DRAINAGE DISORGANISATION IN

ARID AUSTRALIA AND

ITS MEASUREMENT

M.E. Sullivan

Thesis submitted for the degree of Master of Science, in the School of Geography, University of New South Wales, May 1976 U;;i\f-fiotT7 li? ;I.S.W. I

03537 -8.mR.77 LIBRARY ABSTRACT

The concept of drainage disorganisation under aridity is analysed in this study, and factors leading to a state of disorganisation are considered. The methods previously used to measure the level of disorganisation of drainage systems are assessed, and three measures are proposed for this study. One measure is based on the effectiveness of the catchment in generating drainage channels; the other two are concerned with the stream network, one measuring connectivity, and the other the degree of complete­ ness of the hypothetical network reconstructed using Horton's Laws.

In the recognition that these measures are based on different aspects of failure of drainage, and tend to emphasise different factors contributing to the disorganisation, they are applied in turn to three extensive catchments from different parts of the Australian arid zone. Differences between these catchments, and between the three measures are considered.

A study in greater detail has been carried out on a small stream system in western New South Wales from which the processes of channel breakdown are inferred. An explanation of the floodout process, and its function in the disorganisation of arid zone drainage has also been carried out.

Factors contributing to the disorganisation of drainage have thus been examined, and their relative importance assessed within different systems in the arid zone. *. * ACKNOWLEDGEMENTS

I would like to thank many people who have inspired and encouraged me in the preparation of this thesis. In particular I wish to thank my two supervisors, Professor Jack Mabbutt and Dr. Fred Bell, for advice and criticism, and for a great deal of time spent on my behalf.

To my family, my thanks for patience and co-operation, and to

Kevin Maynard, many thanks for much advice and practical help in the preparation of maps and diagrams.

I am also grateful to departments of Australian and State

Governments which allowed access to unpublished survey data or records. CONTENTS

Page

ABSTRACT i

ACKNOWLEDGEMENTS ii

CHAPTER 1 INTRODUCTION 1

Drainage Disorganisation and its Causes 1 Drainage Disorganisation and Aridity 2

CHAPTER 2 MEASURES OF DRAINAGE DISORGANISATION 4 Drainage Terminal and the Extent of Interior Drainage 4 The Degree of Connectivity 8 PvopoirfibvA EA^ecAwe, Ca.Ac/Uw-i&vat , 10 The Consideration of Frequency of Flows 12 Measures of Drainage Disorganisation Used in This Study 14

CHAPTER 3 DRAINAGE DIVISIONS IN THE AUSTRALIAN ARID ZONE 22 Attributes of the Three Measures 22 Drainage Status in the Australian Arid Zone 24

CHAPTER 4 THE GEORGINA-DIAMANTINA SYSTEM 30 The Proportion of the Catchment Hydrologically Effective 30 The Degree of Channel Connectivity 37 The Comparison of Actual and Potential Channel Lengths 47 A Comparison of the Status of Drainage Disorganisation as Indicated by the Three Measures 50

CHAPTER 5 THE FINKE-MACUMBA SYSTEM 53 The Proportion of the Catchment Hydrologically Effective 53 The Degree of Channel Connectivity 60 The Comparison of Actual and Potential Channel Lengths 72 Comparison of Levels of Disorganisation as Indicated by the Three Measures 74 Page

CHAPTER 6 FOWLERS CREEK SYSTEM: A STUDY OF CHANNEL BREAKDOWN 76 Fowlers Creek Catchment 77 Changes in the Channel of Fowlers Creek 78 Implications from Fowlers Creek for Drainage Disorganisation 85

CHAPTER 7 DESERT FLOODOUTS 87 The Occurrence and Form of Desert Floodouts 87 Processes Operating to Form Desert Floodouts 89 Mathematical Models Explaining Floodouts 93 Physiographic Controls of the Location of Floodouts 100

CHAPTER 8 THE RIVER LAKES OF THE WESTERN AUSTRALIAN SHIELD: THE EXAMPLE OF THE LAKE WAY-LAKE COWAN SYSTEM 101 The Present Drainage 101 The Proportion of the Catchment Hydrologically Effective 106 The Degree of Channel Connectivity 109 The Comparison of Actual and Potential Channel Lengths 111 The Comparison of the Three Measures of Drainage Disorganisation 113

CHAPTER 9 EFFECTIVENESS OF THE MEASURES OF DRAINAGE DISORGANISATION 116 The Proportion of the Catchment Hydrologically Effective 118 The Degree of Channel Connectivity 121 The Comparison of Actual and Potential Channel Lengths 124 Application of the Three Measures 127

APPENDIX 1 130

BIBLIOGRAPHY 132 LIST OF TABLES Following Number Title Page

2-1 Estimates of areas of interior drainage 5

2-2 Characteristics related to index of aridity 7

4-1 Subcatchment areas and rainfalls, Georgina- Diamantina system 31

4-2 Landsurface proportions, Georgina-Diamantina system 33

4-3 Relief ratios, Georgina-Diamantina system 34

4-4 Actual and potential channel lengths, Georgina-Diamantina system 48

4- 5 Percentage of channel lengths missing, Georgina-Diamantina system 48

5- 1 Topographic and effective catchment areas, Finke-Macumba system 55

5-2 Landsurface proportions, Finke-Macumba system 57

5-3 Percentage of floodout channels, Finke-Macumba system 62

5-4 Upland catchment areas and tributary drainage, Karinga Creek section of Finke-Macumba system 62

5-5 Lengths of unsupported channel, Finke-Macumba system 63

5-6 Estimated bankfull discharges, Finke River 67

5-7 Width/depth ratios and sediment loads, Finke River 68

5-8 Actual and potential channel lengths, Finke- Macumba system 73

5- 9 Percentage of channel lengths missing, Finke-Macumba system 73

6- 1 Channel dimensions and bankfull discharges, Fowlers Creek 79

6- 2 Channel parameters, Fowlers Creek 79

7- 1 Rates of loss of discharge for ephemeral channels 91

8- 1 Topographic and effective catchment areas, Lake Way-Lake Cowan system 106 Number Title Following Page

8-2 Rainfall data, Lake Way-Lake Cowan system 107

8-3 Landsurface proportions, Lake Way-Lake Cowan system 108

8-4 Actual and potential channel lengths, Lake Way-Lake Cowan system 111

8-5 Percentage of channel lengths missing, Lake Way-Lake Cowan system 112

9-1 Application of the three measures of disorganisation to three sample catchments 116 LIST OF FIGURES Following Number Title Page

2-1 Drainage classification of Australia after de Martonne (1927) 8

2-2 Index of aridity for Australia after de Martonne (1927) 8

2-3 Drainage classification of Australia after Hills (1953) 9

2-4 Drainage classification of Australia after Gentilli (1952) 14

2-5 Stages of breakdown of the channel network 17

2- 6 Difference between actual and theoretical mean segment lengths 19

3- 1 Drainage status in the Australian arid zone after Mabbutt (1972) 24

3-2 Climatic classification of arid and semiarid Australia after Meigs (1953) 24

3- 3 Morphostructural divisions in Australia after Mabbutt (1972) 25

4- 1 Drainage network and topographic catchment, Georgina-Diamantina system 30

4-2 Areas of the three subcatchments, Georgina- Diamantina system, which are effectively drained 30

4-3 Rainfall within Georgina-Diamantina catchment 31

4-4 Landsurfaces and relief in Georgina-Diamantina catchment 33

4-5 Stages of channel breakdown in Georgina- Diamantina system 37

4-6 Drainage terminal of Georgina River 38

4-7 Drainage terminal of Sandover River 39

4-8 Graph showing contributory catchment area to channel length for Sandover, Georgina and Diamantina Rivers 40

4-9 Features of floodplain of Diamantina River 42

4-10 Decrease in flood discharge for Coopers Creek 46

4-11 Graph of actual and theoretical mean segment lengths for Georgina-Diamantina channel systems 48 Following Number Title Page

5-1 Drainage network and topographic catchment for Finke-Macumba system 53

5-2 Postulated reconstruction of former drainage associated with Finke River system 53

5-3 Effectively drained areas of Finke and Macumba subcatchments 56

5-4 Landsurfaces and relief in Finke-Macumba catchment 57

5-5 Stages of channel breakdown in Finke-Macumba system 60

5-6 Rainfall probability graph for Finke River catchment 64

5-7 Sectors of Finke River 66

5-8 Locations of surveyed cross-sections along Finke River 67

5-9 Graph showing change in bankful1 discharge along Finke River 67

5-10 Relationship between width/depth ratio and percentage silt and clay along Finke River 68

5-11 Floodout of Finke River 71

5-12 Graph of actual and theoretical mean segment lengths for Finke-Macumba channel systems 73

6-1 Location of Fowlers Creek and relationship to Lake Bancannia 76

6-2 Fowlers Creek catchment 77

6-3 Plan of alluvial channel of Fowlers Creek showing locations of measured cross-sections and three channel sectors 79

6-4 Cross-profiles of channel of Fowlers Creek 79

8-1 Southern Western Australia showing Yilgarn Plateau and drainage on Plateau 101

8-2 Reconstruction of drainage systems of Western Australia mainly after Beard (1968) 103

8-3 Lake Way-Lake Cowan drainage system 104

8-4 Areas of Lake Way-Lake Cowan system effectively drained 106 Following Number Title Page

8-5 Landsurfaces and relief in Lake Way-Lake Cowan catchment 108

8-6 Stages of channel breakdown in Lake Way- Lake Cowan catchment 109

8-7 Graph of actual and theoretical mean segment lengths for Lake Way-Lake Cowan channel systems 111

9-1 Graph of percentage of channel segment lengths missing from three sample catchments 124 LIST OF PLATES Following Number Title Page

4-1 Vertical air photograph of Diamantina River through Channel Country 42

5-1 Vertical air photographs of Finke River 66

5-2 Changes in channel cross-section down Finke River 70

5-3 Vegetation indicators of channel processes in lower Finke River 71

6-1 Vertical air photographs of Fowlers Creek 79

6-2 Changes in channel of Fowlers Creek 84

6-3 Choked distributary channel 85

8-1 Vertical air photographs of Lake Darlot system 109 1.

CHAPTER 1

INTRODUCTION

Drainage Disorganisation and its Causes

This study is concerned with disorganised drainage within the

Australian arid zone. A disorganised drainage system is here

defined as one which does not reach its potential baselevel as a

connected system. The potential baselevel of a drainage system

is the lowest point within the topographic drainage basin. This basin may thus be more extensive than the effective catchment, which is defined as that part of the drainage basin which supports

some network of channels, although these may not connect with a trunk channel to reach the potential baselevel.

Potential baselevel for most drainage systems is sealevel, but many channel systems, particularly in arid regions^terminate inland, and are accordingly termed interior drainage systems. The potential baselevels of such interior drainage systems may be the floors of topographically closed basins, generally of tectonic origin, or the lowest points of former oceanic drainage systems within which drainage has failed.

Factors other than aridity may lead to disorganisation of drainage.

Examples of disorganised drainage are common in karst landscapes, where surface channels commonly disappear into sinks and the surface network lacks co-ordination. Similarly in areas of recurrent volcanic activity or of frequent mass movement, channels may be blocked by ash 2.

falls, lava flows, slumps or mudflows, and the drainage systems deranged. Reversed drainage, disconnected swamps and cutoff channels are features of such disorganised systems of channel networks.

Drainage Disorganisation and Aridity

Disorganised drainage however, is most consistently associated with aridity where the failure of drainage reflects the inadequacy of rainfall to maintain drainage to topographic base- level; and where tectonic activity causing disruption to the drainage system is more likely to be long lasting in its effect. The impli­ cation of the association between tectonically controlled interior drainage and aridity is not that the two are necessarily directly linked, but rather that while exterior drainage in more humid regions may survive these disturbances by overflowing depressions, or through channel re-routing, interior drainage reflects rur\o overcome vv^pV\c?v\ o( veU’t'f • Rainfall is inadequate to maintain stream discharge sufficient to re-establish the "normal" exterior drainage, and relatively high rates of channel losses through evaporation and infiltration result in continued decrease in discharge downstream across the lowlands, with consequent breakdown of channel networks. Such disturbances do tend to be causally linked with interior drainage in continental interiors or in the centres of mountain-rimmed tectonic basins, which are more likely to be arid because of conditions such as rain-shadow effects which favour low rainfall.

The disorganised drainage systems of arid regions are the remnants of previously organised networks which occupied the basins during more 3.

humid climatic phases. Evidence from desert regions in many parts of the world indicates that more extensive or more organised drainage

systems were active in more humid phases either during the Pleistocene

Epoch or in the Tertiary Era. Disorganisation thus implies the dis­

integration or dismemberment of formerly more extensive drainage, and the breakdown of previously more integrated networks. 4.

CHAPTER 2

MEASURES OF DRAINAGE DISORGANISATION

In attempts that have been made to analyse or classify disorganised

systems of drainage, three criteria have been used. These are:-

i. Consideration of the terminals of drainage systems; that is,

whether the drainage is outgoing to the sea, or interior.

ii. The degree of connectivity; whether or not there are breaks

in the channel network, and the degree to which former

connections have been obliterated where breaks occur.

iii. Consideration of the continuity of floods occurring along

channel systems. With more frequent flows recognisable

channel courses are maintained, but the links of the network

are more difficult to establish if flows are less frequent.

Tk e s e three criteria are not exclusive, as each in turn fits

wtlKIn that preceding it.

Drainage Terminal and the Extent of Interior Drainage

Many geographers have defined and mapped regions of interior drainage and recognised the association with climatic aridity.

Von Richthofen (1877), in his attempt to establish major physiographic regions in Asia, used type of drainage terminal as part of the basis of his categorisation. Central Asia was distinguished from 5.

Peripheral Asia by its interior-basin drainage, associated with

its arid climate.

Murray (1887), in considering the relationship between rainfall and

river discharge, first estimated the total world area of inland 2 drainage as 29,404,056 km . He noted that this correlated well with

his calculated area of arid regions, (defined as having a mean annual

rainfall of less than 250 mm) of 31,232,000 km^.

The first attempt to map interior drainage on a world scale was by

Berghaus (1886), and his map was subsequently used as the basis for

other estimates of areas of internally-drained territory by Gilbert

(1890) and Albrecht Penck (1894). Their estimated percentages of

the areas of continents draining to interior basins are listed in

Table 2-1. The small-scale world map used did not allow accurate

measurements to be made. For the total world area of interior 2 drainage, Gilbert used Murray's value of 29,404,000 km , or 20 percent 2 of the land surface. Penck's estimate was 32,300,000 km , approxi­

mately 22 percent. Their estimates of the area of Australia draining 2 to interior basins differ widely. Gilbert gave a value of 3,800,000 km

or 52 percent of the continent, whereas Penck, using the same map, 2 gave the internally drained areas (Binnengebiete) as about 5,900,000km

or 80 percent of the land surface.

Both Gilbert and Penck, however, agreed that Australia is the con­

tinent with the greatest proportion of its total area drained to

interior basins.

With the availability of world coverage of medium-scale topographic maps, mainly at 1:6,000,000, de Martonne (1927) made the first attempt Percentage of continent occupied by interior drainage Continent Gilbert (1890) Penck (1894)

Australia 52 80

Africa 31 40

South America 7 8

North America 3 4

World Total 20 22

TABLE 2-1 Estimates of areas of interior drainage based on world map by Berghaus (1886). 6.

to classify and systematically map drainage status in these terms.

The more detailed topographic information enabled far more accurate distinction between regions of interior and external drainage than had previously been possible. He divided systems of drainage into the two classes: -

drainage flowing through to the ocean - exoreic

drainage flowing to interior basins - endoreic.

In this study (1927), de Martonne was primarily concerned with mapping the extent of interior drainage within latitudinal zones for the purpose of correlating this with the degree of climatic aridity. He found a maximum of interior drainage at 20-21° latitude in the northern hemisphere, and a southern hemisphere maximum at

29-30°.

De Martonne was also concerned with the relationship between interior drainage and aridity. He used his index of aridity*, a ratio first suggested two years earlier (de Martonne, 1925), and found a good correlation between aridity as measured by this index, and the extent of regions of interior drainage. He correlated index values with the degree of aridity thus:-

*His premise in the development of this index was that aridity is determined primarily as a function of temperature in high latitudes, and of precipitation in low latitudes. Using an arbitrary zero of 10°C, below which endoreism is unknown, and frozen subsoil gives rise to abnormal conditions of circulation, he derived the formula

A = P/ (T + 10) where P = precipitation in mm T = temperature in degrees C Ar = index of aridity and on this basis drew a world map of the index of aridity (de Martonne, 1926). 7.

Index of aridity (A ) < 5 arid

5-20 semiarid

> 20 humid

From observations of drainage pattern, natural vegetation and the general limits of cultivation, de Martonne (1927) also noted that the index of aridity characterised areas hydrologically and botanically (See Table 2-2).

Australia is listed also in this study by de Martonne as the continent having the greatest proportion of its total area occupied by endoreic 2 drainage. This was 64 percent of the land surface, or 4,920,000 km

(See Fig. 2-1) and may be compared with a proportion of 52 percent for Africa, and 33 percent for the world as a whole.

De Martonne and Aufrere revised the map of interior drainage regions in 1942, using the more detailed topographic information then available from world map coverage at a scale of 1:1,000,000. They also used more climatic data than were available in 1927. Many of the boundaries of endoreic regions were altered, and from these more detailed maps the correlation between the index of aridity and the limits of endoreism breaks down further, and it becomes apparent that drainage terminal and climate are only broadly related.

With the revised map of the index of aridity, de Martonne (1942) again noted that Australia is the continent where the disorganisation of drainage is most extensive, and this corresponds well with low values of index of aridity for this continent, (See Fig. 2-2).

De Martonne admitted (1927 : 414) that endoreism is a reflection of Index of aridity Natural vegetation/ Hydrologic climatic type A r represented conditions

0-5 "True deserts", Generally all botanically and drainage hydrographically endoreic

5-10 Steppe climates (dry)

10 - 20 Prairie vegetation Upper limit of endoreism, but varies with relief

30+ Forest vegetation Generally all types drainage exoreic

TABLE 2-2 Botanical and hydrologic characteristics related to index of aridity (after de Martonne, 1927) . 8.

past rather than present conditions, but did not discuss the

sequence of changes which may have occurred to lead to such

endoreism.

The Degree of Connectivity

De Martonne (1927) recognised that independent of the question of the failure of drainage to reach the sea was the effectiveness of the occupation of drainage basins by connected systems. Within the category of endoreic drainage, he recognised another subdivision, for areas where net runoff is nil, and no flow into other streams occurs.

These areas he classed as areic. With areism, de Martonne introduced the concept of connectivity, or the state of implied disintegration of drainage networks, into the classification of drainage regions based on potential terminal.

He also related areism to his index of aridity. Areism was not found in regions in which the index value was greater than 10, and was common in areas with an index value of less than 5.

For Australia de Martonne mapped the sand deserts and much of

Western Australia as areic (43 percent of the continent) within the a-veo^ topped a5 z^oveib which included an additional strip of country along all but the southern margins of the sand deserts, as well as the Lake

Eyre basin. The limit of endoreism corresponded well with an index of aridity of 20, and the limit of areism with an index value of 5.

This can be seen if Figs. 2-1 and 2-2 are compared. From the revised, more detailed map of Australia (1942), as with other areas, there is Fig.2- 1 Drainage classification of Australia de Martonne (1927)

Fig.2- 2 Index of aridity for Australia -frovn de Martonne (1927) Exoreic C? 0 IOOO 1 I______l Kilometres

Index of aridity KXTl 0-5 5-:o 10-20

1927 boundaries

IS42 revised boundaries 9.

shown to be less correlation between the index of aridity and the limits of endoreism and areism.

Just as de Martonne had found it necessary to combine the concept of connectivity with that of drainage terminal, Hills (1953) used these as the basis of the first detailed attempt to systematically classify drainage in Australia. His classification of drainage

(See Fig. 2-3) was basically into

normal drainage to the sea co-ordinated interior-basin drainage

and

drainage areas with salt lakes unco-ordinated riverless areas

With the terms "co-ordinated" and "unco-ordinated" drainage he also used the concept of connectivity as a measure of drainage disorganisation. He classified the Lake Eyre catchment as a whole, like some other tectonic basins, as an example of "co-ordinated drainage". Most of the remainder of the arid zone was a "region of unco-ordinated drainage with salt lakes". This latter area he regarded as having a potential outlet to the sea, but consisting of ephemeral stream systems. He placed the Nullarbor Plain in the category of "riverless areas".

Because of the increasing impetus of the philosophy of climatic geomorphology in the early 1950's, Hills especially considered the Fig. 2-3 Drainage classification of Australia Hills (1953)

10.

relationship of the distribution of landforms to climatic divisions in the Australian arid zone. He particularly studied the relation­ ship of the distribution of drainage classes to climatic zones.

In the driest area (Koppen's BW or Thornthwaite’s EB* climates) two main types of drainage occur, namely nunco-ordinated drainage with salt lakes", and "co-ordinated interior-basin drainage". There is also a small area of "normal drainage to the sea". In a somewhat less arid climatic zone (Koppen BS or Thornthwaite DB’ climates) there is similarly little association of climatic type and drainage class.

"Co-ordinated interior-basin drainage", "normal drainage to the sea" and "unco-ordinated drainage with salt lakes" are all found in this zone. The one "riverless area", the (limestone' based) Nullarbor Plain also occurs mainly within this marginally arid climatic zone.

While de Martonne had related areism and endoreism to aridity, Hills showed clearly that for Australia his classification of types of drainage transgresses climatic boundaries. He noted particularly the geologic causes for this, but did not consider inherited factors.

PyoporLicy of C^ectjv c Ca-lcUn^e-vAt

Methods such as those of de Martonne and Hills, in which drainage is considered at a continental scale, are necessarily broad and quali­ tative. If individual drainage basins are studied as units, the status of the drainage can be expressed quantitatively, and the differences between individual subcatchments can be assessed. Dubief (1953) working in the Sahara, not as part of a broader world study, used detailed topographic data available from 1:100,000 and

1:200,000 maps combined with aerial surveys. For the large wadi systems of the Sahara, Dubief produced a map of the drainage systems and applied a method of assessing the proportion of a drainage basin represented by connected networks (versants ruissellav^s ) , indepen­ dent of the consideration of the drainage terminal.

Dubief's method was a calculation of the proportion of a catchment which is hydrologically effective. He defined this ratio of effective catchment area as that percentage of the total catchment

(topographically defined) which contains an organised drainage network.

Values he obtained for the effectively drained regions of large drainage basins show a broad correspondence with rainfall in the catchments:-

2 63 percent for the Niger - a river basin of 1,890,000 km

draining sporadically to the sea at the southwestern margin

of the Sahara, mean annual rainfall 730 mm

40 percent for the Chad - an interior lake basin of 2,570,000 2 km extending beyond the southern fringe of the Desert, mean

annual rainfall 378 mm

7 percent for the Western Sahara - an interior drainage basin 2 of 2,660,000 km in the arid northwestern part of the Desert,

mean annual rainfall 34 mm 12.

2 5 percent for the Quattara - the basin of 575,000 km

draining towards an oasis well below sealevel in the arid

Egyptian Sahara, mean annual rainfall 12 mm.

The Consideration of -ConVuuniy of Flows

The examination of individual drainage basins introduces the problem

of establishing the drainage network, and determining the degree of

connectivity or organisation. In studies oC 4^ decree con­

nectivity there is a need to decide if links are predictable,

operative or recognisable. The term disorganisation may imply both

temporal and spatial disorganisation of a drainage system, as flows

in many ephemeral channels commonly fail to reach the topographic

baselevel of the system in many flood episodes, and flows in other

channels stop short of their potential baselevel under all flood

conditions within present day climatic regimes.

There is a considerable range in the degree to which a drainage

system is disorganised. The most extreme case of disorganisation

occurs where, although there is no topographic barrier to connecti­ vity, parts of a potentially connected system remain as discrete units, and the potential system may only be inferred from topographic data. A high degree of disorganisation also exists where the only

evidence of former drainage links is given by alluvial fills or by

specific soil deposits, often associated with particular vegetation bands. Less extreme cases of disorganisation occur where the

evidence of drainage connections consists of pans or fragments of the former network. Where former drainage lines are evident, there 13.

may be no historical record of flows through these courses, and as flow data are commonly short-term records and hydrologic data are totally lacking in many parts of the arid zone, it is difficult to determine the degree of connectivity in these instances.

Since recognisable channel links are maintained by more frequent flows, the concept of flow frequency has been used in systems of classification of drainage which are based on the degree of connec­ tivity.

