GEOMETRY OF THE GORGE OF THE NILE, ETHIOPIA FROM SRTM DATA

The Blue Nile in the Ethiopia highlands makes a near-circular loop (referred to as the Blue Nile Bend) by flowing SE as it emerges from Lake Tana, then S and SW before assuming a NW-flowing direction towards the lowlands of Sudan. The river carves a 1.5 km deep canyon known as the Gorge of the Nile and exposes Tertiary-Recent volcanic rocks, Mesozoic sedimentary successions that constitute upper sandstone, upper limestone, lower limestone, lower sandstone, and Precambrian basement. Recent studies suggest that the Blue Nile Bend was formed as a result of uplift in the western flank of the Main Ethiopia Rift that forced the Blue Nile to change its course from SE-flowing to SW and NW. We used the Shuttle Radar Topography Mission (SRTM) data with ~90 m X-Y resolution and ~10 m Z resolution to characterize the geometry of the Gorge of the Nile and understand lithological controls on its geomorphological evolution. Fifty, 90 to 180 km long topographic profiles across the Gorge of the Nile that are spaced at 5 to 10 km were extracted from the SRTM data. Our study showed that: (1) The width of the gorge generally increases from 5 km close to Lake Tana where it drains through Tertiary-Recent volcanic rocks to ~70 km as it approaches the lowlands of the Sudan and exposed Precambrian basement. However, the gorge narrows slightly as it encounters the upper to lower limestone units. (2) The depth of the gorge generally increases downstream from 600 m (on the Tertiary-Recent volcanic rocks) to ~1500 m (on the Precambrian basement). (3) The depth to width ratio decrease as a function of distance from the source except in the upper limestone unit, where a slight increase is reported. (4) The Gorge of the Nile is generally symmetrical except when the Blue Nile changes its flow direction from SE to S, S to SW and SW to NW. (5) The Blue Nile has the greatest gradient (3.5 m/km) in its SE-flowing segment where it carves its way through Tertiary-Recent volcanic rocks. In its S-, SW- and NW-flowing segments (where it runs through Mesozoic sediments and Precambrian basement) the river maintains a gradient of 1.5 m/km. However, the river’s gradient drops to below 1 m/km when it encounters the upper limestone unit. We conclude that, although geometrical characteristics are dominantly related to distance from source, lithology might have played a role in shaping up the Gorge of the Nile.