BY E. C. ANDREWS. 505 



channel bases almost completely to sea-level; nevertheless as its 

 own channel grade is the result of mighty floods, the normal 

 stream of even such a river as the Amazon merely purls among 

 its flood boulders. 



If by experiment the grade of a certain stream channel enter- 

 ing this transitional stage should be ascertained, then the grades 

 of streams of equal volume, but of varying velocities, at this 

 transition stage could be very simply calculated, since power of 

 transportation varies as the sixth power of the velocity. 



, In this connection we may consider the flood grades of several 

 stream types, two characteristically developed in Australasia, 

 and two being common facts of experience the world over : — 

 (1) The ordinary roadside gutter flood-stream. (2) The storm- 

 wave. (3) The flood-glaciers of New Zealand. (4) The New South 

 Wales streams. 



(1) Ordinary roadside streams. 



Miniature examples of eroded lake basins, etc., occur along all 

 disused roads where the wheelruts have determined new stream 

 courses, and the channel occupies the whole valley. Here, after 

 a heavy storm, at the bases of miniature steeps or marked rut 

 convergences, basins will be found with reversed grades lower 

 down stream, flat-bottomed valleys exist having precipitous and 

 straight-bordering walls, and plunging waterfalls, with deep holes 

 at their bases, are common. In areas of weakness, also, such as 

 heaps of mine " slimes," the tiny canons thus formed are all 

 quickly carried, approximately, to base-level* before lateral corra- 

 sion causes noticeable widening of such canons. If now we 

 consider the base only of an ordinary river valley it will be found 

 that both it and the roadside gutter have much in common. 

 The river flood channel has its straight-bordering banks, its 

 rapids, and its depressions below temporary base-level. In the 

 one case the river occupies but a tiny fraction of its valley, while 

 in the other case the stream, in the earlier stages at least, occupies 

 the whole of its valley. This fact throws a flood of light upon 



* i.e., the level of the "bed-rock" supporting the "slimes." 

 36 