Dubief (1953) also used this concept of frequency of flow in a more refined measure of connectivity or degree of disorganisation, based on the degree of dismemberment of the drainage network. He distin­ guished a number of stages in the disintegration of desert channel systems, well suited to application in the major wadi systems of the

Sahara. These stages are:-

1. Flows in the trunk channel only reach potential baselevel

when supplemented by confluent flows in lowland sectors.

2. Floods in the trunk channel no longer reach potential

baselevel.

3. Although flooding may occur throughout the main artery of

the drainage system, no continuous flows occur.

4. Although sectors of the main artery of the system may be

recognisable, it is discontinuous, and has never been known

to flood in all sectors.

5. The main artery of the system is completely obliterated,

and tributary networks remain as independent systems. 14.

This classification was particularly appropriate in Dubief's study

of the surface hydrology of the Sahara, but it depends strongly

upon flow data which are often not available in desert regions.

CWss i fv c.aXvt7*\ \3>

used all three criteria - the baselevel of drainage, the degree of

connectivity, and the frequency of flooding. His classification

(See Fig. 2-4) was an extension of the system used by de Martonne (1927)

into which he introduced the concept of flow frequency with the term

"sporadic" to indicate an ephemeral flow regime.

He classed the Lake Eyre catchment, the major internally-draining

tectonic basin, as "sporadically endoreic", with the exception of the

sand-covered Simpson Desert which, with the remainder of the arid area

to the west was left as "areic". He applied the term "areic, sporadi­

cally exoreic" to the Western Australian shield drainage, and

"sporadically exoreic" to ephemeral channels draining to the sea.

He used "cryptoereic" to describe the sub-surface Nullabor drainage

area classed by de Martonne as "areic", and by Hills as "riverless".

In this study, Gentilli was concerned with a broad qualitative classi­

fication of drainage over the continent, not with a comparison of

individual drainage catchments.

Measures of Drainage Disorganisation Used in this Study

The main aim of this study is to assess the degree of drainage dis­

organisation in selected parts of the Australian arid zone, by Fig. 2-4 Drainage classification of Australia frow Gentilli (1952) Sporadically Areic Sporadically exoreic endoreic

Areic, Sporadically exoreic Cyptoreic

IOOO

Kilometres 15.

applying various measures of this degree of disorganisation.

Arising from this, a second aim is to test these measures and to

consider their advantages and defects. The study is also concerned

with the elucidation of the different environmental controls -

particularly geologic or topographic - involved in the breakdown

of drainage systems in different parts of the arid zone.

Methods of assessment reviewed above have not been found to be

satisfactory for various reasons

Division of drainage provinces into interior and exterior

concerns drainage disorganisation only in its broadest sense -

that drainage has been inadequate to overcome tectonic or

depositional obstacles. This may be shown to be a function

of aridity in the general sense, but the method does little

to explain the mechanism of breakdown of channel networks.

Measures dealing with the drainage basin as a unit, such as

Dubief’s calculation of the proportion of a catchment

effectively drained, are useful when applied over large areas

for general comparisons, but do not differentiate within the

drainage basin. Desert systems are essentially disaggregated,

and a method dealing with smaller units is more appropriate.

Dubief's system of numbering the stages of disintegration of

desert drainage systems is effective in explaining the pro­

cesses involved, but a classification of drainage which relies

so heavily on hydrologic data is inappropriate for much of

the arid zone, since historical records of such flow data are

generally lacking. 16.

If disorganisation of drainage is considered in terms of

geologic or topographic variables, or within the study of

fluvial processes, the method used to measure disorganisation

must be applied at a smaller, more detailed scale than that

of the total catchment, in order to deal with individual

channels.

Any method chosen must be applicable to the study area, and the

available data. For a study of this type in which large areas

within the Australian arid zone are surveyed, airphotographs are

the most suitable consistently available data source, and can be

used in association with altimetric information available from

complete map coverage at a scale of 1:250,000.

Although any classification of drainage systems in Australia must

take into account the potential terminals, this has not been used

as a measure of degree of disorganisation, since it does little to

elucidate the processes of disorganisation, and represents too broad

a categorisation.

The hydrologically effective proportion of the catchment has been

calculated, since although it is a coarse measure of disorganisation,

it does identify regions within a basin that are not presently occupied by organised drainage. This measure, used by Dubief (1953)., is that percentage of the topographic catchment which is occupied by a connected drainage net.

Areas within a drainage basin where some network of channels or floodplain features could be identified on airphotographs, were 17.

delineated. In many areas where no channel exists the course of drainage is clearly discernible on airphotographs as lines of pans or bands of vegetation. The potential topographic catchment was established at a scale of 1:250,000 from airphotographs, airphoto- mosaics, 1:250,000 maps and other available altimetric data. The percentage of this whole topographic catchment comprising areas of connected channel networks was calculated.

First and second order channels (Strahler, 1964) arising on low stony rises or pan surfaces within sandplain or dune areas may be identified on airphotographs or mosaics. Such first and second order channels in which the courses are totally obliterated by the surroun­ ding aeolian material generally occupy areas too small to be mapped within the total catchment, and are thus not considered with the hydrologically effective regions of the catchment.

Descriptive classification of the stages of disintegration of channel networks from hydrologic evidence was used by Dubief (1953) applied to entire catchments. In this study, a method is used which simi­ larly considers the systematic failure of drainage systems. Since hydrologic data are commonly unavailable in the Australian arid zone, this method is based on describing the stages of network disinte­ gration from physiographic evidence. These stages of disintegration or disorganisation of drainage shown in Fig. 2-5 are:-

1. The channel of the trunk stream (the highest order channel

of the system) is not continuous through to potential base-

level, but the floodplain morphology, often as reflected by

vegetation, shows evidence of streamflooding along the whole

course. Fig. 2-5 Stages of breakdown of a channel network, modified after Dubief (1953) Pofenriat topographic \ course

Topographic \ base level

Organised drainage

C ouv.se o ppovve./\l' hal

diact>r>ri£cT£cl

Stage (i) Stage

Stage 18.

2. Breaks occur in the lower course of the trunk stream, and there

is evidence of discontinuous streamfloods. Pans or vegetated

alluvial tracts indicate the former course.

3. The lower course of the trunk stream is not apparent but the

tributary network converges to the course of the trunk

stream.

4. The lower course of the trunk stream is obliterated and the

lower sectors of the tributary network are discontinuous,

their former courses indicated by pans or vegetated alluvial

tracts.

5. The lower courses of the tributary network are obliterated

and upper tributaries disperse as independent systems.

In the Australian arid zone regions of high relief are sparsely distributed and watersheds consisting of ranges and uplands of low relief are commonly widely separated from their topographic base- levels. This fosters poorly developed drainage networks in some subcatchments with more highly integrated networks in other parts of the main catchment where more extensive uplands occur. For this reason a better appreciation of the status of drainage organisation is achieved by considering subcatchments with terminating drainage separately.

Determination of the stage of disorganisation of stream courses can be made using airphotographs. Where a distinct channel or a flood- plain with evidence of streamflooding was recognised, the course was mapped as unbroken. The former courses of many channels are occupied by lines of pans or tracts of alluvial material associated with 19.

denser vegetation. While such pans and alluvial zones now act individually as discrete drainage terminals, they represent the now-discontinuous lines of the earlier network.

The comparison of actual and potential stream length is a graphical method which has been developed to assess the degree of breakdown of drainage in this study. By calculation of the theoretical and actual mean segment lengths for any catchment, this method can be used to compare the relative degree of disorganisation of drainage between catchments with a more disorganised system varying more from the ideal network. Fig. 2-6 is an hypothetical graph showing how this comparison of actual and potential cumulative mean segment length would appear.

For this method it is necessary to construct as much as possible of the potential stream network for the topographic catchment. In areas of sandplain or dunes, where pans or other evidence of former drainage have been obliterated by sand, the probable course of drainage can be mapped using all available height data. In comparing mean channel segment lengths between the theoretical or reconstructed drainage network and the actual network, some estimate of the approximate number of channel segments of each stream order is necessary.

A prediction of the network can be made using Horton’s (1945) Law of

Stream Numbers, Law of Catchment Areas, and Law of Stream Lengths, this last modified to the Law of Cumulative Mean Segment Lengths where the Strahler (1964) method of stream ordering is used. A characteristic of arid zone drainage is the extreme variation in bifurcation ratio between upland and lowland parts of the catchment (Slatyer and Mabbutt,

1964) and thus appropriate bifurcation ratios are needed to calculate the number of channels, relevant to the position in the landscape. Fig. 2-6 Difference between actual and po+e-^V'ccl cumulative mean segment lengths maximum potential

stream order

actual

maximum stream

order attained

Cumulative mean segment length

encl osed area

represents degree

drainage disor ganisation

I n

Stream order 20.

Within each major catchment to be examined in this study several

subcatchments were chosen to proportionally represent each rock type present within the major catchment, in both upland and lowland sectors

of the network, to find bifurcation ratios (Horton, 1945) between

each two stream orders. Appropriate bifurcation ratios were then used in each landscape unit of the drainage basin to find the approximate total number of streams of each order which would be present in the theoretical network. These were calculated using

Horton's (1945) Law of Stream Numbers. A similar method was used to calculate the number of streams of low orders in the network actually existing, however for stream orders which were readily identified on airphotomosaics and 1:250,000 maps, the actual numbers of streams of each order were counted. Length ratios (Horton, 1945;

Strahler, 1964) were used in each catchment to determine the mean lengths of channel segments. Samples of streams from each sector of the catchment on each rock type were measured from airphotographs and maps, and the Law of Cumulative Mean Segment Lengths was used to determine the mean lengths of low order segments. This was carried out for the actual network and the theoretical or reconstructed network, and the cumulative mean segment lengths plotted against stream order, using a line of best fit for each distribution. The difference between these two lines or between the actual and potential lengths represents the degree of disorganisation of the drainage in the catchment.

The accuracy or reliability of the method is least when applied to catchments developed on widely spaced ranges and alternating extensive lowlands. While a reconstruction can be made based on the present density of channels, and the present bifurcation ratios within separate 21.

parts of the catchment, it is impossible to estimate what network of channels may be buried beneath the sediments which fill the low­ land regions. The extrapolation from the existing network to that which may have covered the entire catchment is large, and thus cannot be assumed to be reliable. Even less reliable is any attempt to re­ construct the pattern of lower order channels which may have originated on low ground in the bottom reaches of the catchments.

These three methods of assessing drainage disorganisation are applied in this study within selected sample catchments in the Australian arid zone. As airphotographs, airphotomosaics and maps are the main sources of data it is necessary to know how many orders of streams are missing from each of these. Field checks, and recourse to pub­ lished and unpublished data were used for this purpose. 22.

CHAPTER 3

DRAINAGE DIVISIONS IN THE AUSTRALIAN ARID ZONE

Three measures of drainage disorganisation are applied in this study

to sample catchments representative of the main types of disorganised

drainage occurring within the Australian arid zone. These measures,

described in Chapter 2 are:-

i. The proportion of the catchment occupied by a connected

stream network, i.e. the hydrologically effective part

of the catchment.

ii. The stage of disintegration of the channel network.

iii. The comparison of actual stream lengths with the potential

stream lengths as calculated from Horton's (1945) Law of

Stream Lengths.

Attributes of the Three Measures

Each of the measures stresses a different aspect of drainage dis­

organisation.

Estimation of the hydrologically effective part of the catchment is

a quantitative measure which designates the areas providing runoff,

but not the way in which this occurs. In disorganised systems of

drainage, parts of the topographic catchment (which is defined as the potential topographic catchment) may not be occupied by a connected

stream network. For example obliteration of channels, especially by 23.

sand, within a catchment may render significant parts of the

catchment hydrologically ineffective, or in catchments with a

large proportion of lowland areas which do not contribute to the

channel network, this measure might indicate that a low percentage

of the catchment is hydrologically effective. The method can only be applied to whole catchments.

The stage of disintegration of the channel network is closely linked with the hydrologic continuity of the channel network, although related form criteria are used for recognition of the degree of dis­ memberment of the system back from an original baselevel connection.

The method is not quantitative, since an ordinal scale of measure­ ment is applied, but it is especially valuable in analysing the breakdown of lengths of large-order channels which traverse lowland desert areas not yielding local runoff.

The comparison of actual and potential stream lengths is quantitative and can take some account of the dynamics of channel breakdown by an attempt to reconstruct the channel network lacking from the catchment as a whole. The reliability of the method is dependent upon the maintenance of bifurcation ratios across those parts of the catchment not sampled. It is also difficult to construct the potential network across lowlands in arid areas where these lowlands would not provide runoff, and the method is applied with most validity to upland erosional subcatchments, where reconstruction of the missing parts can more readily be carried out.

The three measures might therefore be expected to express various aspects of the breakdown of channel systems when applied within one 24.

catchment or type of disorganised drainage. They are also likely

to prove to be specifically appropriate for different patterns of

disorganisation, which can be seen from airphotographs to occur

within the Australian arid zone. These patterns undoubtedly reflect

differences in the processes of disorganisation of drainage, for

instance, disorganisation may be due to the general failure of the

drainage network throughout the basin, or to the isolation of remnants

of the network within individual subcatchments, through breaks in the

connecting tributary channels.

Accordingly, each of the three measures is applied within each of the

sample catchments to show aspects of disorganisation within catch­

ments and between drainage types.

Drainage Status in the Australian Arid Zone

Previous studies, such as those of Hills (1953) and Mabbutt (1969)

have shown that climate is not the main factor causing differen­

tiation of drainage patterns within arid Australia, but rather that

structure, topography, and lithology or surface cover are the

factors which most determine local drainage patterns. This can be

seen by comparing surface drainage within the arid zone (Fig. 3-1) mapped after Mabbutt (1972), with climatic boundaries (Fig. 3-2).

In a world study of arid climates, Meigs (1953) distinguished three

sub-classes based on the degree of aridity. His outermost (semiarid, S)

boundary is here used to delimit the arid zone within Australia, as

shown in Fig. 3-2. His boundary of the more arid (A) region within Fig. 3-1 Drainage status in the Australian arid zone -CVo/w Mabbutt (1972) CONNECTED DRAINAGE SYSTEMS DISCONNECTED DRAINAGE SYSTEMS detached tributaries and '//S'//, ' Vv*. y*\ direct external : drainage radiating from uplands, with separate terminals mil indirect external t i m m •ELF : - tributary to aligned playas ('river- lakes') interior

SURFACE DRAINAGE LACKING

areas of limestone or ceolian sand

1000 am wsdksiboundary of semi arid zone Kilometres wiww boundary of arid zone (after Meigs, 1953) Fig. 3-2 Climatic classification of arid and semiarid Australia after Meigs (1953) 40°« £ — Extremely arid A— Arid S - Semiarid a — no marked season of precipitation b — summer precipitation c — winter precipitation 1st digit — mean temperature of coldest month 2nd digit—mean temperature of hottest month 500 km 0 — less than 0°C I__ —____1 1 - 0° to IO°C 2 - 10° to 20°C 3 - 20° to 30°C Boundary of arid zone 4 — more than 30°C Boundary of semiarid zone

120* 130° 140* 150° JL 25.

this is also shown, and it can be seen that the drainage classes clearly transgress the boundary of the more arid interior region.

Drainage status in the arid zone, however, is closely linked to morphostructure, mapped (after Mabbutt, 1972) in Fig. 3-3. His classification of drainage status is based primarily on the concept of the degree of organisation or connectivity of drainage and secondly on whether the drainage is internal or exterior. It is similar to but more detailed than that of Hills (1953) and is based on data from large-scale airphotograph and map coverage of Australia.

Within the framework of the present study, the distinction between internal and exterior drainage is not important, but the classification is well suited for use in a study involving drainage disorganisation.

Three main patterns of drainage occur within arid Australia associated with the three main morphostructural units, and sample catchments have been chosen for study as representative of these. It can also be seen that regions of extreme disorganisation of drainage, where surface drainage is lacking entirely, correspond with regions of aeolian sand or limestone and are also morphostructurally controlled.

As this study is concerned with disorganised or disconnected drainage, the well organised (connected external) drainage of Western Australia is not considered. For this reason also, small areas of organised drainage in the northern and southwestern parts of the arid zone are not studied. The three main patterns of drainage considered are:-

i. Mainly connected (centripetal) drainage ii. Disconnected drainage, mainly radiating from uplands, with

separate terminals Fig. 3-3 Morphostructural divisions in Australia Mabbutt (1972) a o

. o o 26.

iii. Disconnected drainage, tributary to aligned playas

("river lakes").

Mainly connected centripetal drainage is associated with the basins of the central part of the Interior Lowlands of arid Australia. The

Eromanga and Surat Basins make up the eastern and far southwestern parts of the tectonic depression of the Lake Eyre basin. Drainage systems on the gently warped Cretaceous sedimentary rocks comprise generally connected but diminishing channel networks barely main­ tained by somewhat tenuous hydrologic conditions. Flows to the edge of the sand deserts which occupy the lower regions of the basin occur, on an average at one to two year intervals, while floods through to Lake Eyre occur approximately once every two to ten years

(Whitehouse, 1941; C.W. Bonython, 1963; E. Bonython, 1971). The weak, uniform Cretaceous rocks, lithic sandstones and shales, which weather mainly to form heavy clays, favour runoff and the maintenance of the tectonic form and associated centripetal drainage.

Although these channel systems are mainly connected and thus not particularly disorganised, the decrease of the lower parts of the trunk channels reflects the irregularity of streamflooding through to Lake Eyre. The Georgina-Diamantina system, which is typical of the stream systems of the eastern Lake Eyre catchment is studied as representative of this drainage pattern. Connected networks of headward channels lead to broad floodplains on extensive lowlands, in which a series of anastomosing channels are developed before the systems are encroached upon and restricted in their lowermost courses by aeolian sand. 27.

Although the major part of this drainage system is developed within the tectonic basin, it extends beyond the Interior Lowland division in the northwest of the catchment, to include areas of other morphostructural divisions. The Sandover River system drains an area of the Western Plateau, and is representative of the drainage developed on the narrow strike ranges and extensive lowlands of this division. The Georgina River also rises outside the tectonic basin on the Barkly Tableland, an ; intercratonic basin of flat-lying Palaeozoic sedimentary rocks, and the northern half of the Georgina network is developed on this plainland, and thus does not represent drainage of the tectonic basin.

Disconnected drainage radiating from uplands with separate terminals occurs in the structural basins of the Western Plateau of the conti­ nental platform. In these areas considerable difference in relief occurs between the uplands on uplifted cratonic blocks and the structurally determined lowlands of the intercratonic basins.

Palaeozoic rocks, commonly outcrops in the form of isolated linear ranges, or the upwarped margins of the basins are the source of runoff. Drainage systems generated on these uplands have caused filling or choking of the basins, generally obscuring the lower ^Ucd sectors soothe potential outlet of the system cannot be identified, as in the Ngalia, Amadeus and Officer Basins.

This disconnected-tributary drainage type is characterised by partial obliteration of the lower courses of many channels, so that tri­ butary systems rising in upland reaches of the catchment may disperse independently. The generally arenaceous or granitic rocks of these uplands provide coarse-textured sediments which contribute to the choking of the lowland channel sectors. 28.

The western Lake Eyre basin is the major area occupied by this type of drainage. The Finke-Macumba system occupies this western part of the catchment. The Lake Eyre basin, with a total catchment area of approximately 650,000 km is the largest of the structural basins in the Australian arid zone. Within this basin three areas can be distinguished on the basis of drainage status.

Mainly connected centripetal drainage of the Eromanga and Surat

Basins occupies the eastern and southern parts of the Lake Eyre catchment, and the Macumba River system is developed on the south­ western extension of this morphostructural unit.

Disconnected drainage radiating from uplands occupies the structural basins of the continental platform in the northwestern part of the

Lake Eyre catchment. The upper part of the Finke River system which rises on discrete uplands in the Macdonnell, Petermann and

Musgrave Ranges is typical of the drainage of this morphostructural unit.

disconnected drainage tributary to aligned playas is closely asso­ ciated with the extent of the continental shield. On this area of crystalline Precambrian rocks, commonly overlain with aeolian sand, relief differentiation is minimal. Low drainage divides consist of bands of intruded rock, or of gently warped basement rocks. The main generating uplands are the breakaway escarpments of the Old

Plateau (Jutson, 1934), often supporting a gully system of tributaries, linking to form channels which disperse on the sandy surfaces of the Mew Plateau. 29.

On this low-gradient surface with little relief differentiation, breaks occur along the courses of the trunk channels and discon­ tinuous elongate lakes and pans remain, indicating the former drainage lines. The Lake Way-Lake Cowan system is typical of such broken drainage networks, where flows cannot maintain channels linking the lakes, and that system is studied as representative of this drainage type. 30.

CHAPTER 4

THE GEORGINA-DIAMANTINA SYSTEM

The Georgina and Diamantina River systems have been mapped by the

Department of National Development (1966) as two separate drainage

systems with the Sandover River system included within that of the

Georgina. However, the Georgina-Diamantina system occupies a single

topographic catchment which drains to Lake Eyre from the north

(Fig. 4-1) and which functions as three subcatchments with separate river terminals, namely the Sandover, Georgina and Diamantina River

systems. In this study these three subcatchments have been con­

sidered separately.

The Proportion of the Catchment Hydrologically Effective

The topographic limits of the three subcatchments as shown in Fig. 4-1 were drawn from 1:250,000 topographic maps supplemented by unpub­

lished height data obtained from the Geodetic Branch, Department of

National Development and evidence from air mosaics in areas of less determinate relief. These areas are listed in Table 4-1.

The effective catchment areas, or the area within each subcatchment which generates a network of stream channels, were measured from air mosaics and airphotographs, and are also shown in Table 4-1.

Areas of the catchment occupied by a drainage network are shown in

Fig. 4-2. Fig. 4-1 The drainage network and the topographic catchment of the Georgina-Diamantina system

Fig. 4-2 The areas of the three subcatchments of the Georgina- Diamantina system which are effectively drained Georgina

subcatchment

Diamcntina subcatchment

IOO Km

Areas of catchment effectively drained 31.

The Ratio of Effective Catchment

This ratio, which is shown in Table 4-1, differs between the three

component subcatchments. It is lowest in the Sandover subcatchment,

where it is 59 percent, higher in the Georgina subcatchment at 80

percent and highest in the Diamantina subcatchment at 91 percent.

These values for the Ratio of Effective Catchment of between 59 and

91 percent are higher than values obtained by Dubief (1953) for

catchments of slightly higher rainfalls in the Sahara, such as the

Niger or the Chad (p.ll), where values are commonly less than 40

percent. The drainage networks within these three subcatchments can

be regarded as relatively highly effective; however these areas are

considerably smaller than the areas of the catchments studied by

Dubief.

Factors determining Differences in the Ratio of Effective Catchment

Three factors appear to contribute to the differences in the values:

i. Rainfall. There appears to be a direct relationship between

aridity and decreasing values for this Ratio, as indicated by values

obtained in the Sahara by Dubief (1953). Similarly, within the

Georgina-Diamantina catchment, the percentage of effective catchment

area within each subcatchment is seen from Table 4-1 to decrease with mean annual rainfall. However these differences in precipitation are

so small as unlikely to be significant, and there is considerable regional difference in rainfall within each of the subcatchments.

The boundaries of the Georgina and Diamantina subcatchments extend beyond the limit of the arid zone (Fig. 4-3) in the northeast. The uplands of these subcatchments experience average annual rainfalls of Total area Area of Ratio of Average Catchment effective effective annual (km2) drainage catchment rainfall (km^) (%) (cm)

Sandover 45,000 26,500 59 22

Georgina 195,000 156,000 80 24

Diamantina 177,500 161,500 91 25

TABLE 4-1 Areas of subcatchments and rainfalls, for the Georgina-Diamantina system. Fig. 4-3 Rainfall distribution within the Georgina-Diamantina catchment Boundary of arid zone

(after Meigs, 19 53)

Isohyets of mean annual rainfall

Region of catchment with mean annual rainfall exceeding 2 5 cm

Region of catchment with mean annual rainfall less than 25 cm 32.

10 to 20 cm. Not only is the rainfall across the margin of the Barkly

Tableland, the Selwyn Ranges and Isa Highlands in the north of these

subcatchments high, but also reliable, and distinctly seasonal.

