May, 192/ 
The Queensland Naturalist. 
25 
so 1 on as the gradient lessens on the stream, cutting down 
its bed. 
Now, the greater the volume of water the stronger the 
current, and the greater power of transporting rock 
debris to grind away the bottom. Also the steeper the 
grade the more rapid the current, and again the greater 
transporting and erosive power. 
A river thus cuts down its bed most rapidly where a 
combination of grade and volume are at a maximum, and 
it cuts it down until a grade is attained, which just 
permits it to carry away its load of debris. If owing to 
altered conditions the gradient or volume is insufficient 
to provide the necessary current, the debris will be 
deposited, and the river build up its bed. 
We thus have a kind of natural balance, which re- 
sults in a small stream with less volume of water, require 
ing a steeper grade to transport its load of sediment 
than is required by the larger stream, which, moreover, 
being as a rule further from its source, usually has a finer 
sediment to carry. 
A stream thus tends to acquire a peculiar gradient, 
steepest near its head and flattest near the sea, but 
always in process of getting flatter and flatter as the 
general level of the land is reduced. 
The gradient of a stream may thus be some evidence 
of its youth or its old age, or of a rejuvenation owing to 
some alterations in levels by earth movements. 
While the stream is cutting down its bed, the sides 
of the valley it is forming are gradually washed down so 
that the gorge becomes a wider and wider valley, until 
eventually all tire higher ground is washed away, and 
the general level is very little above the sea. In this way 
the narrowness of the valleys may also be some indication 
of the youthfulness or maturity of a stream. But it 
must not be forgotten that the steep grade or narrow 
valley depend originally on the elevation of the land, and 
that a land of low relief near sea level may never 
have been sufficiently elevated to acquire these ^youth- 
ful” characters, namely, steep grade and narrow valleys, 
although the land may really be quite recent. 
The above brief story of the life and work of a 
stream gives us a picture of the process going on in every 
water course, a picture to always bear in mind. Were it 
the whole truth, the matter would be comparatively 
simple, but unfortunately it is not. It is very improbable 
that any block of land was ever so raised, suddenly, and 
