36 Transactions. 



or not. The volume in all probability is now less, the grade 

 of the rivers is less, and yet terraces are formed on a tre- 

 mendous scale. 



Part III. 



The existence of enormous supplies of waste in the valley 

 of a river profoundly influences its action. The energy of a 

 stream is limited, and its excess is chiefly spent on transportation 

 and corrasion. It will therefore be evident that terrace-forming 

 must be connected in some wav with the load a stream carries. 

 If the load is excessive, there will be no energy left for lowering 

 its bed, and hence for forming permanent terraces. Manv of 

 the laws governing streams may be studied by examining the 

 miniature fans and deltas formed at the roadside or in other 

 places after heavy rain. The following order of events is ap- 

 parently true for a miniature fan as for our large rivers : — 



During flood-time the stream is fully loaded with waste from 

 the surrounding country, but drops it on the gently sloping 

 ground, thus raising its bed. Terraces are absent. When the 

 height of the flood is past, the supply of waste falls off — only 

 smaller particles are carried ; and there is an excess of energy 

 left over for corrasion, and the fan is terraced, on a small scale 

 it is true, but the processes and the sequence of events are just 

 the same as on a large scale. If this is so, the degradation of 

 its bed by a river which is fully loaded in flood-time will occur 

 principally as the flood is falling, and will continue till the river 

 is running clear again and carrying no sediment. I have re- 

 peatedly noticed this order of events on shingle fans, and I have 

 received confirmation of these facts from engineers whose busi- 

 ness it is to supervise the fords across the streams on the Christ- 

 church-Hokitika Road. It must be remembered that our rivers 

 when in flood are undoubtedly highly charged with waste, and 

 therefore differ greatly from the condition of ordinary streams 

 when in flood. These may be discoloured by fine particles. 

 and may even move stones along; but the supply of waste on 

 the Canterbury mountains is exceptional in amount, therefore 

 our rivers in flood-time are comparable to the excessively charged 

 streams of a small fan, and the sequence of events is apparently 

 the same, although the conditions are somewhat different. 



I think it can be proved that when the volume of a stream 

 diminishes, the transporting power falls off in a slightly greater 

 ratio than the energy. The result of this will be that, when a 

 stream is fully Loaded,. on a diminution in volume there will be 

 an excess of energy left over for corrasion, and the stream will 

 therefore channel its bed. The explanation of this phenomenon 

 may be due to the fact that with a falling volume the larger 