Winter rainfall is low with only the eastern part of the Diamantina

subcatchment normally receiving more than 2 cm of rain in any of the

three months July to September, but summer rainfall in both the

Georgina and Diamantina subcatchments is higher, with monthly

averages of up to 30 cm. This rainfall is also intense; Jennings

(1967a) notes an average of more than 1 cm of rain per rain-day

within these upland margins of the two subcatchments. Across both

these subcatchments there is thus a gradient from high, seasonal,

intense rainfall to low rainfall with no distinct seasonality.

The rainfall within the Sandover subcatchment is more uniformly low,

with its northern lowland section receiving an average annual rainfall

in excess of 25 cm, and its upland rim in the northeastern portion

of the Macdonnell Ranges and the southeastern part of the Davenport

Ranges, Northern Territory, to the south and west, an average of 20

to 25 cm. Seasonal variations within this subcatchment are less

pronounced than in those of the Georgina or Diamantina.

While mean annual rainfall figures alone are inadequate to explain

differences in the Ratio of Effective Catchment, it can be seen from

Table 4-1 and from a comparison of Figs. 4-2 and 4-3, that those parts

of the three subcatchments which are effectively drained are those within the areas of higher and more effectively concentrated rainfall.

ii. Terrain. Throughout this study the term "terrain" has been used to include variables which determine landform and surface cover within 33.

subcatchments. These are relief - both overall difference in

elevation and local relief associated with local drainage, geology and structure, and are of considerable importance in determining the proportion of the catchment which is effectively drained.

Throughout the catchment, landsurfaces were mapped using air photo­ graphs, CSIRO land systems reports*, and information obtained from the Relief and Landform Map of Australia (Department of National

Development,1969) as shown in Fig. 4-4 and Table 4-2.

The Georgina-Diamantina catchment overlaps two morphostructural divisions, and the three subcatchments show the gross structural and morphological differences between these. The Georgina and

Diamantina subcatchments are mainly developed within the Interior

Lowlands division, and comprise basin-shaped areas with upland rims and gently sloping inner lowlands. The Sandover subcatchment is developed on the Western Plateau, in which catchments generally consist of widely spaced isolated uplands, separated by broad plains.

Within the Sandover subcatchment, in the northwestern part of the

Georgina-Diamantina catchment, are uplands on weathered granitic rocks and schists of broad fold belts^ cx/\cl folded scWsWes cd-lLe-

T?cxve^povl- , with valleys developed along structure lines, and they support a well-integrated drainage network.

Relief ratio, defined (after Schumm, 1956) as the vertical differ­ ence in height between the highest point on the watershed and the

* Stewart; G.A. et al. (1954). Survey of the Barkly Region, Northern Territory and Queensland, 1947-48. CSIRO Aust. Land Res. Ser., 3^, 182pp; Perry, R.A. et al. (1962). Lands of the Alice Springs Area, Northern Territory, 1956-57. CSIRO Aust. Land Res. Ser., 280pp. Erosional surfaces Depositional surfaces Ratio of Ranges Hills Lowlands Sand Alluvium effective catchment

Sandover subcatc:hment 2 Area (km ) 4,500 5,500 15,000 16,000 4,000 59

Percentage area 10 12 33.5 33.5 9

Georgina subcatc.hment 2 Area (km ) 2,000 19,500 56,000 69,000 48,000 80

Percentage area 1 10 29 35 25

Diamantina subcaitchment 2 Area (km ) 0 5,000 122,500 5,000 45,000 91

Percentage area 0 3 69 3 25

TABLE 4-2 Proportions of each landsurface within each subcatchment of the Georgina-Diamantina system. Fig. 4-4 Landsurfaces and relief in the Georgina-Diamantina catchment

34.

stream exit, related to the horizontal distance along the drainage

axis, has been calculated for each subcatchment*. These values are

shown in Table 4-3. This terrain variable could be expected to

influence the Ratio of Effective Catchment, as breakdown of drainage

and associated deposition of sediments might be expected to occur most readily within catchments of low relief ratio. It can be seen

from Table 4-3 however that the Ratio of Effective Catchment does not vary with relief ratio. It is likely that such a measure is too

coarse to indicate real gradients within the subcatchments, which more directly affect sediment deposition.

Gradients within the two parts of the Sandover subcatchment are very different. Across the upland rim of the catchment a gradient of 1 in

350 occurs, but across the lowland plains the gradient decreases to

1 in 3,000. As occurs throughout the Western Plateau, the erosional uplands form a relatively small proportion of the area of this sub­ catchment and the extensive low-gradient plains are mantled with sand.

These sandplains are the result of sediment deposition from a failing drainage system, in turn contribute to the disintegration of the drainage network. On these sandplains no local runoff is generated, and it is this sand-covered plains sector of the catchment which is not effectively drained.

*A11 height data in this catchment has been obtained from Land System Reports: Christian etal., op.cit.; Perry et al., op.cit.; from spot heights on published maps at scale 1:250,000 Australia Topographic Series: Bedourie, Betoota, Birdsville, Boulia, Brighton Downs, Canterbury, Connemara, Cordillo, Duchess, Elkedra, Gason, Glenormiston, Lake Eyre, Machattie, Mackunda, McKinlay, Mount Whelan, Noolyeana, Pandie Pandie, Poolowanna, Sandover River, Springvale, Urandangi; and at scale 1:1,000,000 International Map of the World Series: Cloncurry, Cooper Creek, Oodnadatta. 1 Difference in Length of Relief ratio Catchment altitude (m) drainage axis (m) ah/l AH L

Sandover 450 500,000 1:1,000

Georgina 200 800,000 1:4,000

Diamantina 320 1,000,000 1:3,000

TABLE 4-3 Relief ratios in the three subcatchments of the Georgina-Diamantina system. 35.

The northern part of the Georgina subcatchment is comprised of

erosional surfaces of low relief developed on the flat-lying Palae-

zoic rocks of the Barkly Tableland. The limestones and sandstones

of the southern margin of the Tableland support a densely textured

network of connected drainage lines, while the dissected Tertiary

swamps within this area give rise to a less dense, but integrated

drainage network which is included within the area of effective

runoff although it does not contribute to the channel network of the

Georgina system. This upland rim of the subcatchment transgresses

the boundary of the morphostructural divisions, and gradients of

1 in 400 occur across the edge of the Barkly Tableland on the margin

of the Western Plateau. Through much of the remainder of the Georgina

subcatchment are a series of concentric zones occurring throughout the

eastern Lake Eyre basin (Mabbutt and Sullivan, 1970:38). This series

comprises erosional uplands forming the rim of the basin, rounded low­

lands within these, and sandy plains with corridors of alluvium becoming more constricted towards the centre of the basin. A well-integrated drainage network persists across the erosional lowland surfaces of this subcatchment, where gradients range from 1 in 2,000 across the upper surfaces of the basin to 1 in 5,000 in the lowest parts.

On the margins of the Simpson Desert in the Georgina subcatchment, remnants of a former drainage network occur in channels following the dune swales, but no effective drainage is generated within this area.

The Diamantina subcatchment is developed entirely within the

Interior Lowlands morphostructural division. The concentric zones of uplands, lowlands and depositional plains which occur within the 36.

Georgina subcatchment also extend through the Diamantina subcatchment.

These erosional uplands and lowlands developed on soft impervious

siltstones and lithic sandstones support a densely textured drainage network. Gradients are similar to those in the lower parts

of the Georgina catchment, varying from 1 in 500 to 1 in 5,000 gradually across the basin. The sandplains and dunefields of the

lowest parts of this subcatchment make up the area which is not effectively drained.

iii. Catchment Size. Two factors operate to increase the likelihood that larger catchments in arid areas will be less effectively drained than smaller catchments. As catchments increase in size the pro­ portion of uplands generally diminishes, and that of gentle slopes, which in arid areas are unproductive of drainage, increases.

Increase in catchment size also renders localised rainfall events relatively less effective in maintaining a drainage network across the area.

Although it is impossible to compare catchments in morphologically different divisions, within any morphostructural division in arid

Australia larger catchments will commonly have proportionally larger lowland plains sectors.

Rainfall records for arid Australia, from their lack of compatibility, indicate that rainfall events are frequently isolated, and localised, and thus in larger catchments are unlikely to produce effective runoff over an extensive catchment surface.

While the Ratio of Effective Catchment for the Georgina and Diamantina subcatchments may reflect differences in the areas of the subcatchments, 37.

the value of 59 percent for the small Sandover subcatchment cannot be related to catchment size. At a broader scale, it is likely that the lower proportion of effectively drained catchment for the

Niger and Chad systems is related to catchment size, as these catchments are respectively four and six times as large as that of the Diamantina.

Within this catchment the proportion of each subcatchment effectively drained is apparently determined by a combination of rainfall and terrain factors. Within both the Georgina and Diamantina subcatch­ ments, the distribution of erosional terrain reinforces the rainfall pattern to influence the proportion of each subcatchment effectively drained. Surfaces on steeper gradients within all three subcatch­ ments give rise to integrated drainage networks, and within the Sandover subcatchment where rainfall distribution does not correspond with the distribution of erosional landsurfaces, the effectively drained parts of the catchment are those on upland surfaces with steep gradients.

The Degree of Channel Connectivity

differential Stages of Channel Breakdown

A study of airphoto mosaics of the entire Georgina-Diamantina catchment was carried out to determine the stages of channel breakdown as defined in Chapter 2. Areas of indeterminate channel sections were examined using airphotographs, and these stages of breakdown are shown in Fig. 4-5. The potential baselevel of the Georgina-Diamantina system is Lake Eyre, and stages of breakdown within the channel system are considered relative to this baselevel. Fig. 4-5 Stages of channel breakdown in the Georgina-Diamantina drainage system 100 Km

Numbers (iii) refer to stcge of breakdown of channel

network

8 th ond higher order channels shown 38.

The channel system of the Diamantina is generally well organised,

and shows a high degree of connectivity, with few upland tributaries

flooding out before reaching the trunk channel. The Diamantina

River follows its potential topographic course through to baselevel -

to enter Lake Eyre as the Warburton River - and thus shows no dis­

organisation. This physiographic evidence of connectivity is con­

firmed by hydrological data, as flows in the Diamantina periodically

reach Lake Eyre. Although the trunk channel follows an unbroken

course through to baselevel, two major subsystems, the Georgina and

Sandover systems, fail to connect with the Diamantina network and

although the system does not break down along the trunk channel it

is in a marginal state of disorganisation.

Stage (ii) of disorganisation has been assigned to the Georgina and

Sandover Rivers where breaks occur in the lower courses of the trunk

channels, and their former courses are indicated by pans and vegetated

alluvial tracts. Although these tributaries show the same stage of

disorganisation, the breaks in the Sandover system are more extensive,

and the connectivity appears more tenuous.

The Georgina River drains southwards from the Barkly Tableland and

Isa Highlands with a wide sandy channel form, which gives way across

the flatter-lying lowlands sector to an anastomosing network of channels within a floodplain up to 15 km wide. As occurs in the

Diamantina system, a small number of potential tributaries fail to connect with the main channel system. In its lowest sector this anastomosing channel bifurcates and reunites in a deltoid form

(Whitehouse, 1944) developed in a tectonic basin east of Bedourie

(Fig. 4-6). A system of distributary channels and discontinuous Fig. 4-6 The drainage terminal of the Georgina River Bedourie#

Lake 'Machattie

Bit pa More a. Clay pan)

Birdsville

Simpson Desert

Dune direction rgoon Stream flooding out

alluvial flat

20km 39.

alluvial flats follows the south-southeasterly dune trend for 35 km

to Lake Machattie, the present pan terminal of the Georgina.

Beyond Lake Machattie two former courses (Fig. 4-6) linking the

Georgina with the Diamantina can be identified. A chain of vegetated alluvial flats, interspersed with low sand dunes continues for 40 km to the south-southeast towards the Diamantina. A second discon­ tinuous overflow course linking these catchments can also be

identified from airphotographs. It consists of a closely spaced chain of pans associated with elongate patches of denser vegetation extending for 100 km in a corridor through low sand dunes, and leading southwestwards from the entrance of Lake Machattie to Eyre

Creek, which in turn is tributary to the Warburton River at the point where the Diamantina becomes the Warburton (Fig. 4-6). There is no record of floods in the Georgina reaching the Diamantina.

There is a similar discontinuous course linking the Sandover River system with the Georgina River (Fig. 4-7). The Sandover River is a wide sandy channel draining from the Macdonnell and Davenport Ranges onto a flat sandy plain with some low dunes. Several upland tri­ butary systems fail to connect with the Sandover channel, the most persistent of these, the Elkedra River, flooding out about 50 km onto the plain. The Sandover channel narrows, approximately 40 km across the sandy plain, and breaks down in a floodout zone as a series of distributary channels and alluvial flats, terminating at a series of pans, Winnecke Waterholes.

Airphoto evidence from the floodout zone and pan terminal of the

Sandover River indicate that Bybby Creek, the extension of its course, Fig. 4-7 The drainage terminal of the Sandover River CO H— 40.

is barely discernible as sparsely separated patches of denser vegetation in low-lying tracts within the sandplain extending approximately 50 km beyond the present floodout zone until this drainage is rejuvenated in crossing an erosional plain surface 30 km from the Georgina.

The breakdown of the channel system, where tributary drainage no longer continues to build up the network, can be seen from Fig. 4-8.

In a developing system the Laws of Catchment Area and Stream Length

(Horton, 1945) indicate that contributory drainage area should increase directly with stream length. This can be seen to occur for the trunk channels of all three subcatchments within the Georgina-Diamantina system, until a point at which the contributing catchment area no longer increases proportionately with the length of the channel. This is typical of drainage systems within the arid zone. In each case this occurs on the lowlands of the subcatchment. Here the trunk channel persists then gives way to a distributary system, in which deposition of load reflects the fact that the ratio of sediment load to discharge has increased. In terms of contributory catchment, the

Georgina, and particularly the Sandover, channels are further from an ideal system than the Diamantina, reflecting a higher degree of channel breakdown than occurs for the Diamantina.

Within the Georgina-Diamantina system, although the trunk channel, the Diamantina, reaches its topographic baselevel, it is apparent that the system is beginning to disintegrate. The major tributary drainage from the western side of the catchment does not reach base- level, and the processes in the breakdown of this marginally dis­ organised system can be considered by studying the changes in Fig. 4-8 Graph showing contributory catchment area to channel length for the Sandover, Georgina and Diamantina Rivers Area of catchment (’OOOkm Distance

500 from headwaters

(km) IOOO 41.

floodplain and channel form in the Diamantina River, and by comparing this River with the Georgina River, which fails to reach its topo­ graphic baselevel.

Success and Failure in Channel Country River Systems: The Cases of the Diamantina and Georgina i. Down Valley Changes in Channel and Floodplain Form. The Diamantina and Georgina Rivers are typical of rivers of the Interior Lowlands division. Changes in their channels and floodplains involving a decrease in dimensions as alluviation occurs in a downstream direction, are seen to occur in all rivers within this division. Although both the Diamantina and Georgina Rivers show similar channel and floodplain characteristics, the Diamantina channel course can be seen through to its terminal in Lake Eyre, while that of the Georgina fails to be maintained. As some limited hydrologic data are available for the

Diamantina, and some comparable data available for Coopers Creek, there is an opportunity to supplement physiographic evidence of fluvial activity within these systems.

The Diamantina and Georgina, like other rivers of this catchment are braided across sandy plains or occupy single incised channels on more undulating erosional surfaces on the upper slopes of the catchment.

Below this these channels are wide - 100 to 200 m - and sandy, locally braiding and with low sinuosities. These are apparently sand-bed channels (following the classification of Schumm, 1963). Tributary drainage is well developed in each of these systems. Floodplain gradients in this reach vary between 1 in 300 and 1 in 2,000 but generally around 1 in 500, as calculated from spot heights on published maps at scales of 1:250,000 and 1:1,000,000. As the upper course of 42.

the Diamantina runs parallel with the local trend of the warp axes

of the Eromanga Basin, its course is not deflected, while that of

the Georgina is diverted slightly to the east approximately 200 km

above its terminal by the axis of an anticline trending east-west.

This diversion however appears to have no effect on the form of the channel or floodplain of the Georgina, which remain similar to those

of the Diamantina.

Across the lower sector or Channel Country however, the sandy bedload form of the channels changes to a mixed-load or suspended-load form

(Schumm, 1963). The channels anastomose, and the floodplain widths increase to exceed 20 km. Gradients over these alluvial plains are low, averaging 1 in 5,000 (Whitehouse, 1948:14), mainly the result of alluviation suggested to be caused largely through tectonic subsidence

(Whitehouse, 1944) or back-tilting (Mabbutt, 1967). The features of these lower parts of the Diamantina and Georgina courses have been described from photomaps and airphotographs, and are shown in Plate

4-1 and Fig. 4-9. The channels are narrow 10-50 m, and either strongly sinuous, or less sinuous and strongly anastomosing. Levees are well developed in the anastomosing sectors, indicating bedload deposition, and channel tracts appear to be raised above the general floodplain level probably through bedload accretion. Floodways, many with braided courses are superimposed across this anastomosing channel system, and probably indicate the more direct bankful1 floodwater course. These features are very similar to the shallow mixed-load floodways of Coopers Creek which have been described by Mabbutt (1967).

Many of the channels are distributary and lead to sump basins, which although they may return a little water to the channel system during the drying phase after flooding (Bonython, 1971) serve mainly to hold water away from the main channel system. Fig. 4-9 Features of the floodplain of the Diamantina River, through the Channel Country, approximately 250 km north of Goyder's Lagoon Braided floodway Sinuous channels Less sinuous anastomosing channels

Distributaries leading into sumps Plate 4-1 Vertical air photograph of the Diamantina River across the Channel Country showing the broad floodplain with sinuous and anastomosing channels, billabongs and sand islands.

This photograph has been made available by courtesy of the Director 3 Division of national Mapping3 Department of Minerals and Energy3 Canberra.

43.

These channel and floodplain features are similar to those occurring throughout the Channel Country e.g. as described for Coopers Creek by Whitehouse (1944) and Mabbutt (1967), and are brought about by a combination of three main factors. The tectonic setting is of con­ siderable importance in the development of these floodplains.

Gradients across the region are low, because of back-tilting, and within the Lake Eyre Basin, tectonic depressions with blocking anticlines have the effect of ponding back or diverting drainage

(Mabbutt, 1967). Stream discharges are high as a result of seasonally high rainfalls in the upper catchments, and the soft impervious rocks provide a considerable source of sediment, which can be transported into this zone.

Beyond this anastomosing zone, the channel of the Georgina River fails and the combination of the distributary drainage and high in­ filtration and evaporation losses in the lowest part of the catchment apparently results in a sharp decrease in the flood discharge of the

Diamantina downstream, as noted for instance by Bonython (1963).

Beyond this main zone of anastomosing channels, south of the confluence with Eyre Creek (Fig. 4-6) the whole Diamantina floodplain, and particularly the backplains of fine-textured alluvium diminish in extent, the floodplain narrowing to 100-200 m. Areas of dune sand increase downstream. The southwesterly directed channel beyond this point is barely maintained against sand dunes encroaching from the south-southeast and a river narrowing to 50 m and comprising a single then a bifurcating channel persists to enter Lake Eyre as the

Warburton River (Fig. 4-6). 44.

ii. Particular Conditions of Success and Failure. The success or failure of these channels to reach baselevel as a connected system depends on a combination of several factors.

Flood size is important in maintaining a channel through to baselevel.

Estimated flood volumes reported by Bonython (1963) indicate extremely 9 high discharges, up to 14 x 10 cubic metres for the volume of a 9 flood in Coopers Creek, and 3.5 x 10 cubic metres for a flood in the

Diamantina. From similar rainfall patterns and catchment sizes, it is likely that floods in the Georgina River would be of similar volume to those in the Diamantina. The larger catchment and con­ sequently higher discharge in Coopers Creek apparently ensure its success in reaching baselevel, despite being less favourably placed for other variables.

The relationship between tectonic axes and channel direction is also important in determining a successful course through to baselevel.

The trends of the fold axes within the Eromanga Basin generally run slightly west to slightly east of north, or in an east-westerly direction. The upland expression of these axes in part determines the courses of channels through the Basin. Channels directed approxi­ mately north-south are more favourably placed, since there is little conflict between thier courses and the north-northwesterly trend of sand dunes towards the western part of the Basin. Channels which are diverted in a westerly direction, however, must maintain this course against the obstruction of wind-blown sand. Tectonic depressions, often with blocking anticlines running east-west, contribute to the failure of channels in this division. While the Diamantina River runs parallel with the fold axes through this part of the Eromanga Basin, 45.

until its course is diverted slightly into Goyder's Lagoon, the course of the Georgina is directed to a greater extent by tectonic axes. The broad, anastomosing course leads to a lower basin in which the deltoid of the Georgina occurs (Fig. 4-6), and within this area water is diverted into sumps and broad pans, and sand "islands" occur which apparently indicate the deposition of channel load.

The channel which passes to Lake Machattie beyond this deltoid is very small, mainly consisting of distributary channels and alluvial flats, and representing a diminished discharge, apparently insuf­ ficient to maintain a connected course beyond Lake Machattie.

Although the difference between success and failure in reaching baselevel as a connected system can be ascribed to differences in tectonic controls between the Diamantina and Georgina systems, a combination of factors contributes to the marginal success of Coopers

Creek in maintaining such a course.

Coopers Creek drains a larger catchment than the Diamantina or

Georgina Rivers (in excess of 300,000 km ) , and its flood discharge is considerably greater (p.44). However it is directed towards Lake

Eyre over a long course from the east, and a combination of this direction, tectonic constraints and the choking effect of sand dunes makes it hydrologically less successful than the Diamantina in reaching Lake Eyre. Irregular wet-season flows reaching the margin of Lake Eyre, on an average once in two years (Bonython, 1963), maintain the Diamantina channel through to baselevel. Extreme floods, 9 of an average discharge of 10 cubic metres, clean out channels choked by sand and re-establish the channel form. It has been noted (Nimmo,

1947) that flows in Coopers Creek commonly reach Lake Eyre only after 46.

two successive wet seasons, thus approximately once in three to ten years (Whitehouse, 1941 and 1948; Nimmo, 1947). Flows of the first of these sweep the channel clear of sand, enabling flows of the second season to reach the Lake. Despite no significant differ­ ence in the rainfall in the two catchments, flows in the Diamantina, where the course is almost parallel with the regional dune trend, more commonly reach Lake Eyre than those in Coopers Creek where north-northwesterly dunes trend directly across the channel path.

Flood records suggest that discharge continues to decrease through the lower channel sector. Bonython (1963) for example, notes that a flood volume of 430 million cubic metres (0.35 million acre feet) at Birdsville will barely reach Lake Eyre. This represents a de­ crease in discharge of approximately 185 cumsecs over 325 km, in the time generally taken (about 27 days) for water to travel between

Birdsville and Lake Eyre, i.e. a loss of about 0.5 percent per kilometre. Similar decrease in discharge downstream is shown for a major flood in Coopers Creek in 1963 (Fig. 4-10), also reported by

Bonython.

Tectonic lows lead to the existence of such "overflow" lakes and sump basins as Lakes Yamma Yamma and Hope in the broad floodplain of the Channel Country of Coopers Creek. These act as terminals for all but the most extreme floods and in instances where water passes through, appreciably halt its passage towards Lake Eyre (Bonython,

1963). As noted by Bonython (1960), these large distributary features are generally absent from the Diamantina floodplain, where sump basins are mainly small, and do not withdraw appreciable amounts of water from the main channel flow. Goyder's Lagoon, a depression determined by fold axes, at the zone where the Diamantina River Fig. 4-10 Decrease in the flood discharge of Coopers Creek, after Bonython (1963) \

500 1000 1500 Distance from headwaters (km) 47.

becomes known as the Warburton River does act as a sump, in slowing the passage of floodwaters through the channel (Bonython, 1963), but unlike the large lakes of the Coopers Creek system, it is directly in the channel line, and water passes through it rather than being drawn off laterally from the system.

The Comparison of Actual and Potential Channel Lengths

In applying this method of measurement of drainage disorganisation which is based on the difference between the length of channels now existing and the potential length of channels in the complete network which might have occupied the catchment (see Chapter 2), the three subcatchments have been considered separately, and then in combination.

Channel lengths for all streams in the actual network which occupies the three subcatchments were compared with those of the potential network. For fourth and higher order streams the actual channel lengths were measured from photo mosaics at a scale of 1:50,000 or from maps at a scale of 1:250,000. Three sample catchments in each of the three main subcatchments were used to measure bifurcation ratios and segment length ratios (after Strahler, 1964) and from these the numbers and lengths of lower order segments were estimated.

