Vertisols. Eutric Vertisols and Eutric Fluvisols are common in the downstream plains of the catchment (Henricksen et al., 1984; Ayenew, 2008; Colot, 2012; Miserez, 2013). High runoff response (SMEC, 2008; Mekete et al., 2014) and sediment load (Wondie et al., 2007; Awulachew et al., 2008) of Gumara River occur due to strong degradation of the catchment (Tebebu et al., 2010; Poppe et al., 2013; Steenhuis et al., 2014; Tilahun et al., 2014). ...
In response to anthropogenic disturbances, alluvial rivers adjust their geometry. The alluvial river channels in the upper Blue Nile basin have been disturbed by human-induced factors since a longtime. This paper examines channel adjustment along a 38-km stretch of the Gumara River which drains towards Lake Tana and then to the Blue Nile. Over a 50 years period, agriculture developed rapidly in the catchment and flooding of the alluvial plain has become more frequent in recent times. The objectives of this study were to document the changes in channel planform and cross-section of the Gumara River and to investigate whether the changes could have contributed to the frequent flooding or vice versa. Two sets of aerial photographs (1957 and 1980) were scanned, and then orthorectified. Recent channel planform information was extracted from SPOT images of 2006 and Google Earth. Channel planform and bed morphology (vertical changes) were determined for these nearly 50 years period. The vertical changes were determined based on aggradation along a permanent structure, historic information on river cross-sections at a hydrological gauging station, and field observations. The results indicate that the lower reach of Gumara near its mouth has undergone major planform changes. A delta with approx. 1.12 km 2 of emerged land was created between 1957 and 1980 and an additional 1 km 2 of land has been added between 1980 and 2006. The sinuosity of the river changed only slightly: negatively (-1.1% i.e. meandering decreased) for the period from 1957 to 1980 and positively (+3.0%) for the period 1980-2006. Comparison of cross-sections at the hydrological gauging station showed that the deepest point in the river bed aggraded by 2.91 m for the period 1963-2009. The importance of sediment deposition in the stream and on its banks is related to land degradation in the upper catchment, and to artificial rising of Lake Tana level that creates a backwater effect and sediment deposition in Gumara.
High-altitude forests are very important for local livelihood in the vulnerable environment of the densely populated tropical highlands. Humans need the ecosystem services of the forest and directly impact the forest through livestock herding, fire, and wood harvesting. Nevertheless , temperature-sensitive tree lines in the tropics are scarcely investigated in comparison with higher northern latitudes. In this study, the Erica arborea L. tree line is studied in a tropical mountain in the North Ethiopian highlands: Lib Amba of the Abune Yosef Mountain range (12°04′N, 39°22′E, 3993 m asl). The present tree line and forest cover was recorded by high-resolution satellite imagery from Google Maps and field data (2010–2013), while historical forest cover was studied from aerial photographs (1965–1982) and repeat photography (1917–2013). The aerial and satellite images were orthorectified and classified in forest/non-forest binary maps. The binary forest layers were used to detect forest-cover change and tree line dynamics by image differencing between the three time layers (1965–1982–2010). These maps and a terrestrial photograph indicate two periods of deforestation (1917–1965 and 1982–2013), whereas the forest cover was stable between 1965 and 1982. Deforestation was especially severe (with 63%) between 1982 and 2010, associated with a population increase from 77 to 153 inhabitants per square km. There is significant evidence that the elevation of the E. arborea L. tree line increased from 7 to 15 vertical meters between 1965 and 2010, in an area with decreasing anthropozoogenic pressure.
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