v® 
separated at rare intervals by short lengths of the channel, where water may 
be seen flowing gently over a rocky bed. In the apparently waterless stretches, 
however, water may be obtained by sinking to depths varying with the nature 
of the bottom and the lapse of time since the last heavy fall of rain. The 
appearance of surface water at rare intervals in the bed of the river is due to 
the presence of ‘‘ bars’’ of rock, which cross the channel at these places. 
As the water in its downward progress cannot filter through these rocks as it 
does through the sand and gravel at other parts, it has to rise to the surface to 
pass over them. The change from the normal state of the channels to their 
next condition is a more gradual one. As the supply of water becomes less and 
less, the amount and rate of flow over the rocky bars gradually diminish, until 
finally the supply becomes too small to cause the water to rise over them at all. 
When the river has reached this stage in its desiccation, the isolated and often 
widely-separated waterholes and rockholes become ecoromically 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 tv Gaps aya 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 Macdonnell 
Ranges proper, there are many gaps, the beds of the rivers in these being partly 
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 current of the water flowing through these gaps, 
due to the fact of the streams being confined within nurrow bounds, sweeps ali 
detritus out of them and erodes the river bed to a greater extent here than 
elsewhere, thus often producing deep rockpools which retain large quantities of 
water. Many of these rockholes, owing to their sheltered positions, are practi- 
cally permanent. 
2. Waterholes on the Upper Sides of Bars of Rock.—The second class of 
waterholes are those situated on the 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, &c., 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 not 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 porous sand and gravel, and thus 
prevents the percolation of the water below the surface. When the floodwaters 
