392 PROCEEDINGS OF SECTION E. 



the river diminishes and the alluvial deposits increase in extent 

 till at last the diminished moving force of the currents is unequal 

 to the task of carrying the silt. Then commences a period during 

 which the channel becomes choked up, at and near the place where 

 the velocity receives its first important check — that is, as a rule, 

 at and near the beginning of the delta. During this period the 

 river during floods spreads over the delta, or flows through it in a 

 series of small channels ; but generally either it forms again at 

 the end of the delta, or the channels into which had divided 

 become deeper and better defined. Among the western rivers of 

 New South Wales, instances of almost every stage of this process 

 are to be found. The Darling and the Murrumbidgee maintain 

 well defined channels down to their confluence with the Murray, 

 but on examination of these channels I find that as they approach 

 the Murray their sectional areas diminish. Thus the sectional 

 area of the Darling at Bourke is considerably greater than that of 

 the same river a thousand miles lower down near Wentworth, 

 and the channel of the Murrumbidgee diminishes in section from 

 Wagga to Hay, from Hay to Balranald, and from Balranald to 

 the junction with theMurray. (PI. XXXI.). 



The nature of the relief which these rivers obtain in times of 

 flood affords ample evidence that in the conflicts between the force 

 of the currents and the resistance of the alluvium, the advantage 

 remains with the latter. High floods occur only at long inter- 

 vals ; and those intervals allow the silt to consolidate and vege- 

 tation to spring up. When a flood comes it finds the channel 

 contracted and the resistence to erosion considerable. Hence we 

 find in the case of the Darling that the current through inability 

 to erode its own channel to a sufficient carrying capacity, finds 

 outlets through which the flood water flows into lakes and other 

 natural depressions, as well as filling lagoons and creeks, and, in 

 some instances, overflowing extensive areas of land. In this way 

 during floods. Lakes Poopelloe and Woychugga, near Wilcannia, 

 are filled, as are also the series of lakes and depressions on the east 

 side of the Darling along the courses of the Upper and Lower 

 Tally walkas ; while, in addition, temporary storage on an immense 

 scale is provided by the lakes on the west side of the river near 

 Menindie. The Cato and Tarrion Creeks represent important 

 overflows from the Darling, or Barwon as it is there termed ; but 

 in these instances the overflow is due chiefly, if not entirely, to 

 the rocky bar across the river at Brewarrina. 



Just as the channel of the Darling proves inadequate to the 

 discharge of the flood water, so in the case of the Murrumbidgee. 

 After the latter river passes Wagga Wagga, its valley widens and 

 the extent of the alluvial deposit increases ; while the river spreads 

 in times of flood in a corresponding degree, and tills numerous 

 lagoons and ana branches. Beyond Narrandera, the source of 

 the Murrumbidgee no longer represents the bottom of a valley 



