The Influence of the Gebel Aulyia Dam on the Development of Nile Plankton

Abstract

Although the hydrology of the Nile has been thoroughly investigated by the Egyptian and the Sudan Irrigation Departments, the only detailed biological work published to date is on the algal flora of the lower reaches (Abdin 1946, 1947, 1948, 1949). During the past few years investigations on the middle and upper reaches have, however, been started by biologists from the University College, Khartoum. The hydrological and, apparently, also the biological conditions of the river have been altered profoundly by man, and further considerable changes are yet to be made. The dams at Sennar (Blue Nile), Gebel Aulyia (White Nile) and Aswan (Lower Nile), in addition to the natural barriers, the cataracts and swamps, divide the river into regions, distinctive not only in their utilitarian and geographical aspects, but also showing specific biological features. The distances of 5611 km. from the Delta to Lake Victoria on the White Nile and the 4590 km. to Lake Tana at the head of the Blue Nile, render a biological survey of the river an immense task, and thus investigations must be carried out in sectors defined by natural conditions and transport limitations. One such sector is on the White Nile south of Khartoum (Fig. 1), where the Gebel Aulyia Dam converts the river into a shallow, slow-flowing 'lake', which in places is three miles broad. The dam, built in 1937 44 km. south of Khartoum, provides an irrigation reservoir for Egypt. Each July the dam is closed and the reservoir begins to store water; when full, normally by September, it holds 3-5 x 109 cu.m. As a result, a rise in river level is recorded more than 500 km. upstream of the dam. Not until February is the dam fully opened to allow for the decreasing flow in the Blue Nile, and the water in the reservoir shrinks to normal river level by May. Strong southerly or northerly winds, according to the season, frequently produce impressive waves on this vast, open, temporary 'lake'. The present paper examines the effects of the storage of this large volume of water on plankton production in part of the river upstream of the dam. A series of samples were taken from a place near Khartoum to beyond Gebelein, a distance of 396 km., samples and data being collected at eleven stations, separated by distances varying from 20 to 70 km. (Fig. 1). The river journey was completed between 13 and 18 December 1951, thus ensuring a reasonable comparability of samples in time, an important consideration in flowing water.