1915-] Fauna of the Chilka Lake : Introduction. 11 



force and for protracted periods. We are informed that on occasions, when its 

 violence is extreme, the greater part of Rambha Bay is entirely emptied of water 

 and in February, 1914, the effect of even a moderate breeze was brought home to 

 us by a sudden lowering of the water-level so great that the ' ' Lady of Chilka ' ' 

 grounded at her moorings. Sudden changes of the kind must result in the water being 

 banked up towards the northern end and must produce a considerable admixture of 

 volumes hitherto distinct. Observations made at Barkuda Id. in February, before 

 and after several days' strong breeze, showed a definite rise in density, the salter 

 water having doubtless been brought from the southern end by the wind. 



Tides have of course no effect during the flood season, as at this period the outer 

 channel is filled with fresh water to a level some feet above that of the sea. Even 

 when the lake was at its lowest we were unable to observe any regular ebb and flow 

 in the main area. The influence of wind, indeed, seemed to us sufficient to account 

 for any diurnal changes in level that were actually noted. Any effects that the tides 

 may have had were doubtless masked by this agency, while we made no attempt to 

 investigate less obvious movements. 



In the outer channel tide had of course a slight effect at this season ; but the rise 

 and fall, owing to the narrowness of the sea-mouth, was probably much smaller 

 than in the Bay of Bengal immediately outside. It is clear, nevertheless, that the 

 tides, assisted probably by changes in the wind, must have a much greater effect on 

 the isohalines than is indicated by diurnal changes in water-level, for to this agency 

 in a large measure must be assigned the influx of salt water at the time when the 

 autumn floods have subsided. 



In a lagoon of the size and shallowness of the Chilka Lake evaporation must, 

 especially in a tropical climate, be more than considerable and doubtless plays a 

 great part in the phenomena we have been discussing. We have no means of estima- 

 ting the exact influence of this factor, but it is not unreasonable to suppose that 

 beyond compensating for the comparatively small amount of fresh water that comes 

 from the Mahanaddi system in the dry season, it also plays an important part in 

 inducing an inflow from the sea. 



The great changes in the salinity of the Chilka Lake are due, as has already been 



explained, to the floods of fresh water which enter it 

 Summary statement of annual each ^ the northern end f rom seV eral branches of 



changes in salinity. „ „, , , c , 



the Mahanaddi system ; the annual sequence of events, 



as it concerns the lake as a whole, may be stated briefly as follows : — 



The floods that enter the lake at the close of the monsoon from the Mahanaddi 

 delta expel all salt water from the northern portion, driving it through the outer 

 channel to the sea, and are of sufficient volume to raise the level of the lake some 

 5 or 6 ft. above the mean of the dry season. There being no outlet at the southern 

 end, the comparatively saline water which had accumulated there is banked up by 

 the flood, becoming, however, diluted to a considerable extent both by admixture 

 with water from the north and by surface drainage from the land in the vicinity. 

 Towards the end of the year the floods subside. The first effect of the alteration 



