402 ME. A. CAESON ON THE EISE AND FALL OF LAKE TANGANYIKA. 



observed in other bays on the lake ; the embankments are all largely 

 composed of shingle and are of exceedingly compact formation. 



Diagram-section of embankment near southern end of 

 Lake Tanganyika. 



Now Cameron describes the entrance of the Lukuga thus : 

 "Shortly before noon I arrived at its entrance, more than a mile 

 across, but closed by a grass-grown sandbank, with the exceptiou of 

 a channel 300 or 400 yards wide. Across this there is a sill where 

 the surf breaks heavily at times, although there is more than a 

 fathom of water at its most shallow part." ^ 



Here we have a bank similar to that described at the southern end 

 of the lake, but with a break 300 or 400 yards wide and six feet deep 

 in it, and the top of the bank was probably uncovered in the dry 

 season, when grass would grow on it, for each year towards the end 

 of the rains the lake irises about 2 feet and again gradually falls. 

 Behind this bank we have precisely the conditions necessary for that 

 dense growth of aquatic vegetation which blocked the Lukuga. 

 To appreciate how the river at first became blocked we must re- 

 member that in its normal condition Lake Tanganyika has a 

 considerate Quantity of water to discharge after the rains, but very 

 little (perhaps none) towards the end of the dry season, and the 

 first two months of the half-year rainy season have no appreciable 

 eff'ect in raising its level. The effect of this is that when the lake 

 has regained its normal level the basin of the Lukuga will for several 

 months of the year be only mud pools and be rapidly covered with 

 dense vegetation, while the first rains will only have the effect of 

 assisting to silt up the bed. There will thus be a continual struggle 

 between the choked bed and the waters of the lake swollen after the 

 rains, and for years, perhaps ages, the lake scours in its flood the 

 channel as often partially choked by vegetation. It is therefore easy 

 to understand how in these circumstances — with the bank continually 

 forming across the bay, the dense growth of aquatic vegetation within 

 the bank, and the large quantity of floating Aegetation from all the 

 rivers that enter the lake and which ultimately finds its way to the 

 Lukuga, where it is deposited at the entrance and is jammed into 

 the neck of the outlet — this neck becomes ultimately plugged 

 with a porous stopper as described by Commander Cameron. It 

 may be that these embankments play even a more important part in 

 the blocking up of the Lukuga; but we have at least in Lake 



1 Op. cit. p. 227. 



! 



