RAINFALL AND DISCHARGE OF THE MURRAY RIVER. LIX. 
far distant from the site of the proposed reservoir at 
Barren Jack Mountain, where it is intended to store the 
flood water in times of maximum rainfall, and send it 
down the river channel during periods of low rainfall, both 
for the purpose of irrigation and navigation. Appendix 
“D” (Plate, 22) shows the catchment area of the 
Murrumbidgee River at Gundagai, also lines of equal 
rainfall within the catchment. 
Drainage Area of the Murrumbidgee River at Gundagai. 
—The area of the gathering grounds has been ascertained 
from reliable maps to be about 8,300 square miles, and 
its topographical features, geological formation, and 
forestation are such that a large percentage of run-off can 
be depended upon. It consists of granite, silurian slates 
and quartzites for the most part, with here and there 
patches of basalt and other igneous rocks, also isolated 
belts of limestone. The most effective portion of the 
catchment is on the mountain ranges to the south, which 
are snow-clad in winter, and rise to an altitude of 6,000 
feet above sea level. Going north-westerly, the altitude 
decreases, and the character of the surface becomes 
less rugged and broken, finally merging into gently undu- 
lating plains and river flats. Taking the catchment 
generally, it is of a highly impervious character, with the 
exception of a few tertiary drifts which do not cover any 
considerable area. 
Mean rainfall computations.—The catchment has been 
divided by lines termed isoyets of mean rainfall, the areas 
bounded by each pair of isoyets being ascertained by plani- 
meter and reference to available maps. Taking the arith- 
metical mean of all the rainfall records at the stations 
within the limits of each sub-area, and multiplying the 
resulting mean by the area, a series of products is obtained, 
which, divided by the area of the whole catchment, repre- 