The reconstruction of the potential drainage network was made by con­ necting all floodout terminals and the trunk channels of isolated upland catchment systems to the main channel system, and the same method of measurement applied as with the actual streams. The numbers and lengths of lower order streams which would have 48.

theoretically been supported by this network were also calculated using bifurcation and length ratios.

The results of these comparisons are listed in Tables 4-4 and 4-5, and shown in Fig. 4-11.

i. The Diamantina Channel Network. The Diamantina reaches its maximum order of ten within its upland catchment, and although it continues to receive eighth and ninth order tributaries across the erosional country in the north of the catchment, very few low-order local tributaries enter the lower reaches of the trunk channel. There are few isolated upland catchments flooding out within this system, and there is little difference between the actual and potential channel lengths within the system delimited by the subcatchment boundaries, indicating that this system is not highly disorganised.

From Table 4-4 it can be seen that a very low proportion of low order streams is missing from the actual network, indicating that almost all of the potential catchment is generating drainage. An increase in the proportion of higher order channels missing indicates that what failure occurs in this network is in higher order streams not connecting with the trunk drainage. Twenty percent of the potential channel length is missing (Table 4-5), mainly from the lowland areas.

ii. The Georgina Channel Network. The Georgina River is also a tenth order channel, reaching this order within the upland sector of the subcatchment. A small number of upland drainage systems flood out before reaching the trunk network, and it can be seen from Table 4-5 that there is a 35 percent difference between actual and potential GEORGINA-DIAMANTINA SYSTEM, QLD

Theoretical Actual Percentage Order Average Average of Streams No. of Total Segment Cumulative No. of Total Segment Cumulative Missing From Segments Length Length(L) (L) Segments Length Length(L) (L) Theoretical Network

Sandov er System 1 50275 70385 1.4 1.4 34200 37620 1.1 1.1 32 2 13878 34695 2.5 3.9 9800 19600 2.0 3.1 29 3 3084 13878 4.5 8.4 2790 9765 3.5 6.6 10 4 930 5208 5.6 14.0 780 3900 5.0 11.6 16 5 270 2268 8.4 22.4 250 2000 8.0 19.6 7 6 66 1320 20.0 42.4 64 1152 18.0 37.6 3 7 16 510 32.0 74.4 14 400 28.4 65.0 12 8 4 600 150.0 224.4 2 420 210.0 275.0 50 9 1 480 480.0 704.4 1 90 90.0 365.0 -

Georgi na System 1 647245 906143 1.4 1.4 589140 648054 1.1 1.1 9 2 130649 509531 2.5 3.9 127182 254364 2.0 3.1 3 3 31033 139649 4.5 8.4 27450 96093 3.5 6.6 12 4 6603 36977 5.6 14.0 5860 29350 5.0 11.6 11 5 1430 12012 8.6 22.6 1210 9680 8.0 19.6 15 6 324 6285 19.4 42.0 256 4608 18.0 37.6 21 7 76 1780 23.5 65.5 56 1070 19.3 56.9 26 8 18 600 33.3 98.8 14 425 30.4 87.3 22 9 6 620 10.3 109.1 4 300 77.0 164.3 33 ’ 10 1 620 620.0 729.1 1 520 520.0 684.3 -

Diaman tina System 1 258248 361547 1.4 1.4 245960 295152 1.2 1.2 5 2 63900 159750 2.5 3.9 61440 122800 2.0 3.2 4 3 15974 60701 3.8 7.7 15360 46080 3.0 6.2 4 4 3968 19046 4.8 12.5 3840 17280 4.5 10.7 4 5 1008 8467 8.4 20.9 960 7680 8.0 18.7 3 6 260 5070 19.5 40.4 240 4320 18.0 36.7 8 7 76 1710 22.5 62.9 60 1460 24.3 61.0 21 8 19 1170 61.6 124.5 16 900 56.3 117.3 16 9 4 230 57.5 182.0 3 180 60.0 117.3 25 10 1 330 330.0 512.0 1 750 750.0 927.3 -

Total System 1 955768 1338075 1.4 1.4 869300 980826 1.1 1.1 9 2 208427 703976 2.5 3.9 198422 396764 2.0 3.1 5 3 50090 214227 4.3 8.2 45600 151938 3.3 6.4 9 4 11500 61231 5.3 13.5 10480 50426 4.8 11.2 9 5 2700 22747 8.4 21.9 2420 19360 8.0 19.2 10 6 650 12675 19.5 41.4 560 10080 18.0 37.2 14 7 168 4000 23.8 65.2 130 2930 22.5 59.7 23 8 39 2370 60.8 126.9 32 1745 54.5 114.2 18 9 11 1030 93.6 219.6 8 570 71.3 185.5 27 10 2 950 475.0 694.6 2 1270 635.0 820.5 . - 11 1 420 420.0 1114.6 0 - - 820.5 100

TABLE 4-4 Actual and potential channel numbers and segment lengths in the Georgina-Diamantina catchment bo bo oj C

O *H theoretical

the

from

missing

lengths system.

oj C -t-> -H

CJ *H segments

Georgina-Diamantina channel

of the

of

Percentage network

4-5

TABLE Fig. 4-11 Graph of actual and theoretical cumulative mean segment lengths for the Georgina-Diamantina channel systems ------Cumulotive meon segment length (km) Cumulative mean segment length (Km) -500 -1000 HOOO -100 -500

• *

Actual Theoretical stream stream Diamantina system Sandover system

order

order -100 -500 rIOOO -100 -500 Entire Diamantina stream

stream G.eorgina-

Georgina system order

system order

* • 49.

lengths within this system, which is more disorganised than that of

the Diamantina.

From Table 4-4 it can be seen that a higher proportion of low order

streams are missing from the network than from that of the Diamantina.

This indicates that a higher proportion of the catchment is not

generating drainage, however, as with the Diamantina, most of the

failure of drainage occurs in the trunk systems.

iii. The Sandover Channel network. This system can be seen (Fig. 4-11)

to be more disorganised than either the Georgina or Diamantina

systems. A greater proportion of potential channel length is

missing from this system (Table 4-4 ; 4-5).

It can also be seen (Table 4-4) that failure of drainage occurs

throughout this system. A high proportion of low order channels

is missing from the network, reflecting the fact that a high proportion

of the catchment does not contribute effective runoff. Failure also occurs as higher order streams flood out before reaching the con­ nected network.

iv. The Combined Catchment. This, in fact, can be expressed as the actual and potential channel network of the Diamantina River, and as such takes into account the breaks between the Sandover system and the Georgina, and the Georgina system and the Diamantina, which contribute considerably to the difference between actual and potential channel lengths. Although such a view of the catchment as a whole does not indicate differences between the two morphostructural divisions represented, it better reflects the actual degree of 50.

disorganisation as measured by this method, and which is obscured in considering subcatchments separately.

Reconstruction of the potential drainage network within the Georgina-

Diamantina catchment is considered to be accurate in the upper reaches. On the depositional plains, particularly in the areas now covered by sand, it is not possible to determine the accuracy of this reconstruction, as pre-existing drainage is totally obscured.

However as this is a relatively small proportion of the Georgina and

Diamantina subcatchments, and one in which it is unlikely that channels would ever have existed, the reconstruction of the potential channel system is generally reliable. This method of measuring drainage disorganisation is then most useful for catchments such as those developed within the Interior Lowlands, where much of the catchment is comprised of erosional landsurfaces. It is less likely to be as useful for catchments such as that of the Sandover, developed on the ranges and broad lowlands of the Western Plateau, where a higher proportion of the catchment consists of surfaces now masked by sand or alluvium.

A Comparison of the Status of Drainage Disorganisation as Indicated by the Three Measures

Within the Georgina-Diamantina catchment, the three measures which have been applied indicate that the system is not highly disorganised.

Each of these measures also shows that the Sandover subcatchment is more highly disorganised than either of the other two subcatchments. 51.

Two forms of drainage disorganisation occur within this catchment.

In the Sandover system, a large proportion of the catchment does not

generate drainage. Networks of channels cluster in upland areas, and

few trunk channels reach the main artery. In the Georgina and

Diamantina systems, a high proportion of the catchment surfaces

generate channel networks, but breaks occur in the trunk systems.

This failure of drainage is greater for the Georgina than for the

Diamantina network. The three measures reflect these differences in

disorganisation in different ways.

i. The Ratio of Effective Catchment. This measure indicates the

proportion of a catchment which generates a drainage network. It

reflects the disorganisation of drainage in the Sandover system, but

although it indicates a greater degree of disorganisation for the

Georgina than for the Diamantina, it does not indicate the success

or otherwise of drainage.

ii. The Degree of Connectivity. This measure concentrates on the

ability of the system to maintain a connected channel through to

the potential base level. By this criterion, the Diamantina system

is shown to be an organised drainage system, while the Georgina and

Sandover systems display the same degree of disorganisation.

Although it indicates the success of drainage, and the form of

breakdown of the trunk channel, the method does not show similarity

or difference in the general form of drainage.

iii. The Comparison of Actual and Potential Stream Lengths. This measure attempts to demonstrate how much of the ideal drainage network

is missing. It also shows the Diamantina system to be most highly 52.

organised, and the Sandover system least so, and when applied to the whole system it shows a significant degree of disorganisation.

This method does indicate to some extent both the paUevvi and the

decyr-e-e of di5orcywfSaA\o\n . 53.

CHAPTER 5

THE FINKE - MACUMBA SYSTEM

The Finke-Macumba river system drains the western slopes of the

Lake Eyre catchment (Fig. 5-1), and the topographic catchment overlaps

two main morphostructural divisions of the continent. In the northwest

it forms part of the Western Plateau, and shows features typical of

central Australian section of that division, with isolated uplands

and extensive lowlands with a widespread cover of sandplain or dunes.

The southeastern part occupied by the lower Finke and the Macumba

Rivers is part of the Interior Lowlands, and the relief and drainage

patterns are typical of the structurally younger topographic basin of

Lake Eyre. These relief contrasts have apparently resulted in dif­

fering patterns of drainage disorganisation (Mabbutt, 1973), which

are investigated in the analyses that follow.

The Proportion of the Catchment Hydrologically Effective

The Topographic Catchment

i. The Eastern Limit. In the northwest of the Simpson Desert the

extension of sand dunes has deflected southward the river channels which formerly headed southeast including the Finke, which now follows the approximate western limit of the dunes (Fig. 5-2). It has been necessary to reconstruct the obliterated drainage patterns in order to define the eastern limit of the Finke catchment.

Fig. 5-2 is a suggested reconstruction of the pre-dune topography and drainage based on spot heights from the desert floor, including Fig. 5-1 The drainage network and the topographic catchment of the Finke-Macumba system 3 W

OD Q CEL Fig. 5-2 Postulated reconstruction of former drainage associated with the Finke River system OLD.

Finke Poeppels f \ Corner y. Birdsville ___ ©-----

Warburton

Lake

Approximate boundary of Peera Peera sandridges (after Madigan 1945) Poolonna Lake Warranidirinna Lake Direction of longitudinal dunes Areas of ephemeral Established topographically lakes parallel to low points regional dune trend Present river courses

100 km Suggested former drainage

lines 54.

barometer heights established by Madigan (1945) in his expedition

across the Desert, spot heights from helicopter altimeter traverses

for the Division of National Mapping published on 1:250,000 map sheets*,

and heights from a line of levels through the Desert surveyed by the

Geodetic Branch, Division of National Mapping (unpublished field data).

Approximate contours were interpolated and river courses were drawn

through low points at right angles to them. The drainage lines were

continued through two playas, Peera Peera Poolanna Lake, south of

Poeppel's Corner, and Waranidirinna Lake, further south (Fig. 5-2),

since the east-west alignment of these playas, at variance to the

regional dune trend, indicates that they may occupy positions on

former drainage lines.

The reconstructed contours indicate that the Finke, Todd, Hale, Plenty,

Hay and Field Rivers formerly continued southeast across the northern

Simpson Desert, in prolongation of their courses upstream from the

Desert margin. They are shown as tributary to a north-south trunk

stream following a course slightly west of the present Mulligan River and directed towards a low point just east of Poeppel's Corner. It

is suggested that the Finke did not join the Macumba at this time, but linked with the trunk stream near its confluence with the Diamantina, south of Poeppel's Corner. As these postulated courses transgress the northern boundary of the Tertiary Lake Dieri suggested by David and Browne (1950), they must be younger.

In the northeast and east the boundary of the Finke catchment has been placed at the secondary divide between the postulated former courses

*Australia, Department of National Development, Division of National Mapping - Australia 1: 250,000 series sheets: Dalhousie, Hale River, McDills, Noolyeana, Pandie Pandie, Poolowanna, Simpson Desert North, Simpson Desert South. 55.

of the Finke and Todd Rivers, and in the southeast it has been drawn

along the divide between the reconstructed Finke and Warburton Rivers.

The Macumba is considered to have been deflected southwards by the

extending dunes and a former course has been postulated across the

northern anabranch of the Warburton River; the southern boundary of

the catchment has accordingly been placed along the divide between

the present Macumba and Warburton Rivers.

ii. The Northwestern Limit. The boundary of the catchment is clearly

defined in the north and northwest, where the drainage divide runs

along the Macdonnell, James and George Gill Ranges. However the

western boundary is poorly defined. From spot heights on the pub­

lished 1:250,000 sheets Bloods Range, Lake Amadeus, Mt. Liebig, and

Mt. Rennie and the 1:1,000,000 Lake Mackay and Petermann Ranges

sheets (Australia, Department of National Development, Division of

National Mapping), unpublished heights from the Lands and Surveys

Department, Western Australia, and from a study of airphotographs of

the western part of the system; the catchment boundary has been

placed west of that shown on the published map of Drainage Divisions

and River Basins (Department of National Development, 1966) to include

Lakes Amadeus, Neale, Hopkins and Macdonald (Fig. 5-1). No solid

relief separates the line of dune-ringed pans which occupy the east-

west axis of this catchment from the present Finke River. The areas

of the reconstructed topographic catchment and of its two subcatch­ ments are given in Table 5-1.

The Ratio of Effective Catchment

Areas of effective drainage, that is those generating drainage networks, were mapped from 1:250,000 airphoto mosaics, and these areas are shown Area of Area with Ratio of Region topographic ^ organised effective catchment (km ) drainage (km ) catchment (%)

Total 318,000 n a. n.cc. catchment

Fi nkp 262,000 94,000 36 subcatchment

Macumba 56,000 44,000 80 subcatchment

TABLE 5-1 Areas of topographic and effective catchments for the Finke and Macumba systems. 56.

in Fig. 5-3. These areas of effective runoff were compared with the

topographic catchment areas, and the Ratios of Effective Catchment

are shown in Table 5-1.

In the Finke subcatchment the Ratio of Effective Catchment is 36

percent, and by this criterion the Macumba subcatchment, with 80

percent of its area effectively drained, is more highly organised.

The value for the Finke subcatchment is similar to that of 40 percent

obtained by Dubief (1953) for the Chad catchment in the Sahara, but

this similarity is probably the result of compensating effects of the

factors of rainfall and catchment size. Rainfall in the Chad catchment

is considerably higher than that in the Finke subcatchment, which

should give a higher Ratio of Effective Catchment, however the area

of the Chad catchment is approximately ten times that of the Finke, and

includes a high proportion of unproductive sandy lowland.

Factors Determining Differences in the Ratio of Effective Catchment

i. Rainfall. Average annual rainfalls within the Finke and Macumba subcatchments have been calculated from isohyetal maps*. For the

Finke subcatchment the average annual rainfall is 16 cm; however there is a distinct rainfall gradient across this area. The Macdonnell

Ranges in the north of the subcatchment receive up to 30 cm annually, with a distinct summer maximum, whereas the plains to the south receive less than 12 cm, with less marked seasonal variation (Slatyer, 1962).

Within the Macumba subcatchment the average annual rainfall is 12.5 cm, with no marked gradient or distinct seasonality. Throughout the entire catchment rainfall is highly variable.

*Dept. of Nat. Devel., Atlas of Australian Resources Rainfall map. 2nd edit. 1970; Slatyer, R.O. (1962) Climate of the Alice Springs area. Part III in Lands of the Alice Springs Area, Northern Territory, 1956-57. CSIRO Aust. Land Res. Ser., 6, 109-28. Fig. 5-3 Effectively drained areas of the Finke and Macumba subcatchments Lake 57.

Although average annual rainfall within the Macumba subcatchment is

less than the rainfall within the Finke subcatchment, the Ratio of

Effective Catchment is considerably higher and thus amount of

rainfall apparently does not determine the Ratio of Effective

Catchment.

ii. Terrain. The terrain types in the catchments as distinguished

by landforms and superficial deposits are shown in Fig. 5-4 and

related areas given in Table 5-2. The contrasting patterns within

the two subcatchments reflect the differing morphostructural settings

previously referred to.

The northern part of the Finke River system is typical of the drainage

of the central Australian part of the Western Plateau. Uplands are

restricted in extent and widely spaced. The Macdonnell and Krichauff

Ranges in the northern part of the catchment are broad belts of

ranges, generally comprising a series of narrow ridges. The gradient

across this northern upland section, to as far south as Henbury

(Fig. 5-7) is approximately 1 in 400 (from Perry et al., 1962), and

these uplands generate most of the drainage network of the subcatchment

supporting a densely textured network of channels. In the western part

of the Finke subcatchment, a narrow upland rim, mainly the Ellis,

Rawlinson and Petermann Ranges, gives rise to limited systems of

upland drainage.

Systems of integrated drainage rising in the uplands tend to break down in floodouts on the lowland slopes, or to lead to playas in the

lowest parts. The gradient across the lowland surfaces south of

Henbury varies between 1 in 1,200 and 1 in 1,600, and little drainage Erosional Depositional surfaces surfaces Ratio of Ranges: *Hills Lowlands: effective relief and relief greater Tablelands: less Sand Alluvium catchment than relief than 100 m 30-100 m 30 m

Finke subcatchment 2 Area (km ) 42,000 12,000 15,000 18,000 13,000

Percentage area 16 4.5 5.5 69 5 36%

Macumt>a subcatchment 2 Area (km ) 2,000 4,000 30,000 10,000 10,000

Percentage area 3.5 7.5 53 18 18 80%

* Mainly rounded hills in the Finke subcatchment, and extensive tablelands in the Macumba subcatchment.

TABLE 5-2 Proportion of each landsurface within the Finke-Macumba catchment. Fig. 5-4 Landsurfaces and relief in the Finke-Macumba catchment Alluvium or 100 km evaporites 1------58.

is generated across this area. In the lowest parts of the Finke subcatchment are extensive areas covered with sand. In the east the

Simpson Desert on the gently sloping surface of the Eromanga Basin consists of parallel sand dunes aligned in a south-southeast-north- northwest direction. The course of the Finke River follows the western margin of the Desert, and the present drainage entering the

Desert comprises streams trending parallel in their lower courses with the longitudinal dunes, and flooding out on the low gradient surfaces before reaching Lake Eyre. In the western part of the Finke subcatchment systems of integrated upland drainage are completely obliterated across the broad lowland sector where the gradient falls

to 1 in 2,000.

The Macumba River system and the lowest part of the Finke system are developed on silcrete-capped tablelands and extensive stony plains within the Eromanga Basin structure. Only the easternmost part of

the subcatchment within the Musgrave Range consists of ridge-like terrain. This drainage is better organised throughout than that of

the main part of the Finke system for two main reasons. Firstly the

proportion of the catchment which is developed on erosional land-

surfaces is 64 percent, with only 18 percent of the Macumba subcatch­ ment occupied by sand-mantled lowland surfaces (Table 5-2 and

Fig. 5-4). The rocks underlying these erosional surfaces are flat-

lying or very gently dipping and hence largely homogeneous, weak and

impervious, and thus give rise to well-developed drainage networks

throughout that part of the catchment. The lowest part of the Macumba

subcatchment consists of alluvial plains, and that part of the sub­

catchment which does not support a network of drainage is the dunefield

on the low-lying surface between the Hamilton and Alberga Rivers (Fig.

5-2). 59.

Secondly the relatively steep fall of the Macumba enables the drainage network to be maintained. The tectonic basin of Lake

Eyre reinforces the structure of the Eromanga Basin, and results in a centripetal drainage pattern. The Macumba system makes up part of this centripetal drainage, developed on a broadly uniformly sloping surface. Lake Eyre lies towards the western side of its asymmetrical basin (Wopfner and Twidale, 1967) and consequently the average fall of the Macumba subcatchment is relatively steep at 1 in 550.

It can be seen from a comparison of Fig. 5-3 and 5-4 that the part of each subcatchment not occupied by an effective drainage network is that part consisting of sand-covered lowland surfaces, where channels are choked and obliterated. These areas are more extensive within the Finke subcatchment.

In terms of relief distribution and values of the Ratio of Effective catchment, the Finke subcatchment is similar to that of the Sandover

(Chapter 4), also developed on the Western Plateau of the continent, and the Macumba subcatchment is similar to the Georgina or Diamantina subcatchments, these latter both also developed within the Interior

Lowlands division. The distribution of relief, and the relative proportions of landform types are similar throughout each morpho- structural division, and are the factors which determine the effective drainage within catchments.

iii. Catchment Size. This factor appears to influence the proportion of the catchments effectively drained. The area of the Finke subcatch ment is four times that of the Macumba subcatchment, and it can be seen from Table 5-2 that a very high proportion of this difference in 60.

area comprises depositional lowland surfaces within the Finke

subcatchment.

The Degree of Channel Connectivity

Differential Stages of Channel Breakdown

The Finke and the Macumba Rivers would have been tributaries to a

common trunk channel in the reconstructed drainage system suggested

above. However disorganisation of that postulated system has produced

a new vestigial network. While it represents a stage in the breakdown

of a more extensive network, the present Finke-Macumba system is

itself undergoing further disintegration. As the channel course which the Finke now follows is an established course, this study of

the breakdown of connectivity of the channel system is based on the

contemporary courses of the Rivers, in which the Macumba is potentially

tributary to the Finke.

Links between channel systems have been examined and classified by the system described on p.18, using airphotographs and maps of the entire catchment. The stages of breakdown of the three main channel systems are shown in Fig. 5-5.

i. The Macumba River follows a connected course through to its baselevel in Lake Eyre, and by this measure is not disorganised.

ii. The Finke has been assigned a stage of breakdown of (iii) in that the potential course of the River is not followed beyond the floodout. It can be seen that in many reaches above this point Fig. 5-5 Stages of channel breakdown in the Finke-Macumba drainage system 4 4 61.

continuing processes such as dune extension tend to obliterate the channel, which is maintained only by episodic flooding; for instance dunes which extend across the margins of the active floodplain at

Henbury or New Crown (Fig. 5-7) are washed away during flood. In the sector below the floodout the potential link between the Finke and the Macumba is only partly discernible, as dunes and loose sand encroach upon pans. Lines of more dense vegetation mark out the swales which apparently formed the upper part of this link, but the lower course cannot be traced through to Lake Eyro.

iii. Karinga Creek in the western part of the catchment has been assigned a stage of disorganisation of (iv) in that the axial lowland above Karinga Creek reflects the almost total obliteration of sections of the former channel by dune deposits across the lowland. The link between Karinga Creek and the Finke, not followed by floods in historic time, is identified from lines of pans with fringing dunes on the airphotographs.

This western part of the Finke system is typical of much of the drainage of the central Australian region of the Western Plateau, in that upland tributary drainage systems disintegrate on the low gradient plains of the lowlands. Factors controlling the failure of potential drainage channels to reach the line of playas running west to east across the axial lowland are of two kinds:

i. The inadequacy of the uplands to generate some order of tributary

system which will persist across the lowlands to reach the

trunk drainage. 62.

ii. The large lowland distance over which tributary drainage must

pass to reach the trunk system, i.e. spacing of relief.

In this study a comparison was made between channel systems in this

western part of the catchment and those in areas of more co-ordinated

drainage within the Finke-Macumba network. In Table 5-3 the total

numbers of streams which flood out at each order are shown, and it can

be seen that ike cUv>^\el5 \kcvm$e$ trrdcr +o Dr°\ev chvvn'v\ > sl-wzs . V\e»Acc- Ci^A ki^Ue-*- ovdev' C-kav,*els o*c **ove likely to contribute to the connected network, while a high proportion

of streams of lower order fail to do so. The average area of upland

necessary to support a fifth order catchment has been calculated from

sample catchments which develop beyond fifth order. The upland areas

of "successful" fifth and sixth order networks throughout the catch­

ment have been compared with the available upland catchment areas on

similar rock types in the western section of the Finke-Macumba system

(Table 5-4). Most of the uplands here consist of single ranges too

narrow to provide the area necessary to allow the development of fifth

order catchments. However fifth and sixth order catchments do

develop on the broad granitic complex of the Musgrave Ranges and

to a lesser extent on the multiple quartzite ridges in the Petermann

Ranges. Available upland catchment area will only partly explain

the lack of successful drainage from this part of the Finke-Macumba

system, and thus the second factor, spacing of relief, must also be

considered as a factor in this disorganisation of drainage.

