THE RISE ATfD FALL OF LAKE TANGANYIKA. 401 



26. The EisE and Pall o/Lake Tanganyika. By Alexander Caeson, 

 Esq., B.Sc. (Communicated by E. Kidston, Esq., E.R.S.E., 

 F.G.S. Bead April 27th, 1892.) 



[Abridged.] 



After alluding to certain observations of Cameron, Livingstone, 

 and Stanley on the changes of level of Lake Tanganyika, the writer 

 states that the most interesting point in connexion with the rise 

 and fall of the lake is the question : — How is it possible for a great 

 lake to rise 30 feet above its normal level by the blocking up of 

 its outlet? 



This is to a large extent accounted for by Cameron. He says : 

 " On going down the river [Lukuga] we found that it was blocked 

 by vegetation similar to that on which we had crossed the Sindi, and 

 also to that over which we had had to pass to reach the shore from 

 our boats at the southern end of Lake Tanganyika, and which I after- 

 wards found existed on Lakes Kassali and Mohrya. The presence of 

 this mass of vegetation is easily accounted for. Every day that there 

 is a gale of wind, and the consequent sea on the lake, blocks of this 

 peculiar growth are detached, and such as survive the passage to the 

 outlet getting jammed together commence to grow, and form a sort 

 of porous stopper in the neck of the outlet." ^ He then quotes Sir 

 Samuel Baker's experience on the Kile when the whole channel "was 

 blocked up and the expedition to Gondokoro impeded for a year, the 

 country above the stoppage having been converted into a series of 

 swamps and shallow lakes, while Egypt was suffering from all the 

 evils of a low Nile." 



Only those who have seen this vegetation will be likely to give 

 due weight to Commander Cameron's argument. The writer has 

 seen the channel of the Shire blocked by the same vegetation. 

 But in order fully to explain the blocking up of the Lukuga it is 

 necessary to take into account another phenomenon observable 

 on Lake Tanganyika. At the southern end of the lake there is a 

 series of promontories formed of uptilted strata and between them 

 deep bays with shallow water, while the valleys between are only 

 a few feet above the present level of the lake. Across one of these 

 valleys, close to the water, the lake since its recent fall has left an 

 embankment about 20 feet high, completely closing up the valley : 

 so much so that when the missionaries occupied the hill overlooking 

 it and the people made gardens in the valley (which had not before 

 been cultivated), the first heavy rains made a lake of it 4 feet deep at 

 the lower end and a channel had to be cat through the embankment 

 to carry off the water. The sketch on the following page shows the 

 form of this embankment, which is probably 300 feet long and about 

 20 feet above the level of the valley. The same phenomenon has been 



1 'Across Africa' (1885 ed.), p. 555. 



