THE VICTORIAN NATURALIST. 101 



AN EXAMPLE OF STREAM CAPTURE NEAR 

 MELBOURNE. 



(With map.) 

 By E. O. Thiele. 

 {Read before the Field Naturalists^ Club of Victoria, 9th Jtdy, 1906.) 

 The area under consideration in this paper is famiHar to most 

 members of this Club, as it embraces chiefly part of the Ringwood 

 district, so well known to our botanical collectors. My desire is 

 to direct attention to an interesting geographical feature of stream 

 development, which is well illustrated in that neighbourhood, and 

 is therefore within easy reach of town. 



Stream action is recognized as one of the important agents 

 which contribute to the determining of the varied surface features 

 of a country. The factors, however, whose joint action produces 

 some particular type of scenery are numerous and varied, and the 

 problem is generally a complex one. 



No attempt is here made to trace out how the combined action 

 of various natural agencies finds expression in the landscape and 

 natural history of the area. One geographical feature only is 

 dealt with here, and this refers to slow readjustments which are 

 to be observed in the natural drainage system of the area con- 

 sidered, and are common to all districts where running water 

 takes its place as an important natural agent of change. 



Rivers and streams, in common with all other natural features 

 on the surface of the earth, are subject to a succession of slow 

 changes, which in time may completely alter or modify the 

 original drainage system of the area to which they belong. A 

 river system may be enlarged in two important ways — either by 

 the headward erosion of the main stream and its tributaries, or by 

 the addition to its basin of land at its lower course. This latter 

 growth may be the result of delta formation, or it may be due to 

 the uplift of the land in the form of a coastal plain. Similarly a 

 river system may be diminished by the submergence or drowning 

 of its lower course, or by the encroachment of a more active con- 

 tinuous drainage area in the upper part of its basin. 



The area drained by a river is therefore not a fixed quantity, 

 but that it is subject to a slow and constant change is well 

 shown when its past history is examined. It is found that the 

 water parting of two adjoining river basins is not an immovable 

 line, but it is subject to successive shifts in a direction controlled 

 by the relative activity of the streams on either side. Such 

 encroachments are generally known as river captures. The study 

 of stream development endeavours to trace back the life-history of 

 a river system to its early youth, and note the successive vicis- 

 situdes through which it has passed. Very recently Messrs. 

 Kitson and Baragwanath gave an interesting account of changes 