To assess this effect of relief spacing the distance of upland catchments from the axial line of playas leading to Karinga Creek was contrasted with the critical length of unsupported channel, here defined as the maximum distance a channel persists beyond its upland Number of Percentage of Stream Number of channels which channels which order segments flood out flood out

Finke System

1 9652 ? ?

2 2440 510 22

3 660 176 27

4 112 71 63

5 43 22 50

6 9 3 33

7 1 1 100

Macumba System

1 17372 ? ?

2 4437 256 6

3 1267 89 7

4 175 28 16

5 70 5 7

6 18 1 6

7 4 0 0

8 1 0 0

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catchment without receiving tributary drainage. To determine this critical value, which appears to be related to local relief (see

Table 5-5), measurements from airphotographs and photomaps were made throughout the Finke-Macumba catchment, on all channels which flood out before reaching the main system. The average distance over which channels persisted beyond their lowest confluence was calculated for streams derived from similar upland areas in each local relief* class.

This distance correlated well with factors of upland area and relief in that in no case did the standard deviation exceed 0.26. It is seen from Table 5-5 that the distances required to be traversed from the bounding uplands to the axial drainage line is exceeded throughout the western section of the catchment, for uplands of each dimension and relief class, and thus the remote bounding uplands do not drain to Karinga Creek.

Hydrologic Records Confirming Physiographic Indications of Connectivity

Unfortunately few hydrologic data are available to supplement physio­ graphic evidence of channel breakdown. Although potentially part of the same channel network, there is no historical record of flows in the Finke River reaching the Macumba. It has been suggested that this occurred at, or just after the end of February 1921 (T.H.G.

Strehlow, 1970, pers. comm.). Bonython (1963) also cites unsubstan­ tiated reports that such flooding may have occurred, but no reliable

*Local relief has been used in several comparisons. This has been calculated from spot heights along traverse lines on 1:250,000 maps and from land system descriptions (Perry et al., 1962). Where no other source was available, the 1:5,000,000 Relief and Landform Map of Australia (Dept. Nat. Devel. 1969) has been used. Measured within western Calculated from flood- (Karinga Ck) subcatchment Local out channels throughout of Finke system relief Finke-Macumba catchment: class average length of Average distance Minimum distance unsupported channel of upland catch­ between upland (m) (km) ments from catchment and drainage artery drainage artery (km) (km)

0-30 3.0 51 30

30-100 3.5 38 5

100-200 4.0 58 25

200-400 4.5 94 77

TABLE 5-5 Comparison of average lengths channels will persist beyond receiving tributary drainage, with the distance between upland rim and axial drainage artery in the Karinga Creek subcatchment. 64.

records of this exist. The major flood in the Finke in 1967, reported by Williams (1971) did not reach the Macumba, nor did the floods in 1974, generated by the second highest rainfalls recorded in the catchment, of in excess of 75 cms (30 ins) at both Alice Springs and Oodnadatta. If such flooding did occur in 1921, it followed a

12 month period in which Strehlow states that over 100 cm (40.02 in) of rain fell at Alice Springs, and a similar amount at Oodnadatta.

Bureau of Meterorology records confirm the possibility of this having occurred, since at the end of March 1921, 102.49 cm (40.39 in) of rain had been recorded over a 15-month period at Alice Springs. Estimated from percentage probability (Fig. 5-6) using 97 years of rainfall record, this would represent a rainfall recurrence of about 800 years.

However the rainfall in 1974, representing a recurrence event of approximately 100 years resulted in flooding only to the distributary zone where water from floods of much less magnitude also normally disperses.

Observations by Williams (1971), supported by those of Bonython (1963), indicate that Finke floodwaters reach the distributary channels near the South Australian border at four or five year intervals - based on rainfall probability. Data collected by Mabbutt and Schumm (unpub­ lished) indicate an average of three flows per year in the piedmont reaches of both the Finke and Hugh Rivers. Flows reach Finke township five times in four years on an average, but these may represent flooding from the Hugh, or Lilia Creek, independent of flows in the

Finke. Their data also suggest that Finke floodwaters reach the distributary zone once in four years. Fig. 5-6 Rainfall probability graph, for rainfall within the Finke River catchment 245

cms

24-5

2 45

Percentage probability 65.

Williams (1971) noted that the major flood of 1967 reached this floodout zone (as did flooding in 1974) and did so with the aid of flows in the lower tributaries, and that although some thousands of cubic metres of water stood at a depth of one to two metres within the floodout zone for up to two years, flows did not persist beyond this zone, but were effectively halted by the sand barrier across the very low-gradient surface. It appears that if flows do persist beyond this zone it is with a recurrence interval of the order of

800 years (Fig. 5-6).

Downstream Decrease in Discharge

These changes in the form of the channel and floodplain accompany a decrease of discharge downstream. This factor also contributes to

controlling the location of the floodout.

Downstream Changes in the Finke River Channel

The Finke River is typical of drainage systems of the central

Australian part of the Western Plateau in that it floods out before

attaining its potential topographic baselevel, and a study of down­

stream changes in channel form can provide some indication of the mechanism of the floodout process, of the location of the floodout and of the relationship between channel form and the stage of dis­ organisation of drainage.

A general description of the changes in a downstream direction of a desert river has been given by Leopold and Miller (1956). The effect

of channel losses, mainly through infiltration and some evaporation,

is to increase the ratio of sediment load to discharge downstream.

This is reflected in the development of a zone of aggradation described by Schumm and Hadley (1957) as commonly taking the form of a steep fan-like sector, over which characteristic "bedload channel" (Schumm,

1963) conditions such as braiding, and high width/depth ratio, prevail. Downstream again channel gradient diminishes and sinuosity increases when following the deposition of most of the sandy bedload, distributary channels of the "suspended load" type (Schumm, 1963) with low width/depth ratio, are formed. With changes in discharge, the zone of aggradation may migrate, or the river and its tributaries may incise into the aggraded bed.

While this description of a typical arid stream course is generally true of the Finke River, the local geological environment imposes some variations. Three lowland sectors can be delimited on the basis of channel form or floodplain plan (see Fig. 5-7). The piedmont sector is characterised by alternating reaches of a straight, shallowly incised single channel, and a more sinuous, less incised group of braiding channels. The second shoaling sector appears to be the zone of major deposition of bedload, with a third sector made up of sinuous distributary channels representing the final distributary or flooding-out stage of the river.

Downstream Decrease in Discharge

These changes in the form of the channel and floodplain accompany a decrease of discharge downstream. This factor also contributes to controlling the location of the floodout.

It is characteristic of streams in arid regions that discharge initially increases downstream along the trunk channel (as is the case with streams in more humid regions) but beyond leaving the upland Fig. 5-7 Sectors of the Finke River channel Macdonnell

DISTRIBUTARY Plate 5-1 Vertical air photographs of the Finke River. A. Showing the straighter, slightly braided sandy channel in the shoaling sector with a more densely vegetated inner floodplain, and the outer floodplain trimming the dune boundary. B. Showing the anastomosing distributary channels above the terminal floodout, and lines of vegetation marking swales where water is backed up during floods

The photographs have been made available by courtesy of the Director Division of National Mapping3 Department of Minerals and Energy3 Canberra.

67. area, and especially when tributaries no longer con­ tribute to the flow, discharge progressively decreases downstream. This has been shown to occur in the Finke

River for bankfull discharges calculated from the Manning Formula using channel data at measured cross- sections (see Fig. 5-8). This data was mainly from a survey of the lowland course of the Finke carried out by Mabbutt and Schumm (Mabbutt,in press), supplemented by two measured cross-sections and by information listed in Williams (1970).

It can be seen from Table 5-6 and Fig. 5-9 that the bankfull discharge of the Finke begins to diminish before the Hugh River - itself a wide, sandy channel draining from the eastern part of the Macdonnell Ranges - enters as a left-bank tributary. Immediately below the Hugh confluence the bankfull discharge of the Finke channel increases sharply, but then steeply diminishes as no further major tributaries enter the trunk channel.

Minor increases occur at New Crown - discharge received through Goyder Creek - and below the entry of Coglin Creek, but these additions are not sufficient to main­ tain the channel which disappears into the dunefield of the Simpson Desert near the South Australian border.

The relationship between channel sediment and width/ depth ratio has been noted by Schumm (1960 b, 1963) for rivers of the United States Great Plains, where the relationship

F 255 M-1.08 occurs in which of silt and clay in channel perimeter Location of No. on Distance from Approximate upland sources bankfull * cross-section Fig.5-8 discharge (km) (cu.m/sec.)

Hermannsburg 1 89 600

Henbury 3 233 820

Idracowra 4 322 970

Above Hugh confluence 5 435 340

Below Hugh confluence 6 436 480

Horseshoe Bend 7 467 460

Crown Point 8 523 330

Finke Railway crossing 9 547 300

New Crown 10 588 60

Below Coglin Creek 11 607 107

Mayfields Swamp 13 612 42

Willyunpah 14 622 30.5

South Australian border - 700 no channel

* Mainly from data from Mabbutt and Schumm (Mabbutt, in press).

TABLE 5-6 Estimated bankfull discharge at surveyed cross- sections along the Finke River, ^owvv Fig. 5-8 Locations of surveyed cross-sections along the Finke River rmannsbur Fig. 5-9 Graph showing the change in bankfull discharge along the Finke River Below Hugh confluence

Above . Hugh ^ confluence

Below Coglin Ck

Bonkfull discharge (cumsecs)

Distance from upland source (km) 68.

The relationship between channel sediment and width/depth ratio was

examined at the measured cross-sections along the Finke River shown

in Fig. 5-8 in which the load of the channel changes downstream.

These data, listed in Table 5-7, were mainly collected by Mabbutt

and Schumm (unpublished). In this instance the equation

F = 260 M-0'82

was obtained (correlation coefficient 0.74) (see Fig. 5-10); a result

similar to that obtained by Schumm (1963) for rivers on the United

States Great Plains, indicating that downstream changes in the form

of the Finke reflect changes in sediment load, and hence discharge.

The Three Lowland Sectors

The Piedmont Sector. This represents the greatest part of the length

of the Finke floodplain, extending from Henbury Station at the

southern limit of the James Range to the vicinity of Horseshoe Bend,

south of the Hugh River confluence. In the upper reaches of this

sector the Finke has a gravelly to sandy bedload channel, with a floodplain generally 300 to 600 m wide, and marked out by river red gums (Eucalyptus camaldulensis). Below the Palmer River confluence the Finke channel is a wide sand-bed channel with a width/depth ratio of 45 to 60 (Table 5-7). The Finke also attains its maximum bankfull discharge at Idracowra, within this sector (Table 5-6). The sinuous and locally braiding channel up to 280 m wide and with sinuosity approximately 1.2 to 1.4 alternates with a more incised straighter form, where the channel is constricted by older terraces or rock outcrop. The single straighter channel is about 150-200 m wide, incised into a floodplain about 400-600 m wide. Below Idracowra sand dunes encroach on the eastern marign of the floodplain and the course of the Finke is hemmed in by the Simpson Desert. Location of No. on Width/depth Weighted mean percent silt-clay cross-section Fig. 5-9 ratio (F) in perimeter (M)*

Hermannsburg 1 18 41.0

Below Hermannsburg 2 29 37.0

Henbury 3 43 5.1

Idracowra 4 60 10.0

Above Hugh confluence 5 61 7.6

Below Hugh confluence 6 10 8.0

Horseshoe Bend 7 39 21.0

Crown Point 8 48 7.8

Finke railway crossing 9 54 5.6

New Crown 10 62 8.5

McDills Swamp 11 36 35.0

Below Coglin Creek 12 40 7.5

Mayfields Swamp 13 2 90.0

Willyunpah 14 4 70.0

* Mainly from Mabbutt and Schumm (unpublished).

TABLE 5-7 Width/depth ratios and channel sediment load at surveyed cross-sections along the Finke River, Fig. 5-10 The relationship between channel width/depth ratio and the weighted mean percent of silt and clay in the channel perimeter, along the Finke River Width/Depth

Percent silt 'cloy (M ) 69.

The Shoaling Sector. Below Horseshoe Bend the Finke channel becomes less sinuous (measured sinuosities 1.1 to 1.3) within a floodplain comprising two levels. An inner sandy floodplain thickly vegetated with river red gum rises to an outer floodplain with an undulating surface. This outer floodplain consits of finer textured sediments, and it supports dense stands of coolibah (Eucalyptus microtheca).

On the outer margins of this floodplain, active dunes occur, especially on the eastern side. The combined floodplain width throughout most of this sector is 1.0 to 1.5 km.

The channel is sandy, and up to 200 m wide. In its straighter reaches the channel braids, with up to five minor channels mostly 30 to 60 m wide. Through much of this sector, the outer floodplain which is en­ croached upon by dunes or which trims the current dune boundary, is bounded or traversed by an apparently seldom-used channel, about the width of the active channel, but without the lines of living vege­ tation at the banks. This discontinuous channel line, which may be a flood scour route, is straighter than the active channel.

Within this reach a considerable amount of sand enters the Finke channel from the wide, sandy channel of Lilia Creek, the consequent channel widening and braiding is apparent for more than 20 km down­ stream, however the channel becomes shallower. From the entry of

Goyder Creek there is a sharp change to a broad active floodplain with a single shallow sandy channel and this in turn gives way abruptly to a "floodout" system. Just above its junction with the

Finke, Goyder Creek also changes abruptly to a distributary system, apparently due to ponding back of its course by sand deposited along the floodpath of the Finke. Its shallow sandy channel carrying 70.

coarse sands and grit derived from granite and sandstone to the west

suddenly divides into a series of narrow sinuous anastomosing dis­

tributaries, following deposition of a large quantity of sand.

For a short distance beyond the Goyder confluence the Finke attains

its maximum width/depth ratio of 62 (Table 5-7), as the channel

becomes shallower, probably due to increased sand deposition. This

wide shallow channel form which is the maximum extent of the shoaling

is maintained only for a distance of 12 km, before the distributary or

floodout zone begins.

The Distributary Sector. The change to distributary channels is

abrupt. The braiding sandy channel diminishes to about 200 m wide,

then splits into a group of tightly sinuous, anastomosing channels

each between 10 and 100 m wide. One main drainage channel persists,

with minor active channels, while on the southwestern edge of the

floodplain a higher-lying channel occurs - apparently a flood channel.

Levees occur along the distributary channels, which have the effect

of maintaining the distributary pattern.

Immediately south of the beginning of this distributary sector the

narrow anastomosing channels flood into an ephemeral swamp near the

base of low stony tablelands to the west. Some distributary channels which have followed a course through sand dune country to the east

enter this floodout basin along the swales. Airphotographs of this

area indicate that without the entry of the tributary Coglin Creek,

the Finke channel would not have extended beyond this floodout zone.

Coglin Creek drains an area of duricrusted finer-textured Mesozoic

sedimentary rocks which provide a source of clay and silt with minor coarser sediments, and the character of this sediment load is Plate 5-2 Changes in channel cross-section down the Finke River. A. In the piedmont sector near Henbury Station, a wide shallow channel with gravelly and sandy bed. B. In the shoaling sector near Crown Point, a wider shallow straight sandy channel. C. In the distributary sector near the confluence with Coglin Creek, a narrower and deeper more sinuous channel with sandy and silty bed.

71.

reflected in its more sinuous channel with a much lower width/depth

ratio than is found in the tributaries to the Finke in the upstream

sectors.

Beyond the swampy lowland marking the entry of Coglin Creek, the

Finke enters its terminal floodout (Fig. 5-11). This is a confused

system of anastomosing channels which enter or drain from a sinuous

main channel in which the initial width of up to 200 m diminishes to

no more than 50 m within 20 km. No major tributary drains from the

tablelands to the west. Pans occur where water backs up in the swales

during flooding and returns to the channel later, indicated by lines

of coolibah along these swales. Beyond this zone two channels emerge

(see Fig. 5-11) each transgressing the local dune trend. The more

easterly of these channels spreads floodwater into surrounding dune

swales, degenerates within 15 km, and finally floods out into three

dune swales before reaching the South Australian border. The western

channel persists for a longer distance between dunes, and it is

probable that if flows in the Finke ever reached the Macumba they

did so through this channel.

The location of the floodout is determined by hydrologic and physio­

graphic factors. Decreasing bankful1 discharge along the Finke

results in floods of this stage diminishing entirely in this zone;

however larger floods might be expected to persist further. The

geographical location of this zone, however, is largely determined

by physiographic constraints. In this region the Finke, with its

discharge already diminished, is hemmed in against tablelands to the

northwest by dunes encroaching from the south-southeast. This entry

of wind-borne sediment, together with sediment carried from these Fig. 5-11 The floodout of the Finke River .2rS CD —^ '■*- cn o ,2c$ >0) w

o—

E o Plate 5-3 Vegetation indicators of channel processes in the

lower Finke River.

A. River red gums (right bank) give way to

coolibahs as the channel load changes from sand

to silt and clay. View downstream near Charlotte

Waters.

B. Dense stands of coolibah mark the floodout

zone, where floodwater is ponded. View downstream

near Coglin Creek confluence.

72.

tablelands by local streams constricts the course of the Finke in this area.

The Comparison of Actual and Potential Channel Lengths

As the Finke-Macumba system now follows an established course, with tributary systems directed towards this course, the comparison of actual and potential channel lengths has been carried out for this established system.

The two other methods of measuring drainage disorganisation have shown that within this system there are three subcatchments showing differences of landsurface distribution and hence of effective catchment, and of breakdown within the channel system. For this reason, the Finke-Macumba system has been considered in this measure firstly as three separate systems - Karinga Creek, the remainder of the Finke system and the

Macumba - and then as an entire network.

In developing bifurcation ratios to estimate the numbers of third, second and first order streams in the network, it was recognised that there was greater diversity due to contrasting lithologies in this catchment than in the Georgina-Diamantina system (Chapter 4). Differ­ ences in hydrologic and relief factors occur between catchments on quartzites, granites and silcrete-capped tablelands or stony plains, and these ratios were obtained from at least three sample catchments on each of the various rock types. The values for granitic rocks

(the only class for which some comparison was available) compared well with those noted by Woodyer and Brookfield (1966). 73.

In constructing the potential network some floodout terminals could be readily connected with the trunk system. In the Karinga Creek catchment however, the almost complete absence of channels rendered this method inadequate. The height and extent of bordering uplands

(Table 5-4) were used to estimate the order of potential tributaries.

From this, from comparable catchment size, and from the remaining isolated networks of upland drainage, it was concluded that the Karinga

Creek catchment was potentially occupied by a seventh order drainage network, similar to the Finke system upstream of the Karinga Creek confluence.

The theoretical number of streams within the catchments are compared with the actual number now present, in Table 5-8, and the comparisons are represented graphically in Fig. 5-12. The degree of disorganisation expressed as the proportion of channel segments or lengths missing from the theoretical network is shown in Table 5-9.

From these results it can be seen that the Karinga Creek system is highly disorganised, the remainder of the Finke system much less disorganised, and the Macumba system barely disorganised. When the entire catchment is considered, the breaks between Karinga Creek and the Finke system, and between the Finke and Macumba Rivers influence the measure of the length of channels missing from the network, and show the whole system to be significantly disorganised. These broad comparisons are shown in Table 5-9.

i. The Karinga Creek system is extremely disorganised. It can be seen from Table 5-8 that a very high proportion of channel segments is missing from the network at all stream orders, indicating both FINKE-MACUMBA SYSTEM, CENTRAL AUSTRALIA

Theoretical Actual Percentage of Streams Order Average Average Missing From No. of Total Segment Cumulative No. of Total Segment Cumulative Theoretical Segments Length Length(L) (TO Segments Length Length(L) CL) Network

Karinga Creek System 1 4130 4130 1.0 1.0 980 490 0.5 0.5 75 2 1180 2950 2.5 3.5 280 476 1.7 2.2 75 3 340 1054 3.1 6.6 80 163 2.1 4.3 77 4 85 705 8.3 14.9 16 90 5.6 9.9 81 5 23 536 23.3 38.2 5 100 20.0 29.9 78 6 4 330 82.5 120.7 2 80 40.0 69.9 50 7 1 220 220.0 340.7 0 - - 69.9 100

Finke 5 ystem 1 6594 6916 1.04 1.04 4980 3084 0.6 0.6 24 2 1750 4670 2.7 3.7 1422 2928 2.1 2.7 19 3 485 1586 3.3 7.0 380 936 2.5 5.2 22 4 139 1199 8.6 15.6 96 638 6.6 11.8 31 5 63 1528 24.3 39.9 38 889 23.4 35.2 40 6 8 690 86.2 126.1 7 520 74.3 109.5 13 7 1 729 729.0 855.1 1 600 600.0 709.5 0

Macumba System 1 17954 17415 0.97 0.97 17372 16677 0.96 0.96 4 2 4775 11935 2.5 3.5 4437 10649 2.4 3.4 8 3 1365 4914 3.6 7.1 1267 4434 3.5 6.9 7 4 210 1659 7.9 15.0 175 1260 7.2 14.1 17 5 74 1560 21.0 36.0 70 1400 20.0 34.1 5 6 19 954 50.0 86.0 18 900 50.0 84.1 5 7 4 1160 290.0 376.0 4 1160 290.0 374.1 0 8 1 ' 300 300.0 676.0 1 300 300.0 674.1 0

Combine d System 1 28678 28461 1.0 1.0 23332 20251 0.83 0.83 29 2 7705 19555 2.5 3.5 6139 14053 2.2 3.1 20 3 2190 9554 3.4 6.9 1727 5538 3.1 6.2 21 4 434 3563 8.2 15.1 287 1988 6.9 13.1 34 5 160 3624 22.7 37.8 113 2389 21.1 34.2 29 6 31 1974 63.7 101.5 27 1500 58.3 92.5 13 7 6 1660 277.0 378.5 5 1760 352.0 444.5 17 8 2 820 410.0 788.5 1 300 300.0 744.5 50 9 1 110 110.0 793.5 0 - - 744.5 100

TABLE 5-8 Actual and potential channel numbers and segment lengths in the' Finke-Macumba catchment P -H C

p bo C CD

P -H T3 C cd CD *H

t/i system. PC CD 6 bO (/)CD

CCD c cd

X Finke-Macumba

o Cp

O the CD

bO of

cd p o P CD

a, network cr> LO PJPI CQ < H Fig. 5-12 Graph of actual and theoretical cumulative mean segment lengths for the Finke-Macumba channel systems Cumulative mean segment length (km) Cumulative mean segment length (km) -0-5 ------rIOOO -100 ------500 -500 -1000

• I I

Actual *

i

Theoretical stream Macumba system i

i

Karinga system

5 i

order i

I

Creek i

r -100 -500

1000 r -100 -500 Entire Macumba

1000 stream

Finke-

system Finke order stream system

order 74.

that large areas of the catchment are unproductive of runoff, and that the trunk systems fail to reach the connected network.

ii. The Finke system is also missing streams at all orders from its network. The larger numbers missing at fourth and fifth orders represent failure of trunk channels on lowland parts of the catchment, however the lack of appreciable numbers of first and second order streams indicates that areas within the catchment are not generating runoff.

iii. The Macimba system shows a much lower proportion of streams missing from the network at each order. Very few low order streams are missing, indicating that most of the catchment is generating drainage, and what failure occurs is of higher order trunk channels on lowland surfaces.

Comparison of Levels of Disorganisation as

Indicated by the Three Measures

Within this system all three measures indicate similarly the relative degrees of disorganisation within the catchment. Differences due to morphostructural differences within the catchment are reflected by the three measures.

i. The Proportion of the Catchment Hydrologically Effective distin­ guishes between the part of the catchment developed on the Western

Plateau and that part on the Interior Lowlands. Localised effective drainage and a large unproductive area of catchment occurring in the 75.

Finke subcatchment, and especially in the western part of this sub­ catchment developed entirely on the Western Plateau, contrast markedly with a mainly effective catchment area for the Macumba system on the Interior Lowlands. This measure does not take into account the connectivity of the drainage networks generated, but does distinguish between the subcatchments, and shows the Finke system to be more disorganised.

ii. The Degree of Channel Connectivity is a useful measure to dis­ tinguish the form of breakdown of the channel systems. No account can be taken, using this measure, of the former drainage courses, and it is most effectively used where there is failure in the trunk channel of the system. It is more difficult to apply this measure to a system in which failure occurs at the catchment margins, as occurs in the

Karinga Creek system, and in this case the level of disintegration of drainage is not indicated.

iii. The Comparison of Actual and Potential Channel Length involves consideration of a reconstruction of the ideal drainage system which might be expected to occupy the catchment, and hence allows con­ sideration of both the areas of breakdown and the degree of failure.

