THE NEW ZEALAND SOUND BASINS 35 



flood waters which at times surmount the stream banks and spread 

 over the surrounding country are for the most part observed to be 

 stagnant and to form "back-water" masses, in which some aggrada- 

 tion goes on. If some previous channel or natural valley is found 

 by this trespassing mass, a considerable amount of corrosion may 

 occur even at these higher points. This is, of course, in the broads 

 of the stream. In the narrows there is little chance for back water, 

 except in sheltered corners opposite the cutting curve. 



Fig. 10. — Lower Milford Sound. Note the immense double slope on the left. 



All this is, of course, quite natural. However high the flood, 

 however much it overflows its banks, we expect the maximum strength 

 to occur at or near the central channel portion, while away from the 

 drainage channels we expect aggradation and even stagnation. 



With the recession of the mighty flood, we do not expect a continua- 

 tion of basin excavation below baselevel. On the contrary, we look 

 for aggradation at these points until the normal stream-channel 

 slope has been restored. Nor do we expect the drought-stricken 

 stream to rouse to action the huge loads of flood bowlders littering 

 its channel, and with them to tear away the sides and bottom, even 



