AND OTHER WATER WAVES 223 



ished, and the increasing width of the river 

 further diminishes the amplitude of the wave. 

 Thus the very perfection of the mechanism of 

 wave-transmission down-stream in rivers usually 

 prevents the accumulation of a wave-face suffi- 

 ciently steep to be visible. The rise of a river 

 before the arrival of the actual flood -water which 

 originates the rise is, moreover, less obviously 

 wavelike in character than the rise of level in a 

 river due to the tide, for in the former the rise is 

 accompanied by increased velocity, just as it would 

 be if the river were running at a steady, but higher, 

 level. In the case of the first rise of a river due 

 to tide, however, there is a slackening of the 

 current. 



We see, therefore, that causes commonly co- 

 operate to mask the progressive waves due to floods 

 in non -tidal rivers. 



In the upper reaches of certain rivers, however, 

 the sudden arrival of a wall of water, travelling 

 down -channel at a great speed, is a normal occur- 

 rence after heavy rain in the hills where the river 

 and its tributaries take their rise. Thus, on the 

 River Tees, during one summer and autumn the 

 roll-wave, as it is called, was seen on no less than 

 six occasions by Mr. F. R. Glyn, E.R.G.S. The 

 following description of the phenomenon is taken 



