Speight. — Terrace-development of Canterbury Rivers. 35 



broad terraces to be seen in the lower reaches of the Avon and 

 Heathcote Rivers. 



The elevation of the land is always considered a most important 

 point in causing terrace-development, but this is chiefly in those 

 cases where rivers have been near their base-level. Subsequent 

 elevation causes them to form terraces owing to the restoration 

 to them of their power of corrosion. This is the case of the 

 Avon and Heathcote terraces just mentioned. 



Now, the Canterbury rivers have a remarkably steep grade, 

 and a depression of the land would hardly be felt in their upper 

 and middle portions. I think it very probable that if the land 

 were lowered till the shore-line corresponded with the main 

 line oi railway, the erosive power of the streams near the gorges 

 would be only very slightly altered. Further, if terracing were 

 due to elevation it should be progressive upstream from the 

 coast, whereas the contrary is the case : the terraces are highest 

 in their upper portions. 



I do not think that change in the height of the land has 

 materially affected the erosive power of these rivers. Near the 

 sea-coast it has undoubtedly exerted some influence, and the 

 raising of the bed of the Waimakariri near Belfast is most pro- 

 bably due to continued depression of the land. 



The Volume and Load. 



Other causes must therefore be sought to explain the river 

 terraces. If we consider change in volume, we are forced to 

 conclude that our rivers have shrunk in volume from what they 

 were in the glacier epoch. If our mountains were higher, they 

 would intercept more snow, and the average volume of the rivers 

 would be greater. The largest rivers of Canterbury, such as 

 the Waitaki and Eakaia, drain the highest portions of the Alps ; 

 further, the Rangitata, with a comparatively small drainage- 

 basin, is nearly as large a river as the Waimakariri with a large 

 drainage-basin, because the small area supplying the Rangitata 

 is an area of high mountains, where the glaciers are larger. 

 Our rivers are therefore smaller than they were, and they would 

 not be likely, therefore, to be able to terrace their beds were 

 this not accompanied by a marked diminution in the load. 



It is important to notice here the different grade of the 

 plains — that is, of the old glacier rivers as compared with the 

 grade of the present rivers. They are all, without exception, 

 running on a gentler gradient now than formerly. If we except 

 the hypothesis of elevation along an axis through the outer 

 range of mountains, we are forced to conclude that the last 

 important cause — viz., the load of the river — is the predominating 

 factor in determining whether the rivers could terrace the plains 



