liiGGiNSON. — On Floods in Lake Districts. 181 



While watching the water rising on the morning of the 18th, I 

 observed that a mark which I had placed on the shore became rax^idly 

 covered and shortly afterwards exposed again, the surface of the lake being 

 at the time perfectly calm, not a ripple ruffling it. 



As I found it difficult to account for this, I made more careful observa- 

 tions for upwards of an hour, the result of which was that I ascertauied 

 there was a rise and fall of three inches in the level of the water at intervals 

 of five minutes, which was maintained with perfect regularity. This rise 

 and fall was extremely gradual, so that it was quite imperceptible on the 

 smooth surface. After 1 p.m. a breeze sprang up, preventing a fiurther 

 continuance of my observations. I was unable at the time to account for 

 this disturbance satisfactorily, but was inclined to attribute it to a 

 shght earthquake, possibly too faint to be noticed. 



In conversation with Mr. Worthington, the Meteorological Observer, the 

 next morning, I mentioned what I had remarked. He informed me that he 

 had himself noticed the same rise and fall on a larger scale, after one of the 

 heaviest floods experienced ; consequently it at once pointed to the floods 

 being in some manner the cause, though at first sight it seemed impossible 

 for any flood to have such an effect on a body of water nearly fifty miles 

 in length and of great depth. 



Having repeatedly thought over the matter without being able to 

 account for it in a satisfactory manner, I put together a few facts relating 

 to the natm-al features of the lake with reference to this flood, in order to 

 assist me in coming to some conclusion, which I therefore beg to suggest to 

 you. I have also tabulated the effect of this flood in conjunction with some 

 that have occurred in the European Alps, in districts bearing similar 

 features, the information being taken from Beardmore's " Manual of 

 Hydrology." 



Lake Wakatipu is nearly fifty mUes in length, and varies between one- 

 and-a-half and three-and-a-half mUes in breadth, its area scaled from the 

 map being 113 square miles. Its drainage area is about 1,200 square miles, 

 principally at the northern extremity, where the water-shed is the main 

 range of Southern Alps drained by the rivers Dart and Eees. These two 

 rivers flow into the extreme northern apex of the lake, having a drainage 

 area of 400 square miles or a third of the whole commanded by the lake, 

 including the portion covered with perpetual snow and glaciers. 



The rise of two feet in the level of the lake means 1,823 cubic feet of water 

 per minute per square mile impounded. At the same time the river Kawarau 

 at the outlet was discharging 500 cubic feet per minute per square mile 

 based on the rainfall for the first sixteen days in November. This makes a 

 total of 2,323 cubic feet per minute per square mile run off' the drainage area 



