16 HORN EXPEDITION — PHYSICAL GEOGRAPHY. 



When the river has reached this stage in its desiccation, the isolated and 

 often widely-separated waterholes and rockholes become economically important 

 features. The principal waterholes occurring in the beds of the rivers and creeks 

 may be described under the three following headings : — 



1. Rockholes confined to gaps and gorges. 



2. Waterholes on the upper sides of bars of rock. 



3. Waterholes not associated with rocky bars, but occurring in the rivers 



where the bed is impervious. 



1. Rockholes confined to Gaps and Gorges. — These occur in the many gaps 

 and gorges through which the rivers and creeks have forced their way in their 

 southerly course from one longitudinal valley to the next. In the quartzite ridge, 

 for instance, forming the southern boundary of the south McDonnell Ranges 

 proper, there are many gaps, the beds of the rivers in these being pai'tly occupied 

 by pools of usually excellent water. Such waterholes occur in the Redbank, 

 Finke and Ellery Creek Gorges, and in Simpson and Emily Gaps. 



The great strength of the curi'ent of the water flowing through these gaps, 

 due to the fact of the streams l)eing confined within narrow bounds, sweeps all 

 detritus out of them and erodes the river bed to a greater extent here than else- 

 where, thus often producing deep rockpools which retain large quantities of water. 

 Many of these rockholes, owing to their sheltered positions, are practically 

 permanent. 



2. ]Vaterholes on the Upper Sides oj Bars of Rock. — The second class of water- 

 holes are tliose situated on tlie upper side of some rocky bars. If the flow of 

 water over the portions of the river channels in which these occur is strong, then 

 the eddy in front of the bar causes the removal of sand and gravel from this side, 

 and leaves, as the water subsides, a depression filled with water. Unfortunately, 

 however, at a later period a light rain often causes a flow of water just strong 

 enough to carry sand, etc., into this hollow and thus obliterate it. For this and 

 other reasons one cannot always depend upon getting water on the surface at these 

 places, even though one may have seen on a former occasion a fine pool of water at 

 the same place. A very good example of this class of waterholes occurs in the 

 Finke near Henbury Cattle Station. 



3. Waterholes 7wt Associated with Rocky Bars. — The third class of waterholes 

 owe their existence to their being situated over the portions of the river beds where 

 fine silt or mud takes the place of poiuus sand and gra\'el, and thus pi'events the 