In this case, the measure indicates failure of the catchments of

Karinga Creek and the Finke, and also indicates the relatively greater degree of channel failure for Karinga Creek than for the Finke or

Macumba systems. 76.

CHAPTER 6

FOWLERS CREEK SYSTEM: A STUDY OF CHANNEL BREAKDOWN

The two preceding chapters in this study of drainage disorganisation

have been mainly concerned with the consideration of large catchments

and with the breakdown of drainage systems at a broad scale. At this

level the influence of major structural controls on the form and

degree of drainage breakdown can be compared, but little consideration

can be given to the changes in channel and floodplain form which

accompany this breakdown. In the case of the Finke River the changes

in channel form and the nature and position of the floodout are known

in a general way. In order to elucidate the stages involved in

channel breakdown, a sample study has been carried out on a small

river channel which floods out over a short distance, where the

forms of the channel and floodplain, and the associated alluvial

sediments might indicate the processes involved.

The channel chosen for this study in greater detail is that of

Fowlers Creek (Fig. 6-1). Fowlers Creek rises in the northeastern

part of the Barrier Range, New South Wales, and floods out on the

adjacent lowlands of the Bancannia Trough, before reaching its

potential baselevel of Lake Bancannia. Over a distance of 20 km

Fowlers Creek undergoes changes in its channel form which the Finke

River, for example, undergoes over a distance of 300 km.

The catchment of Fowlers Creek falls within Meigs' (1953) Aa 23 climatic classification - arid, with mild summer and winter tempera­ tures and with no distinct rainfall seasonality. Mean annual rainfall throughout the catchment is approximately 20 cm, with slightly higher rainfalls for the summer months (October to March) than for the Fig. 6-1 The location of Fowlers Creek and its relationship to Lake Bancannia Lake Bancannia

: The" / ^Selection

6th order channels

Stream flooding out

Lake or pan

Floodout zone

The Selection Broken Hill

NEW SOUTH

WALES Sydney 10 km

LOCATION 77.

winter period (Bell, 1973). In both seasons much of the rainfall occurs as short bursts of high intensity rain, however wet spells of three days duration or longer are responsible for the larger rainfalls, i.e. over 4 cm, and these occur generally less than once per year (Bell, 1973).

Fowlers Creek Catchment

Fowlers Creek forms part of the Lake Bancannia drainage, of the

Builoo-Bancannia drainage division (Dept, of Nat. Devel., 1966).

Lake Bancannia lies at an elevation of 120 m within the Bancannia

Trough, and Fowlers Creek floods out 16 km south of the Lake.

Although a line of smaller pans and a low-lying zone with denser vegetation across the Bancannia Trough indicates the former course of the Creek (Fig. 6-1), this course is mainly obliterated by vegetated sand accumulations, and no floodflows from Fowlers Creek now reach

Lake Bancannia.

The catchment of Fowlers Creek (Fig. 6-2) was mapped from published topographic maps (Australia 1:250,000 series Broken Hill and Cobham

Lake; N.S.W. Australia 1:25,000 Fowlers Gap Field Station Sheets 1 and 2) and airphotographs. The potential catchment is an area of 2 2 775 km , of which 450 km effectively contributes to the channel network.

On average, flows occur in Fowlers Creek two to three times per year after local heavy falls of rain of 5 to 10 cm, and water flows through to the floodout zone once in one to two years, following rainfall Fig. 6-2 The catchment of Fowlers Creek

78.

episodes in excess of 10 cm throughout most of the catchment

(Mabbutt, 1973). Rainfall sufficient in amount and extent to cause

Fowlers Creek to flow through to its terminal floodout invariably

causes floods in Sandy and Telephone Creeks (G. Laurie*, pers. comm.)

which are tributary to Fowlers Creek within this floodout zone, and

drainage into this zone is a good example of Dubief’s (1953) criterion

of draiiage disorganisation in its third stage - when flows do not

reach biselevel even with the aid of lower tributaries.

The uplind area of the catchment, within the Barrier Range, is

effective in generating runoff. A densely textured network of

channels is developed, and the system attains its maximum order of

eight within the upland sector. The materials within the upland

catchment consist of calcareous shale, slightly silicified shale and

partially silicified sandstone of the Teamsters Creek Subgroup (Ward

and Sullivan, 1973) and give rise to a high proportion of fine

textured sediment as well as sand and gravel. The piedmont slopes

on the margin of the Barrier Range are gravelly, and the lowest

sector of the catchment is a plain surface of sandy or finer textured

alluvium on the Bancannia Trough, with gradients between 1 in 200

and 1 in 500.

Changes in the Channel of Fowlers Creek

The breakdown of Fowlers Creek involves changes in the form of the

channel, a reduction of channel size, the splitting of the channel

*Graham Laurie is Manager of The Selection Station, the property on which the floodout channels of Fowlers Creek terminate. 79.

into distributaries, and the dying out of these distributary channels (Plate 6-1).

To study these changes, fourteen cross-sections were measured along the alluvial channel of Fowlers Creek (Fig. 6-3). Nine of these sections were along the presently active channel, and five along a distributary course which is apparently abandoned, since river red gums (Eucalyptus camaldulensis) lining this channel are dead and reaches have become choked with silt. At each of these sections cross profiles were measured, and .the slope of the channel reach levelled. Sediment samples were collected from the channel perimeter, the levee (where present) and the backplain, for analysis of their silt and clay content (See Appendix 1). The channel plan was also studied on airphotographs in each of these reaches to see if changes in channel form were associated with changes in the load carried.

The changes which occur downstream in the alluvial channel of Fowlers

Creek can be seen from Fig. 6-4 and Tables 6-1 and 6-2. The most obvious change is a decrease in the size of the channel. This can be seen from Fig. 6-4, and the consequent decrease in calculated bankfull discharge can be seen from Table 6-1. Associated with this is an increase in the proportion of silt and clay carried by the channel

(Table 6-2), but with little change in texture along the floodplain until very close to the drainage terminal, where silt and clay makes up almost the entire sediment content of the backplain and levees.

On the basis of floodplain plan and channel form, three sectors can be distinguished along the alluvial channel of Fowlers Creek

(Fig. 6-3). Calculated # Channel Width Depth Slope Roughness Bankfull Bankfull section m m coefficient velocity discharge m/sec cms

Active channel

1 91.5 1.46 0.02 0.06 3.06 404.4 2 22.88 3.14 0.0026 0.04 2.31 163.8 3 30.48 1.52 0.0020 0.05 0.90 41.4 4 15.24 0.91 0.0009 0.03 1.39 16.2 5 9.15 0.91 0.0041 0.03 1.48 11.8 6 26.84 0.85 0.0042 0.04 0.44 10.1 * 7 21.65 1.92 0.0011 0.04 1.24 51.1 8 6.56 1.31 0.0011 0.03 0.05 0.41 9 2.29 0.46 0.0011 0.06 0.14 0.14

Abandoned channel 10 9.15 3.66 0.0013 0.04 2.18 71.4 11 8.84 1.98 0.0029 0.03 2.85 49.6 12 8.54 1.34 0.0014 0.03 1.45 16.3 13 11.90 0.87 0.0016 0.03 1.40 14.2 14 7.01 0.21 0.0009 0.03 0.64 0.9

* The calculated discharge and velocity in this cross-section are unreasonably high. It seems likely that, as the bank had collapsed in this region, the channel width and depth values may not represent the bankfull channel section. 1 u -l # Manning Formula v = 1.5 d3 s n and roughness coefficients after Leopold and Maddock, 1953.

TABLE 6-1 Channel dimensions and calculated bankfull discharges for the measured cross-sections along Fowlers Creek. Silt+clay percentage Width/depth Channel Sinuosity ratio section Channel Levee Backplain

Active channel 1 13.25 60.05* 80.81 62.7 1.03

2 53.95 none 82.72 7.1 1.24 * 3 18.14 61.15 - 21.3 1.25

4 14.25 60.83 82.87 16.8 1.43

5 31.38 none - 10.0 2.67

6 55.83 59.37 80.79 31.6 1.00

7 64.98 70.83 80.71 11.3 1.14

8 78.72 91.48 80.03 5.0 1.89

9 89.87 - - 5.0 1.55

Abandoned channel

10 84.14 - - 2.5 1.20

11 72.10 80.79 89.98 4.5 1.12

12 70.78 92.32 80.80 6.4 1.80

13 52.80 82.12 77.94 13.6 1.17

14 90.75 96.36 93.26 33.4 1.21

* Sandy rises, not continuous levees.

TABLE 6-2 Channel parameters for the measured cross-sections along Fowlers Creek. Fig. 6-3 Plan of the alluvial channel of Fowlers Creek, showing the locations of the measured cross-sections, and the three channel sectors smMmmnBammmkmmm

Selection Tank

Zone in which levees occur Fig. 6-4 Cross-profiles of the channel of Fowlers Creek

W hcv'C- CeAltfcpS-C V^\CX5 OC C U V" V£(K V^oI-iC-OlV^S

f ia.H KjKfsr

ACTIVE CHANNEL SECTIONS Plate 6-1 Vertical air photographs of Fowlers Creek A. Showing the wide, straighter sandy channel upstream of the floodout with one reach showing a change in course, by meandering along older lines of crevassing. B. Showing the active and abandoned channels, narrower and more sinuous in the floodout zone, with anastomosing and distributary channels.

Reproduction by permission of the Department of Lands3 Dew South Dales.

80.

A\- the ou+\eA .f.,.ow1 ~ h~l{s the channel is wide, generally more than

20 m, and shallow; the width/depth ratios mainly exceed 20, and there is an associated high proportion of sand and gravel in the channel perimeter. This sector is thus mainly of a bedload form (Schumm,

1963). Bedforms include sandy bars with minor gravel, and flatter reaches of increased concentration of gravel. Sinuosity is low, less than 1.25 throughout this sector, and no levees occur, but flood deposits include sandy rises or longitudinal bars (Williams, 1971) along the channel in some areas. Splays of sand and gravel across the banks also indicate overbank flows, and the edge of the channel is commonly not sharply defined. The measured and sampled cross­ sections 1 and 2 fall within this sector.

Section 1 is typical of most of this reach. The perimeter

of the bedload channel is mainly of sand and gravel, although

the fine textured sediments derived from the upland catchment

provide a significant proportion of finer grades. The width/

depth ratio is high, and the channel slope is relatively

steep.

Section 2. This trenched reach appears to be degrading by

downcutting. The bed and bank materials are more silty than

through most of the uppermost sector. The channel has

adjusted its width/depth ratio to the load of very fine

sand and silt*supplied. It is probable that the source of

additional silt and clay to the channel in this reach is

overland flow of sediment from the floodout deposit of

Telephone Creek on the west.

*Wentworth size gradings used (Folk, 1968). 81.

The middle sector of the alluvial channel of Fowlers Creek appears to be a transition form between the bedload channel upstream and the distributary sector downstream. In this reach the channel is generally narrower, with varying depth, but with width/depth ratios mainly between 10 and 20, within the range of mixed load channels

(Schumm, 1963). The sediment load becomes increasingly less sandy downstream, and sinuosities are generally higher than in the upper sector. In this sector levees begin to develop, as paired rises, initially broader on the western bank. These levees reach to 5 m in height, and 200 m in width, with gentle slopes to the backplain. In many reaches the apparent bankfull level of the channel is well below the floodplain surface. Undercutting of the banks during floods has caused collapse of the inner edges of the levee, and the channel is entrenched well below this collapsed levee. Within this sector also, crevassing of the levees occurs, and in some reaches this appears to have led to changes in the channel course.

Section 3 represents a section in which the channel has

altered course along a line of crevassing. The present

course follows a wide meander, and is not bounded by trees,

while two earlier straighter courses, since largely filled

with silty sediment, are marked by lines of river red gums.

No levees are present along the most recent channel.

Section 4 is a reach in which undercutting has caused

collapse of the bank, and the bankfull channel is approxi­

mately 5 m entrenched. In this reach the channel course

has altered slightly. A more recent channel, probably cut

during a high-flood event, has resulted in the present 82.

course consisting of two channels both apparently downcutting, separated by a low island of mixed-texture

sediment. The channel perimeter consists mainly of

gravel and coarse sand, and is coarser than might be

expected within this transition zone. The width/depth

ratio is low, relative to the load texture, and the channel

shape is similar to other sections through this middle

sector.

Section 5 is also one in which the course has altered

several times. Here a former channel, marked out by

parallel lines of still-living river red gums, has been

blocked and the initial obstruction is no longer visible,

but is covered with a thick deposit of silty sediment. The

latest channel course follows a wide meander, and erosion

of the cliffed bank has caused aggradation within this

reach, including the formation of longitudinal bars of

gravelly sand, although the channel perimeter consists of

fine to very fine sand, and silt. Levees occur along this

reach, eroded on the concave bank of the meander.

Section 6. This section across the steepening gradient at

the edge of the distributary sector of the floodplain, shows

signs of active infilling of the channel. Bank erosion has

been pronounced, and the straight channel is wide and shallow

with a sediment load of very fine sand and silt. Levee rises

in this reach are slight, but a dense network of channels

in the backplain has been derived from crevasse-channels. 83.

Section 7 is beyond the reach where the main abandoned

channel acts as a distributary branch during flooding.

A fallen river red gum at this section has blocked most

of the former channel line, and debris including tree

branches and sediment has accumulated on this. A second

channel has developed along a former crevasse line, and

it appears that the stream is aggrading across both the

older and newly-cut channels. Undercutting of the low

levees, especially that on the western side, has occurred,

and the channel bank is very poorly defined, making profile

measurement difficult. The sediment load is very fine sand

and silt, and the width/depth ratio falls within the mixed

load classification.

The lowest sector of the channel is the distributary zone, in which the main channel splits into a number of smaller channels. Here the levees are discontinuous, and smaller than those along the main channel being generally less than 3 m high and less than 100 m wide.

The gradient in this part of the floodplain is very low, and the channels are typically of the suspended load form (Schumm, 1963), with the perimeter of silt and clay and with width/depth ratios less than 7.

In this zone also are several abandoned channels, mainly filled with silty sediment. The presently active channels are marked by lines of living river red gums, while the abandoned channels are identified by fringes of dead or dying trees of the same species. 84.

Sections 8 and 9 are typical of this zone. The sediment

load becomes increasingly fine towards the drainage terminal,

and the channels are narrow and entrenched. Levees are very

low and indistinct. Although each of these sections is in

a relatively straight reach, the distributary channels are

generally sinuous, and mainly terminate in minor hollows

with prominent microrelief.

Sections were also measured along the abandoned channels (Fig. 6-3), however no regular pattern in downstream variation in channel form could be discerned, and the sediments were mainly very fine sand and silt. It can be seen from Tables 6-1 and 6-2, that while a regular decrease in bankful1 discharge, and change in channel form can be observed for the active channel, no such pattern exists for the abandoned channels.

In extreme floods all channels - active and abandoned - are used by the water (G. Laurie, pers. comm.) and silt is deposited in all of these drainage terminals. Infilling of the abandoned channel can be seen to occur through progressive decrease in depth, with consequent increase in width/depth ratio (Table 6-2), accompanied by an overall decrease in channel size and bankfull discharge.

It can be noted from the state of the river red gums along these channels that infilling is a rapid process. Many channels with dying trees are almost entirely silted in, while active channels with still immature trees lining their banks are rapidly becoming filled (Plate

6-2) . Plate 6-2 Changes in the channel of Fowlers Creek A. Silty perimeter and relatively low width/depth ratio, at section 2. B. Wider sandy and gravelly channel at section 3. C. Main, active, silty channel in the distributary zone, at section 7. D. Main abandoned channel, infilled and lined by dead river red gums, at section 13.

85.

At the ends of the terminal channels there is an accumulation of

flood debris which appears with successive floods to build back

into the channel, still further choking the system (Plate 6-3).

Implications from Fowlers Creek for Drainage Disorganisation

The form of breakdown observed along the channel of Fowlers Creek

is repeated in channels of any order which flood out before reaching

their baselevel, throughout the arid zone. The Finke River carries

throughout a much higher proportion of sand in its load, and conse­

quently develops a true bedload form through much of its length,

however similar changes occur in this river, at a much larger scale,

to those observed in Fowlers Creek. For instance, the load supplied

by a tributary channel causes changes in the Finke channel similar

to those in Fowlers Creek. The entrenchment of the channel of

Fowlers Creek with the entry of finer textured sediments from the

floodout of Telephone Creek is similar to the change in the Finke

channel where the Palmer River enters, carrying a load of finer

textured material (Fig. 5-7).

The form of the distributary sector and the floodout terminal are

also similar in both Fowlers Creek and the Finke. The main channel

gives way to smaller silty channels, sinuous and distributary, which

finally die out in basins and hollows within a major swampy zone.

From these drainage terminals it can be seen that the aiAop-VW* o( a

decreases of cUa

As deposition of debris occurs within the terminal channels, the

channels become blocked. More frequent flows in the upper channel Plate 6-3 A choked distributary channel near section 14, showing silt and debris accumulation. A. Upstream. B. Downstream to the choked terminal.

86.

reaches can clear this material, but in the lowest parts where discharge is small, these distributary channels are readily blocked by their own alluvial loads. While some of this sediment may be removed or re-deposited by wind, successive flows through to the terminal channels will increase the amount of alluvium, which could only be removed by an exceptionally large flow of water. This perhaps occurred in the Finke River, during the major flood reported to have reached Lake Eyre in 1922. 87.

CHAPTER 7

DESERT FLOODOUTS

The Occurrence and Form of Desert Floodouts

Floodouts occur within all divisions of the Australian arid zone.

The form of desert floodouts has been described broadly for the

Finke River (Chapter 5) and in greater detail for Fowlers Creek

(Chapter 6), as specific but typical examples of this geomorphic phenomenon. In general the position and detailed form of the

floodout owe determined by a combination of local physiographic

factors along with the decrease in stream discharge, and at a general

level the processes which determine the changes in channel form can also be considered.

An integrated network of upland drainage disintegrating on the plains as a floodout is the common pattern of breakdown of major streams within much of the arid zone, and especially on the Western Plateau, where isolated uplands and extensive sandy lowlands occur. Floodouts also occur on a smaller scale in most tributary channels which fail to connect with a higher order channel, throughout the arid zone.

The process of flooding out involves the splitting of the main channel into a number of smaller distributary channels with loads of silt and clay in suspension, following the deposition of the remaining coarser load carried by the stream. These distributary channels subsequently die out resulting jVomthe hydrologic failure of the stream system. The form of the floodout, and its relation­ ship to the potential baselevel of the system is shown in Fig. 5-11 88.

and Plate 5-1 for the Finke River, and in Figs. 6-1 and 6-3 and

Plate 6-1 for Fowlers Creek.

As discharge decreases along the channel there is a reduction in the amount of bedload sediment transported by the stream f0 selecA'^ dcj>o&d'o^\.'TUis takes the form of shoaling within straight sandy bedload channels, resulting in the development of more sinuous mixed-load channels, commonly with levees. These narrower, more sinuous channels split into a number of mixed-load or suspended-load dis­ tributary channels, which are generally narrow and sinuous with levees developed along them. In most instances there is a marked change in the channel sediment in this distributary reach, which may be reflected by a change in vegetation, as occurs in the case of the Finke River.

The distributary channels commonly die out in sumps or flood basins, and the number and location of active distributary channels change with time, as is evidenced by the floodout zone of Fowlers Creek.

Redistribution and some removal of the sediment deposited along the channel may occur through wind action, resulting in modifications to the build-up, including spreading of deposits and blocking and re-routing of channels. Finer textured material deposited within the channels serves to still further choke these distributaries by back-filling.

Disorganisation of drainage through this floodout process is self- perpetuating, in that unless there is a major flood which removes the debris, continued accumulation of sediment will occur. In turn this means that successive major floods must be more extreme in order to remove this debris build-up and persist through the baselevel. It is 89.

also unlikely that floodwaters spread through several distributary

channels will persist through to baselevel as readily as in a

single channel.

Processes Operating to Form Desert Floodouts

The decrease in discharge in arid zone streams as channel losses tend

generally to exceed flow inputs, is a critical factor in the formation

of floodouts. With this decreasing discharge, the velocity of flow,

and hence the capacity to transport bed sediments similarly becomes

less. Discharge decreases progressively downstream from the point

at which tributary channels no longer contribute to the flow. This has been •f\o\ccl \r> 4-Ue FmXWc. KiO’dvr (p\cd?buMj Table 5-3, Fig. 5-11) and

in Fowlers Creek (Table 6-1) for bankfull discharges calculated from the Manning Formula using channel data at measured cross-sections

(Figs. 5-10 and 6-3). In this respect arid zone streams are unlike those of more humid regions where, although velocity is adjusted to decreasing slope downstream, there is generally an increase in dis­ charge through tributary inflows to maintain the capacity of the streams to transport bedload sediments.

Decrease in discharge is brought about in two main ways. Transmission losses through infiltration and evaporation occur especially from the bedload , and evaporation losses occur from sump basins which in many instances do not discharge back into the channel on the falling stage of floods (Bonython, 1963). 90.

In the floodout region, a point is reached where velocity is at a critical limit and no longer able to support the transport of bed­ load. The remaining bedload sediment is deposited, some aloiAoj the levee rises characteristic of the floodout zone, and sowe £w\- like form of increased gradient (noted by Schumm, 1961), an example of which was noted for Fowlers Creek. This is in part a "resultant" form: small flows would tend to build back the zone of deposition, while larger floods probably move material from this region further downstream. Stream velocity would potentially increase immediately downstream of such fan-like deposits, as there is a steepening of the floodplain gradient. It is possible that scouring in this reach of increased velocity could initiate the formation of the multiple dis­ tributary channels. These channels appear to re-adjust their gradients to their sediment loads by increasing sinuosity. Width/depth ratios decrease as sediment is deposited up and over the channel banks, confining these distributaries by their own levees. Crevassing of these levees occurs, producing still more confined distributaries.

The fan-like form of increased gradient in the lower floodplain appears to correlate with the downstream extent of 1^-3 year flood (seec.cy p-64-,77). return period/ Flows from more extreme floods should persist beyond this zone. For instance, extreme seasonal rainfall in the Georgina-

Diamantina and Cooper catchments enables flows to persist beyond the zone of anastomosing channels and sump basins which mark the beginning of a potential floodout zone, to reach Lake Eyre. 91.

Records of Downstream Changes in Discharge

From discharges calculated or estimated for the Finke River and

Fowlers Creek, from other exmaples of floodout channels in the

Australian arid zone and from other published records which include the measurement of transmission losses in channels in Australia and the United States of America, some general appreciation is gained of the magnitudes of losses in discharge (Table 7-1). For six major rivers in arid or semiarid regions of Australia, mean down­ stream loss in discharge is 0.88 percent per kilometre for bankfull or "normal" flood flows. This agrees well with an average value of

0. 7 percent per kilometre found by Cornish (1961) in a study of bankfull channel losses in semiarid Oklahoma, USA. In this study,

Cornish noted that the loss was approximately uniform throughout the channel length.

Large percentage losses per kilometre apparently occur in floods exceeding bankfull stage. This may be seen in Table 7-1 from data collected for the lower reaches of the Lander, Finke and Diamantina

Rivers, in which the average loss for more extreme floods was cal­ culated as 2.07 percent per kilometre. Further particulars of these floods are:

1. Lander River. For April-May 1970, the discharge in the Lander

River at Moreton's Yard (N.T.A. Wat. Res. Branch, unpublished records) was 10,200 cu.m/sec. approximately 2.37 times the bankfull discharge at this section. Flood data summarised by Leopold, Wolman and

Miller (1964) indicate a recurrence interval of about 20 years for a flood of this magnitude. Losses over a 140 kilometre reach were

2.5 percent per kilometre, compared with a calculated 0.7 percent per kilometre for bankfull floods. Rate of Distance Source River Flow regime loss of over which of discharge losses data (%/km) occur (km)

Diamantina bankfull 1.01 135 Bonython, 1963

Qld. 15 yr.flood 1.90 135 M M

Coopers normal 0.20 385 Nimmo, 1947

Creek median 0.22 385 Bonython, 1963

Qld. 15 yr.flood 1.20 770 it u

Finke, bankful1 1.0 300 Mabbutt and Schumm,unpub., N.T. field survey 15-20 yr. 1.8 500 Williams, 1970 flood Lander, bankfull 0.7 150 N.T.A. Water Res.Branch N.T. 20 yr.flood 2.5 150 it tt it

Darling, bankful1 0.55 800 Laurenson, 1962 N.S.W. Murrumbidgee, bankful1 1.30 240 Laurenson, 1962 N.S.W. Fowlers Creek bankfull 2.50 40 field survey N.S.W. sandy channels, bankfull 0.7 648 Cornish, 1961 Oklahoma, U.S.A.

TABLE 7-1 Rates of loss in stream discharge for some ephemeral channels in Australia and the United States of America. 92.

ii. Finke River. For floods in the Finke channel during February-

March 1967 (Williams, 1970) discharges were directly estimated at several sections. At Henbury (with local heavy rain) the estimated peak discharge was 8.54 times the normal bankfull value. At

Idracowra, the peak was 1.23 times the bankfull discharge, and at

Finke township it was 3.96 times bankfull discharge.

If the extreme value at Henbury is regarded as atypical, since much of this was probably due to extreme local runoff, the flood represents an event with a general recurrence interval of about 15-20 years.

Downstream decrease in discharge for this flood was approximately

1.8 percent per kilometre, compared with a decrease of 1.0 percent for bankfull flows.

iii. Diamantina River. For the April-May 1963 floods in the Diamantina

River (Bonython, 1963) the peak discharge at Birdsville was approxi­ mately 2.85 times bankfull discharge, indicating a recurrence interval of about 15 years. Losses from this flood were 1.9 percent per kilometre, compared with bankfull losses of 1.0 percent per kilometre.

The above loss values are consistent with the findings of Sharp and

Saxton (1962) who noted that percentage transmission losses in eighteen rivers on the Great Plains, USA, increased with the magnitude of flows. These increased transmission losses for floods of higher magnitude is also consistent with the observations (e.g. for the

Finke River or for Fowlers Creek) that the floodout zone represents a terminal for floods of a wide range of magnitudes greater than calculated bankfull stage. 93.

Mathematical Models* Explaining Floodouts

The hydraulic processes associated with floodout development may be described quantitatively from studies carried out by geomorphologists and hydraulic engineers on stable and unstable channel systems.

These should provide a basis for deriving mathematical models to explain the occurrence of floodouts in more precise, quantitative terms.

As discharge decreases downstream there is a consequent decrease in velocity, associated with a decrease in competence. At some point in the downstream decrease in velocity, a critical limit is reached where the remaining bedload tends to be deposited. This zone of

aggradation is the upper limit of the floodout zone, and beyond this the width/depth ratios should be adjusted to equilibrium with channels carrying their total load as suspended load. Two mathematical models can explain the location of zowcs iv> ■Wvi/v'S ^fowiov-jjhtc- parameters.

In the derivation of the two models which follow, the following symbols have been used:

d = mean grain size diameter of bedload, in inches

D = channel depth, in feet

M = percentage of silt+clay in channel sediment load

*These models have been derived using other established but empirical relationships, including those of Lacey (1934), Schumm (19603, b) Maddock (1969) and Henderson (1961). As the coefficients of all such established equations relate to F.P.S. units of measurement, this convention has been followed here, and metric units avoided. Conversion back to metric units would involve changes in coefficients but not in exponentials. 94.

P = channel perimeter, in feet

Q = bankfull discharge, in cubic feet/second (cusec)

s = slope or gradient

v = stream velocity, in feet/second

w = channel width, in feet

Model (A)

Maddock (1969) noted that for stream velocities in the low range, the velocity associated with the onset of bedload sediment movement could be expressed in terms of mean sediment diameter and stream depth. For decreasing discharge, and hence decreasing velocity, this equation can be used to represent the velocity below which no bedload sediment can be transported.

0 375 0 125 His equation v = 9.6 d D , in which (d), the mean sediment

grain size diameter is expressed in feet can be modified to express

(d) in inches thus:

,0.375 _0.125 v 24.38 a L) (1)

When this velocity is reached, all load which can be transported is

silt and clay. Schumm (1960) expressed the percentage of silt-clay

in a channel load as M. Thus below this critical velocity M = 100.

From Schumm's (1960) equation channel form may be related to sediment

load transported thus:

-1.08 w/D = 225M ••• (2) 95.

For the critically low velocity

1 no w/D = 225/ 100

= 225/ 145

= 1.55

.’. w = 1.55 D . .. (3)

But v = Q/wD ... (4)

At the critical velocity, from equation (3)

v = QD 2/ 1.55 ... (5)

Substituting in equation (1)

-2 0 375 0 125 QD = 1.55 x 24.38 d D

37.78 d0*375 D°-125

.*. Q = 37.78 d0,375 D2,125 (6)

Thus from equation (6) it can be seen that a set of hydrologic conditions can exist in a situation in which discharge is decreasing, such that deposition of bedload and associated flooding out must occur; when Q = 37.78 d D

Application of Model (A)

The model was used with data collected at Fowlers Creek, just upstream of the floodout zone, where bedload is very coarse sand (Wentworth class) and channel depth is 2.80 feet. 96.

D = 2.80 2.125 D = 8.92

d = 0.08 0.375 d = 0.92

37.78 d

i.e. The Model would predict that the floodout should occur where discharge falls to 311 cusecs. Calculated bankfull discharge

(Table 6-1) from a section just upstream of the floodout zone is

360 cusecs.

This result would tend to indicate that the floodout form is in

equilibrium with a flood of approximately bankfull stage. While

floods of greater magnitude occur, they are sufficiently infrequent not to affect the location of the floodout zone. It would be ex­ pected that such floods however should carry bedload sediments into the channels of the distributary zone. This has been noted in the

Finke River distributary channels. Whitehouse (1941) also describes

sandy beds in the anastomosing channels of the Diamantina and Cooper systems following major floods.

As bankfull discharge and regional slope are two independent hydro-

logic variables, the development of a floodout zone may also be

explained in terms of these variables.

Henderson (1961) related gradient (2) and bankfull discharge (Q)

in stable channels by the equation s = 0.44 d^*^ Q ... (7)

Thus for any sediment grain size s = k Q (where k is a constant). 97.

It has been shown that for floodout streams discharge (Q) increases then decreases downstream. Hence slope or gradient (s) should decrease then increase in accordance with the relationship k/ Q^'^.

Stream gradient can be adjusted to a limited extent in response to discharge since gradient is readily decreased by increasing sinuosity, and conversely can be increased by channel straightening, to compensate for some loss in discharge.

As regional gradients decrease downstream, a critical point is reached where channel slope can no longer be increased to compensate for decreasing discharge. Beyond this point velocity becomes too low to support bedload, and deposition of this bedload occurs.

An example of this attempt to adjust stream gradient can be seen from the Finke River. From Fig. 5-7 it can be seen that meandering is most apparent near Idracowra and Horseshoe Bend - the two sections where bankfull discharge is at its peak (see Table 5-6, Fig. 5-9) while channel straightening occurs just above the Hugh confluence, and downstream of Horseshoe Bend where discharge is decreasing.

The critical relationship between slope, discharge and other channel parameters can also be explained quantitatively: 98.

Model (B)

Henderson (1961) related slope (s) and channel width (w) to mean sediment grain size (d) and discharge (Q) for stable alluvial channels thus: ,1.15-0.46 s = 0.44 d Q ... (7)

w = 0.93r\ 07 d.-0.15 Qn 0.46 ... (/'o'v8J multiplying equations (7) and (8) to eliminate Q

ws = 0.44o /i/i x 0.93o 07 d Q„0.46, / d,0.15 nQ0.46

.’. w = 0.41 d/s

/. d = ws/ 0.41 ... (9)

From equation (1), the critical velocity at which bedload transport ceases is given by

v - 24.38 d0-375 D0-125

This velocity can also be expressed in terms of some associated critical discharge, since v = Q/wD

0 375 0 125 .*. Q/ wD = 24.38 (ws/0.41) * D (10)

Equation (10) is an expression of discharge at which bedload transport should cease, in terms of the geomorphic parameters of channel width, depth and slope.

Application of Model (B)

This equation may be applied to data obtained from Fowlers Creek and from the Finke River near their floodouts.

For Fowlers Creek3 a channel section was measured slightly upstream of the floodout, between sections 5 and 6. No sediments were analysed 99.

from this section. In this reach, the channel is straight, and at

the section its width was 4.54 m (15 ft), depth 0.91 m (3 ft) and

slope approximately 0.002.

From equation (10)

1.375 0.375 _1.125 Q 34 w s D 1 375 0 375 1.125 34 x (15) x (0.002)u°/D x (3)

34 x 41 x 3.4 x 0.09

= 426.5

i.e. The discharge at which bedload transport should cease is 426.5 cusecs (11.8 cu. m/sec). Estimated bankfull discharge at this section was 11 cu. m/sec., and the floodout begins immediately downstream of this section, with the development of now-abandoned distributary channels.

For the Finke River the equation has been applied to the section below the entry of Coglin Creek, where channel width is 100 ft, depth 2.5 ft, and slope approximately 0.002.

1.375 0.375 n1.125 Q 34 w s D 1 375 0 375 1.125 34 x (100) x (0.002)u* :>/'J x (2.5)

34 x 562 x 2.8 x 0.09

= 4815 i.e. The discharge at which bedload transport should cease is 4815 cusecs (134 cu. m/sec). Calculated bankfull discharge in this section is 107 cu. m/sec, here within the beginning of the floodout zone. 100.

Physiographic Controls on the Location of Floodouts

It can be seen from their application to the Finke River and

Fowlers Creek floodouts that these models do explain, and to some degree predict the occurrence and location of floodouts, based on such geomorphic parameters as slope, grain size of sediments and channel dimensions.

Floodouts occur when, in conjunction with decrease in stream dis­ charge, the gradient of the lowlands of the catchment reach a critically low value. For channel systems such as those of the

Diamantina River or Coopers Creek within the Eromanga Basin structure, a tendency towards this process occurs when the regional gradient of the lowland sector of the catchment decreases, probably due to tectonic back-tilting (Mabbutt, 1967). For streams which flood out within sandy lowlands, continued accretion within these lowland sectors results in a gradual decrease of the regional gradient, with subsequent flooding out. Other physiographic factors however may operate to accelerate this build-up of sediment and control the location of the floodout. For instance in the Finke

River, constriction of its course in the lowest reaches by dune encroachment from the south-southeast forcing the channel back against a stony tableland on the west, has particularly contributed to the floodout location. 101.

CHAPTER 8

THE RIVER LAKES OF THE WESTERN AUSTRALIAN SHIELD:

THE EXAMPLE OF THE LAKE WAY - LAKE COWAN SYSTEM

The Present Drainage

The southwestern part of the Western Australian Shield, referred to by Jennings and Mabbutt (in press) as the Yilgarn Plateau geo- morphic province, is occupied by drainage of which the trunk elements consist of a series of mainly salt lakes (Fig. 8-1). For this reason the area has been known as Salinaland (Jutson, 1934). The

lakes occupy the main shallow valleys across the Yilgarn Plateau, and the salt lakes are isolated from the oceanic drainage.

The lakes are commonly aligned, but mostly not connected by channels, and each has a network of tributary channels. Many of the lakes are fringed or impounded by sand dunes, while some are connected by lines of smaller pans or ancient alluvium.

The present Salinaland drainage system was mapped broadly by de Martonne (1927) as partly areic and partly endoreic. Gentilli

(1952) re-mapped the area on the basis of observed river regimes and noted two regime classes within the area: areic, for the Salinaland part of the Yilgarn Plateau, and areic, sporadically exoreic for the western part of the area, where lakes are associated with coastal drainage systems.

The Reconstruction of Former or Potential Drainage Lines

It has become clear that these lakes represent what Gibson (1909) Fig. 8-1 The southern part of Western Australia, showing the Yilgarn Plateau and the drainage on this Plateau

102.

described as a deranged (i.e. disorganised) drainage system, and are hence generally known as "river lakes", although other suggested origins for the lakes have been published: as wind-planed salt flats (Woodward, 1897), as former ice-scoured depressions (Campbell,

1906) or as depressions on marine planation surfaces (Montgomery,

1914, 1916). In order to apply the three measures of drainage disorganisation used in this study it is necessary to reconstruct drainage systems on the Yilgarn Plateau.

Various lines of evidence have gradually enabled reconstruction of the former drainage. Earlier reconstructions used height data to establish the direction and continuity of fall, and the position of drainage divides. Such reconstructions were also based on logical consideration of the shape and orientation of the lakes and of their tributary systems (Gregory, 1914; Jutson, 1934; Hills, 1953).

Alluvial deposits between lakes indicate former flood connections

(Jutson, 1934; Mabbutt, 1963; Sofoulis and Mabbutt; 1963), the form and lithology of subsurface deposits give the position and direction of drainage (Mabbutt, 1963; Morgan, 1966; Bunting, van de Graaff and

Jackson, 1974) and the evidence of regional hydrology and the hydrologic and salinity regimes of many of the lakes has confirmed links between them (Chapman, 1962; Morgan, 1966). Submarine canyons off the coastline in the south of Western Australia are consistent with the outlets of major systems of drainage having existed during

Tertiary times (von der Borch, 1968). Observations of exceptional flooding linking lakes not normally connected have also been used in a reconstruction of the drainage (Browne, 1934). 103.

Recently, the mapping of soils on a continental basis at a scale of

1:2,000,000 (Atlas of Australian Soils sheets 5, 6 and 10 of

Western Australia, Northcote et al., 1967, 1968; Bettenay et al., 1967) and the mapping of vegetation of Western Australia at a scale of

1:250,000 (Beard, 1969 , 1972 ) have allowed reconstruction of former drainage lines more confidently and on an integrated scale (Beard,

1973 mapped 1968; Mulcahy and Bettenay, 1972; Bettenay and Mulcahy,

1972).

The reconstruction of drainage given in Fig. 8-2 is modified from a map of Tertiary drainage systems by Beard (1973). Some tributary drainage lines have been completed and some alterations made on the basis of the direction of fall. In this amended reconstruction use has been made of unpublished height data, obtained since 1968, from the Department of Lands and Surveys, Western Australia.

Where this reconstruction of fossil drainage links with active existing rivers, it is possible that subsequent alluviation may have masked the fossil drainage. Hence this reconstruction may not rep­ resent the actual Tertiary drainage, but rather the potential course which would be followed should exceptional flooding now link the system.

The reconstruction shows a broadly dendritic drainage, but with many trunk channels showing structural control. The grain of the meta- morphic belts determines the direction of many long valleys, approximately northwest-southeast, and results in ranges of low hills parallel with the valley lines. There is a main drainage divide through the Yilgarn Plateau (Fig. 8-2). This consists of a broad low Fig. 8-2 Reconstruction of the drainage system of Western Australia, mainly after Beard (1973)

104.

hill belt in the south, running slightly west of north, and turning to the east along a belt of tabular hills on sandstones and basalts at the northern margin of the Salinaland province. The lake systems tend to drain either to the west and southwest towards the present coastal drainage, or to the southeast where there was formerly an outlet to the sea.

The Lake Way - Lake Cowan System

A reconstructed Lake Way - Lake Cowan system in the east of the area

(Fig. 8-3) has been chosen as the sample catchment for the application of the three measures of drainage disorganisation within the

Salinaland province of the Yilgarn Plateau. This catchment is part of the system which drained to the southeast towards the shoreline of the Eocene sea, now the inner margin of the Nullarbor PIAin.

The system is typical of the drainage on the Yilgarn Plateau in that it consists of long narrow salt lakes, each with its own network of tributary channels, and with some of the lakes connected by chains of smaller pans. The catchment is also typical in its physiography in that it consists of low featureless divides with sandplain surfaces.

Because of these featureless divides sectors of the catchment boundary are indefinite, and in outlining the catchment it has been necessary to project the boundary across these surfaces from distinct watershed areas. Other features typical of the Plateau are restricted flanking hill belts along the valleys, low broken lateritic plateaux and lateritic breakaway zones, and long stony plains on granites. Along the main valley lines are lacustrine and alluvial deposits, including limestones, with areas of sandplain, and with dunes fringing many of the lakes. Relief throughout the catchment is low, generally less Fig. 8-3 The Lake Way - Lake Cowan drainage system Lorna Glen L. Carnegie

)L.Way Annean Stanford L. Austin Sandstone

Leonora XCaref Barlee

1Moore Kalgoorlr

L. Cowan

Norseman PerthW^(

L ■ Oundps Bunbury

IOO km

Lakes and streams Lake Way-Lake Cowan catchment

Catchment boundary indefinite

Isohyets of mean annual rainfall —30 values in cm

Towns or settlements

@ Climate stations used 105.

than 30 m, and the longitudinal gradient is typically low, at approximately 1 in 2,000 with the lateral gradients higher at 1 in

150 to 1 in 700.

The choice of the catchment is also justified in that good airphoto coverage is available for the entire catchment, and in that there is little doubt in the reconstruction of the Lake Way - Lake Cowan drainage system. A good indication of former drainage lines in the northern part of the catchment is given by the evidence of super­ ficial geology, slopes and relief from a survey carried out in the

Wiluna-Meekatharra area (Mabbutt et al., 1963). Airphoto mosaics of the area show that several of the lakes are connected by chains of smaller pans or by lines of denser vegetation marking former river courses. Where lakes are separated by areas of sandplain and all traces of former drainage have been obliterated, the numerous spot heights now available (Department of Lands and Surveys, Western

Australia, unpublished) have enabled potential links, as indicated by fall, to be completed. This reconstruction of drainage agrees closely with those of Morgan (1966) and Beard (1973).

This system ceases to be traceable on the inner margin of the

Nullarbor Plain where Eocene marine limestone now indicates its former outlet. As the system is now cut off from oceanic drainage there has been no alteration of the former network through more, recent capture by regressive drainage Aeve\opM ovc 4-Le 106.

The Proportion of the Catchment Hydrologically Effective

The catchment boundaries were mapped from 1:250,000 maps* and airphoto mosaics* The areas of effective drainage i.e. generating a network of channels, were mapped from airphoto mosaics, and are shown in

Fig. 8-4. Those parts of the catchment which do not generate drainage networks are mainly on the margins of the system, and occupy positions along what would normally be expected to be the watershed of the catchment, with smaller areas along the main valley lines. This pattern is repeated in both subcatchments.

The Ratio of Effective Catchment was calculated for each subcatchment and for the entire catchment, and these values are shown in Table 8-1.

A Ratio of Effective Catchment of 78 percent in this catchment is re­ latively high for the Australian arid zone, being similar to that in the Georgina and Macumba catchments and much higher than values for the Sandover and Finke catchments (Tables 4-1 and 5-1).

Factors Determining the Ratio of Effective Catchment i. RainfaVl. Average annual rainfall throughout the Lake Way - Lake

Cowan catchment is approximately 23 cm (Arnold, 1963; Bur. of Met.,

1968; Dept, of Nat. Devel., 1970), with isohyetal patterns (Fig. 8-3) indicating slightly higher rainfall values in the south and west of the catchment. This shows no correspondence with the location of effectively drained parts of the catchment.

*Australia 1:250,000 Topographic Series sheets: Balladonia, Barlee, Boorabbin, Cundeelee, Duketon, Edjudina, Glengarry, Kalgoorlie, Kingston, Kurnalpi, Lake Johnston, Laverton, Leonora, Menzies, Minigwal, Neale, Norseman, Plumridge, Rason, Ravensthorpe, Robert, Sandstone, Sir Samuel, Throssell, Widgiemooltha, Wiluna, Youanmi, Zanthus. Lands and Surveys Department, Western Australia, 4 miles to 1 inch airphoto mosaics as above. ' Total area Area of effective Ratio of effective Catchment 2 (km2) drainage (km ) catchment (%)

Lake Way 100,000* 78,700 78.7 subcatchment

Lake Cowan 400,000* 311,300 77.8 subcatchment

Total 500,000* 390,000 78 catchment

* These figures are rounded since catchment limits are not able to be accurately defined.

TABLE 8-1 Areas of topographic and effective catchments for the Lake Way-Lake Cowan system. Fig. 8-4 The areas of the Lake Way-Lake Cowan catchment which are effectively drained N A

100 Km I------1

Areas of catchment effectively drained 107.

The five climatic stations within or on the margin of the catchment

(Table 8-2 and Fig. 8-3) show a trend towards a summer rainfall maximum in the north of the catchment and a winter rainfall maximum in the south. However neither trend is reflected in the location of areas of the catchment effective in generating runoff, and apparently any seasonal flooding which results from these rainfall maxima is not sufficient to maintain effective drainage. There is no correspondence between rainfall seasonality and effective drainage.

Rainfall intensity, as indicated by the amount of rain per rain day, shows a slight increase northwards and eastwards within the catchment, however again there is no trend corresponding with the pattern of effectively drained parts of the catchment, and there appears to be no relationship between rainfall and the Ratio of Effective Catchment.

ii. Terrain. Slightly more than 20 percent of the Lake Way - Lake

Cowan catchment does not generate channel networks, and this area consists of sand covered surfaces. On the catchment margins sandplain occurs on eroded granite plains and on the lateritic old plateau

(Jutson, 1934) surface. In the lower parts of the system reworked alluvial material is deposited as sandplain and dunes. In each case the sand deposition is both a cause and a function of the lack of effective drainage, as residual sandplain on the lateritic surfaces, and as transported and deposited alluvium in the former stream courses.

Catchment form and landsurface type both contribute to the distribution of effectively and non-effectively drained parts of the catchment.

Gradients along the trunk drainage lines are low. The relief ratio Mean annual Summer Winter Rain per Climate rainfall rainfall rainfall rain day Station (cm) Oct.-Mar. Apr.-Sept. (cm)

Wiluna 24.0 13.5 10.5 0.7

Lorna Glen 24.5 14.8 9.7 0.8

Leonora 19.2 7.1 12.1 0.6

Kalgoorlie 22.2 8.4 13.8 0.5

Norseman 25.6 8.8 16.8 0.5

TABLE 8-2 Rainfall data for the Lake Way-Lake Cowan catchment. 108.

(after Schumm, 1956) calculated from spot heights on the catchment boundary and in the lowland reaches, is 1 in 2,100 for the Lake Way subcatchment, and 1 in 2,000 for the Lake Cowan subcatchment. The lateral gradients towards the trunk valleys are much steeper however, ranging from 1 in 700 to 1 in 150 for each subcatchment, and drainage systems developed on the steeper valley margins bring in sediment which cannot be moved by the trunk channels across the flat longi­ tudinal surfaces, and accumulates in the lowlands.

A high proportion of the landsurface of the catchment generates channel networks. The distribution of landsurface types is shown in Fig. 8-5, and the proportions of these within the catchment are shown in

Table 8-3. In the extreme north of the catchment broad hills with long stony slopes are developed on sandstones, metasediments and granites. These provide the main upland surfaces in the catchment, and give rise to a dense channel network. The steep breakaway margins of the lateritic plateaux and the exposed underlying granitic surfaces support channel networks, although the broad sandy plateau surfaces do not. Within this catchment the lateritic plateaux are mainly small dissection remnants, and their breakaways generate networks of channels tributary to many of the lakes. Stony plains, chiefly on eroded granites, make up the main part of the catchment, and give rise to sparser but integrated channel networks. Limestone surfaces on the margin of the Nullarbor Plain do not contribute to surface runoff.

The proportion of erosional surfaces within the catchment can account for the relatively high value of 78 percent for the Ratio of Effective

Catchment. The form of breakdown of drainage also accounts for this.

As the extended network disintegrates along its length on the low- Landsurfaces in the Area Percentage Lake Way-Lake Cowan catchment (km2) area

Erosional landsurfaces

Hills and ridges on sandstones, metasediments and granites 13,350 2.7

Lateritic breakaway zones 22,000 4.4

Stony plains on granites and metasediments 264,900 52.9

Limestone plains 20,000 4.0

Depositional landsurfaces

Sandplain 64,000 12.8

Dunefield 16,000 3.2

Alluvium and evaporites 100,000 20.0

TABLE 8-3 Proportions of each landsurface within the Lake Way- Lake Cowan catchment. Fig. 8-5 Landsurfaces and relief in the Lake Way-Lake Cowan catchment 'L.Cowan Nullarbor

100 km

Erosional surfaces Depositional surfaces

Hills on sandstones and metasediments Sandplain Lateritic breakaway Dunes zones Plains on granitic Alluvium or rocks evaporites

Plains on limestone

Catchment boundary 109.

gradient slopes, each system thus formed consists of a playa segment with its own network of tributaries (Plate 8-1), and hence effectively drained, with small sandy areas between these segments not occupied by a drainage network.

iii. Catchment Size. This catchment is larger than others studied within the Australian arid zone, and might therefore be expected to show a lower value for this ratio. However the value is higher than those obtained for smaller catchments such as the Finke and Sandover.

This reflects the form of breakdown of drainage. Within the Lake Way -

Lake Cowan system the distance from the catchment margin to the axial lowland is not a critical factor in breakdown which reflects catchment size, and breakdown through disintegration of the axial drainage along its length does not result in a low Ratio of Effective Catchment as the size increases.

The Lake Cowan subcatchment is four times as large as the Lake Way subcatchment, with no difference in the Ratio of Effective Catchment, and it appears that catchment size is relatively unimportant in determining this Ratio with this type of drainage system.

The Degree of Channel Connectivity

Stages of degree of channel breakdown (modified after Dubief, 1953) were measured from airphoto mosaics and airphotographs. These stages are shown in Fig. 8-6. Within this catchment no indication of a potential trunk channel appears on the airphotographs, and the two subsystems are isolated. Thus the stage of breakdown of the system is (v). Fig. 8-6 Stages of channel breakdown in the Lake Way-Lake Cowan drainage system L. Way

L.J/Mason L %^ond6 Dariot

Youanmi

; L. Carey

Leonora

L. Rebecca

Yindarlgooc/a ■xvdZr—?--^- 'Lefroy ^ Nail arbor yf

'Johnston \Lakes

Cctchment boundary Actual streams or lakes

Catchment boundary Series of pans or soil

indefinite type indicating drainage lines

Stoge of disorganisation Hypothetical continuation of

(modified after Dubief ) drainage lines

Lower order • streams shown ore

J 5th order, except those shown 4 Plate 8-1 Vertical air photographs within the Lake Way- Lake Cowan catchment. A. Showing a well organised tributary system of Lake Darlot, rising in a lateritic breakaway zone. B. Showing Lake Darlot, elongate, salt crusted, and encroached upon by sandplain.

Photos by permission of the department of Lands and Surveys3 Western Australia.

110.

In the two main subcatchments breaks occur in the trunk channels, and

the evidence of former stream courses which remains consists of chains

of small pans, isolated by areas of sand. The two subcatchments each

therefore show a stage of breakdown of (ii).

The lower part of the system is hence disconnected and highly dis­

organised, with river lakes forming drainage terminals and with sand

accumulations both causing hydrologic breaks and obliterating drainage

lines. This is in strong contrast with the networks in the upper

parts of the catchment, which comprise organised and connected

tributary systems. This difference in connectivity within the network, and the low connectivity of the trunk drainage in this morphostructural division can be explained by three factors.

Catchment shape is an inherited factor influencing connectivity. The

subcatchments are long and narrow, following the old valley lines.

The lake chains are elongated along these old shallow valleys, such that the resulting short lateral drainage across the steeper side walls of the valleys reaches the axial lowland, carrying sediment load

into the trunk channels. The long trunk lines across the low-gradient surfaces are not maintained against this alluviation, and breaks occur

in the trunk channels.

Low gradients and low relief contribute to failure of the trunk drainage. In this southeastern part of the Plateau withdrawal of the sea to the southeast following the Eocene marine incursion probably extended the drainage courses without rejuvenation. This system is now remote from the steeper fall of the oceanic drainage and the overall gradient of 1 in 2,000 is low. In less than 5 percent 111.

of the catchment is relief greater than 30 m and nowhere does it reach 100 m, while more than 50 percent of the surface is plainland, with very low relief. The drainage divides are low with few steep slopes to generate high-energy runoff. These conditions favour ponding of the lakes through blocking of drainage lines by sand, and dismemberment of the shallow systems into a series of lakes and pans.

Surface cUposcts also favour this blocking of the trunk drainage.

Most of the surfaces of the Yilgarn Plateau are lateritic or granitic, and sand or sandy soils are derived from these surfaces. Under arid conditions this blown sand has been deposited in the lower parts of the system, impounding lakes and obliterating channels.

The Comparison of Actual and Potential Channel Lengths

The reconstructed drainage network of the Lake Way - Lake Cowan system was used to compare actual and potential channel lengths.

The system is pinnate in form, and hence there is a distortion of the bifurcation ratios obtained from either the trunk channels or from the tributary network. The lengths of the trunk channels were measured by connecting the river lakes along their topographic courses.

The probable numbers of lower order stream segments and their lengths have been calculated using the higher bifurcation ratios derived from the tributary drainage entering the lakes. These data are listed in

Table 8-4 and the graphical comparison of actual and potential cumulative segment lengths is given in Fig. 8-7. LAKE WAY-LAKE COWAN SYSTEM, W.A.

Theoretical Actual Percentage Order of Streams Average Average Missing From No. of Total Segment Cumulative No. of Total Segment Cumulative Theoretical Segments Length Length(L) (L) Segments Length Length(U) (L) Network

Lake Wjiy System

1 7460 7460 1.0 1.0 6530 2610 0.4 0.4 12 2 2580 4260 1.65 2.6 2150 1720 0.8 1.2 17 3 893 2590 2.9 5.5 670 1140 1.7 2.9 25 4 309 1514 4.9 10.4 206 659 3.2 6.1 33 5 104 1238 11.9 22.3 52 489 9.4 15.5 50 6 32 646 20.2 42.5 16 283 17.7 33.2 50 7 2 640 320.0 362.5 5 500 100.0 133.2 - 8 1 800 800.0 1162.5 0 - - 133.2 100

Lake Cc wan System 1 13656 13656 1.0 1.0 12950 5180 0.4 0.4 5 2 4984 7974 1.6 2.6 4154 3323 0.8 1.2 17 3 1819 5457 3.0 4.6 1364 2320 1.7 2.9 25 4 664 4382 6.6 11.2 443 2170 4.9 7.8 34 5 241 2314 9.6 20.8 120 852 7.1 14.9 50 6 79 1920 24.3 45.1 40 872 21.8 36.7 50 7 6 2000 333.0 378.1 11 1342 122.0 158.7 - 8 2 470 235.0 613.1 0 - - 158.7 100 9 1 400+ 400+ 1013.1 0 - - 158.7 100

Total S ystem 1 21116 21116 1.0 1.0 19480 7390 0.4 0.4 8 2 7565 12234 1.6 2.6 6304 5043 0.8 1.2 17 3 2712 8047 5.0 5.6 2034 3460 1.7 2.9 25 4 973 5896 6.1 9.7 649 2830 4.4 7.3 33 5 345 3552 10.3 20.0 172 1340 7.8 15.1 50 6 111 2566 23.1 46.0 56 1155 20.6 35.7 50 7 8 2640 220.1 376.0 16 1842 115.0 150.7 - 8 3 1270 457.1 833.0 0 - - 150.7 100 9 1 400+ 400+ 1233+ 0 - - 150.7 100

TABLE 8-4 Actual and potential channel numbers and segment lengths in the Lake Way-Lake Cowan catchment Fig. 8-7 Graph of actual and theoretical cumulative mean segment lengths for the Lake Way-Lake Cowan channel systems 1000 Lake Way Loke Cowan subsystem subsystem

cumulative

me on

segment

length

stream order stream order

Lake Way Lake Cowan

system

cumulative meon segment length Theoretical

Actual

stream order 112.

The difference between the actual and potential channel lengths can be seen from Fig. 8-7 to be large, especially for the higher stream orders, and this comparison indicates that the Lake Way - Lake

Cowan catchment is highly disorganised, compared with sample catch­ ments from other divisions of the Australian arid zone (Figs. 4-12 and 5-14). As occurred with the other two measures of disorganisation, the comparison of actual and potential channel lengths shows the two parts of the catchment to be very similar in their degree of disorgani­ sation, and in the form of network breakdown.

It can be seen from Table 8-5 that approximately 60 percent of the potential channel length throughout the catchment is missing from the system. A higher proportion of channel length is missing from the network at higher stream orders than at lower orders, and this mainly represents the long sectors missing from the trunk drainage lines between the lakes, and the potential ninth order channel which no longer exists. There is also some failure within the potential tributary networks.

The reconstructed drainage consists of long trunk channels with short lateral tributary networks. Most of the lakes receive fifth or sixth order tributaries with some of these potential tributaries flooding out before reaching the lakes. The lower proportion of channel lengths missing from the lower order segments reflects the better organisation of the tributary networks, derived from the hills or lateritic break­ aways which form the low divides on the margins of the former valleys, parallel with the axial drainage. The main losses of channel length from the network are the sectors along the trunk drainage lines where the outlets of the lakes are blocked and the former connections oblit­ erated. a3 £ P -H

ooj •H P CD U o CD X P

CD X P o6 G P W) g •rH m m •H E co X PW) c CD system. g c

P -H XS G CD m Oj

in Cowan

p G0 E 0tiO co Way-Lake 0 c c G

X Lake

o p

o the 0

fcoO of pG

CJ G 0

X, network

LOI 00 W X m < E- 113.

The Comparison of the Three Measures of Drainage Disorganisation

As each of the three measures distinguishes a different aspect of drainage disorganisation, they give different results when applied

to the Lake Way - Lake Cowan system, where disorganisation is a

result of breaks along the axes of the system, but not breakdown of

the entire network. Together the measures show that a high proportion

of the catchment surface generates runoff, and that disorganisation

of the system is low when expressed in terms of effectiveness of

catchment. The network however is disconnected, and the method of

measuring disorganisation through connectivity shows a high stage

of breakdown. The measure of comparing actual and potential channel

lengths also indicates that a high level of disorganisation occurs,

particularly in the trunk sectors. Together the measures reflect the

control over drainage breakdown of the form of the catchment, which is

determined by the structural setting and landscape evolution of the

Yilgarn Plateau.

The original drainage consisted of long broad valleys, not excessively

branching, and with low, flat, broad divides and low interfluves,

inherited from the low density drainage system developed across a

much larger continental area. The Plateau surface consists of ex­

tensive plainlands with minor narrow hill belts of metamorphic rocks,

and with areas of lateritic plateaux and discontinuous flat-lying

Palaeozoic sedimentary rocks which form tabular hills. This planation

surface with its associated deeply weathered materials was developed

during the long humid climatic phase during late Mesozoic to

Tertiary times (Jutson, 1934). The crystalline rocks of the Shield

formed a tectonically stable uniform basement for this planation and 114.

lateritisation. The drainage system developed at this time was sparse and pinnate, controlled by the relatively minor belts of

Archaean schists, greenstone and whitestone (Sofoulis and Mabbutt,

1963). This drainage system was part of a larger continental system.

Australia and India broke away from the Gondwanaland land mass during the early Triassic, and separated during the Cretaceous with the formation of the western coastline of Western Australia (McElhinny,

1970). Australia, however, remained part of the Antarctic land mass, and the low gradients of the valleys across the Yilgarn Plateau reflect this more extensive drainage system.

Rifting and associated warping as Australia separated from Antarctica in the mid-Eocene (McElhinny, 1970; Veevers, 1971) shortened and rejuvenated the drainage of the Yilgarn Plateau. Dissection of the laterite resulted in the formation of breakaway zones which run parallel with the main valley lines, leaving laterite plateaux on the drainage divides. The incursion of the sea during the Eocene resulted in drowning of many of the lower valleys directed to the southeast.

The present inland drainage systems were elongated and their slopes reduced still further following the withdrawal of the sea to the southeast after Eocene times, and the fluvial regimes gave way to lacustrine with the onset of more arid conditions. The relative drifting of the Australian continent away from the pole has been associated with an increase in aridity. This northward drift which followed the breaking up of the Gondwanaland continent has altered the latitude of Perth, Western Australia, from 57°S during the

Mesozoic (Embleton, 1972) to 42°S by the Miocene (Wellman, McElhinny and McDougall, 1969; McElhinny, 1970). Jennings (1967b) and Lowry (1970) 115.

have noted that by Miocene times the climate of southwestern Australia was arid, and has not been much more humid than at present for any long periods of time since then.

It is reasonable to suppose that this increased aridity led to increased sedimentation in the fluvial systems. Bore logs through many of the present playa and aeolian surfaces (Bunting, van de Graaf and Jackson, 1974) indicate that fluvial systems gave way to lacustrine environments during the Miocene, which with progressive infilling led to the formation of the present river lakes and their associated aeolian features. Lunette deposits of Pleistocene age (J. Bowler, pers. comm.) along many of the salt lakes of the Yilgarn Plateau consist of both sand sized and silt or clay sized grains (Bettenay,

1962), implying that during their formation lakes stood periodically at high water level and were periodically dry. This indicates that although there were periods of high runoff into some of the lakes during the Pleistocene, fluvial regimes were not resumed along these systems after the Miocene. The accumulation of Pleistocene sands

(Bettenay and Mulcahy, 1972) effectively blocked off the outlets of the lakes which have since remained as disconnected drainage terminals. 116.

CHAPTER 9

EFFECTIVENESS OF THE MEASURES OF DRAINAGE DISORGANISATION

The three measures of drainage disorganisation used in this study enable the recognition of different aspects of the failure of drainage.

One measure is based on the catchment, one on the continuity of the channel system, and the third on the completeness of the network.

Because of this they respond differentially to the level of disorgani­ sation, depending on the pattern of the failure as exemplified in the three major catchments studied, where different patterns of dis­ organisation reflect different morphostructural settings and related controls.

The results of the application of these three measures of disorgani­ sation within catchments chosen from the three morphostructural divisions within the arid zone are summarised in Table 9-1.

In the central part of the Interior Lowlands morphostructural division, the Georgina-Diamantina catchment, mainly co-incident with the

Eromanga Basin, but extending into a contrasted morphostructural setting, is the catchment in which the highest levels of organisation of drainage occur for each of the three measures, and it has been suggested that this is due to factors of geology, terrain and climate.

The landscape is developed within a saucer-shaped basin, with its centripetal drainage reflecting youthful uplift. The catchment is comprised of soft rocks, which are readily erodible and which provide a large area of sloping erosional surfaces. The pattern of rainfall also favours the generation and maintenance of channel networks, with the highest, seasonally concentrated rainfalls on the margins of the catchment. Measure of drainage Catchment Disorganisation Morphostructural division subcatchment Ratio of Stage of % of on which effective break­ channel developed catchment down lengths (%) missing

Diamantina 91 (0) 20 Central Interior Lowlands

Georgina 80 (2) 35 Mainly central Interior Lowlands

Sandover 59 (2) 42 Central Australian part, Western Plateau

Georgina-Diamantina 0 Mainly central Interior Lowlands

Macumba 80 (0) 8 Central Interior Lowlands

F inke 36 (3) 45 Mainly central Australian part, Western Plateau

Finke-Macumba 3 Central Interior Lowlands and central Australian part, Western Plateau

Lake Way-Lake Cowan 78 5 59 Yilgarn Plateau of Continental Shield

TABLE 9-1 Comparison of the results obtained when the three measures of drainage disorganisation are applied to the three sample catchments. 117.

In contrast, the central Australian part of the Western Plateau, as represented by the Finke-Macumba catchment, is the morphostructural

setting in which the lowest proportion of effective catchment and high levels of discontinuity and incompleteness of the network occur.

Factors of terrain mainly determine this high degree of disorgani­ sation. The uplands, commonly uplifted cratonic basins or Shield blocks, are widely spaced and comprised mainly of hard rocks which are not readily erodible and which have remained stable for long periods of time. The build of the central Australian part of the

Western Plateau apparently reflects its previous drainage history.

Rivers such as the Lander, the Officer and the Finke were presumably tributaries in formerly more extensive drainage systems (see for example Fig. 5-2 for the Finke). In these systems the catchments were apparently made up of large upland areas, long sloping surfaces and extensive plainlands. Disorganisation and fragmentation of these earlier systems has resulted in the present pattern of widely spaced uplands and very extensive lowlands, now surfaces of sand accumulation, with the present drainage networks occupying long narrow catchments, generally comprising relatively minor uplands and more extensive sandy lowlands. As all these basins are situated well inland, rainfall is low, and reworking of sand by wind is common throughout their lowland sectors, where the trunk channels fail, and the systems become dis­ connected and incomplete.

On the Yilgarn Plateau the three measures indicate a level of disor­ ganisation intermediate between those of the other two morphostructural divisions. The landscape has remained stable for a long period, and relief is subdued. The present drainage consists of long narrow catchments with low longitudinal gradients, in which the channel 118.

connectivity is low, and the stage of breakdown high. Although marginal

sandplains do not generate runoff, the short valley sides and erosional

landsurfaces of the edges of the old plateau are effective in

generating channel networks, and this is reflected in the Ratio of

Effective Catchment.

The comparison of the relative effectiveness of the three measures in

the three cases above allows the interpretation of the way in which

they respond differentially to various factors which contribute, in different degrees, to drainage disorganisation.

The Proportion of the Catchment Hydrologically Effective

This is a measure of the runoff response to rainfall locally as reflected by the existence of a drainage network, but not of its continuity. The measure focuses on local areas, and hence is inde­ pendent of the shape of the catchment and the disposition of high and

low ground within it. Effectively drained areas of a catchment can be mapped, or expressed as a percentage of the catchment area (i.e. the Ratio of Effective Catchment), and the factors which influence the locations of areas of effective drainage can be considered.

Factors Determining Areas of Effective Drainage

Three local factors are mainly responsible for determining areas of effective catchment; rainfall, and the two terrain factors of slope, and surface material. 119.

Rainfall. This may affect local runoff response by virtue of its

amount, intensity or seasonal occurrence. Although the results

obtained by Dubief (1953) for catchments in the Sahara indicate a

direct relationship between mean annual rainfall and the Ratio of

Effective Catchment, this relationship is less definitely shown for

the catchments studied in Australia. Within these catchments, rain­

fall patterns in as far as data was available for detailed comparison,

did not correlate in any way with the areas effectively drained. A

higher mean annual rainfall within the Georgina-Diamantina catchment

than in either of the other catchments studied may be correlated with

higher values for the Ratio of Effective Catchment. The relationship

is not clear however, since rainfall variation throughout the

Australian arid zone is small, and because other factors have more

direct influence.

Slope is a terrain factor of considerable importance in influencing

the generation of local drainage networks. Sloping surfaces are able

to maintain runoff, and resist the deposition of alluvial sediments,

and thus are effective in generating channel networks. The continuity

of sloping surfaces is also important in maintaining such networks

through the catchment. The proportion of sloping surfaces on the

rounded basin sides of the Georgina-Diamantina catchment is much higher

throughout than that within the Finke-Macumba catchment, where large

lowland plains occur; and the proportion of the Georgina-Diamantina

catchment which is effectively drained is correspondingly higher than

that in the Finke-Macumba catchment. Within the Lake Way-Lake Cowan

catchment the areas of effective catchment similarly correspond with

the long gently sloping surfaces along the former valleys and the

shorter steeper lateral slopes. 120.

Within the catchments it can also be seen that this measure reflects

the continuity of sloping surfaces. In the Diamantina subcatchment,

only the lowest section consists of flat-lying land, and 91 percent

of its area is occupied by effective drainage; whilst the Sandover

subcatchment with a greater extent of flat-lying land has only 59

percent of its area contributing to the drainage network. Similarly

within the Finke-Macumba system, the Macumba subcatchment developed

on the sloping surfaces of the Interior Lowlands has a Ratio of

Effective Catchment of 80 percent; and the Finke subcatchment, with

large lowland plains has only 36 percent of its surface generating

runoff.

Surface material is also an important factor in determining the

Ratio of Effective Catchment, as can be seen from the locations of

areas of effective drainage within each catchment. Erosional

surfaces, which commonly correspond with sloping landsurfaces, tend to generate runoff, whilst depositional landsurfaces, in particular

sandy lowlands, do not. For the catchments studied, the highest value for the Ratio of Effective Catchment occur within the Georgina-

Diamantina catchment, where only the central lowland area, and the

lowlands of the Sandover system represent surfaces of sand deposition; whereas the Finke-Macumba catchment with a much higher proportion of sandy lowland surfaces shows correspondingly lower values for the

Ratio.

Within each of these catchments the contrast is more marked, and the relationship between a high proportion of depositional lowland and low values for the Ratio of Effective Catchment can be seen. The

Sandover and Finke subcatchments, each with large sandy lowland 121.

surfaces, show considerably lower proportions of the landsurface

generating runoff than do the Georgina, Diamantina or Macumba sub­

catchments, each developed mainly on erosional terrain.

This measure is most useful in comparing catchments or subcatchments

with similar rainfalls and of similar sizes. It is therefore extremely

appropriate for comparing areas within one catchment. In general

larger catchments have a higher proportion of their surfaces not

effective in generating runoff, and the measure is most usefully

applied to medium sized catchments. In this study it has been 2 effectively used with catchments to 500,000 km .

The Degree of Channel Connectivity

This descriptive classification by numbering the stages of disinte­

gration of the channel network is a measure of continuity of the

system, that is, the ability of the whole system to persist through

to baselevel as a connected system. The measure directs attention

to the failure of the channel system, particularly the trunk stream,

and therefore does not take into account the proportion of the

catchment which contributes to this trunk drainage.

Although applied initially by Dubief (1953) to the long wadi systems

of the Sahara, in conjunction with hydrologic data, the measure has been used in this study on purely physiographic criteria, and it has been shown that it can be successfully applied in this way. 122.

In the classification of stages of breakdown, the measure is directed to a consideration of the form of the channel breakdown, and there­ fore does not adequately distinguish between degrees of failure given a similar form of breakdown. An example of failure in one form but to a different degree is seen within the Georgina-Diamantina system, where the Sandover and Georgina channels show a similar form of breakdown, and hence by this measure, a similar stage of break­ down; yet the magnitude of the failure is far greater in the Sandover system, and the failure is spread over a greater part of the network than is the case in the Georgina system.

Factors Determining Channel Connectivity

This measure, like the Ratio of Effective Catchment, is strongly influenced by factors of terrain, including slope and surficial material which have been noted as important in the generation and maintenance of channel networks, but particularly by the shape of catchments and the disposition of uplands.

Catchment form including shape and the disposition of upland surfaces within the catchment, and the consequent factor of the length a channel must persist to maintain a connected network, is a critical factor in this measure.

It has been shown (Chapter 5) that the length a channel will persist beyond its upland catchment, and hence the likelihood of reaching its potential baselevel, varies directly with stream order, which in turn varies with the area of generating surface. Long distances over which the channel must be maintained, and relatively small areas of upland catchment will lead to a greater degree of breakdown of the 123.

network. Basin-like catchments, rounded in form, in which a large proportion of the catchment is generating runoff and throughout which tributaries are directed towards the trunk channel, are most likely to build up a system which persists through to baselevel. Long narrow catchments, not picking up ancillary drainage are the most vulnerable to the failure of the trunk channel to persist.

This contrast in catchment form is seen between the Georgina-Diamantina catchment, with rounded basin-like form, and a high level of channel connectivity, and the Lake Way-Lake Cowan catchment, elongated, and with a low level of connectivity. Similarly the distribution of upland surfaces evenly throughout the Georgina-Diamantina catchment, with its high level of connectivity can be contrasted with the isolated, widely spaced pattern of uplands within the Finke-Macumba catchment, with its consequently greater level of channel breakdown.

In the central part of the Interior Lowlands the Diamantina system, occupying a rounded basin-like catchment, persists to reach Lake Eyre while the narrower, more elongated Georgina system, situated on the margin of the basin structure, fails to maintain its course through to baselevel. The Sandover River, from a catchment with widely spaced isolated uplands, also fails to reach baselevel as a connected system.

This measure is most effectively applied to entire medium or large sized catchments, in which breakdown along the trunk channel does reflect the processes of disorganisation occurring throughout the catchment. It is particuarly appropriate to assess the level of breakdown of streams which flood out. 124.

The Comparison of Actual and Potential Channel Lengths

This measure is based on the level of "completeness" of a drainage

network. Disorganisation of the drainage system is a function of

its not conforming to an ideal channel network. The measure takes

into account breaks in both the trunk channel and the tributary

network, as segments may be obliterated, or may fail to develop within any part of the system.

This measure incorporates elements of the other two, in that complete­ ness of the theoretical network involves both an effective contributory network, and connectivity of the channel system. These elements are reflected in the level at which failure of stream segments occurs -

for low order tributaries this reflects failure of regions of the catchment to generate runoff; for higher order segments it indicates failure of the trunk systems, often reflecting deposition in the

lower reaches of the catchment.

Although this measure theoretically accounts for all occurrences of breaks in a channel network, the difficulty of reconstructing potential networks in areas now blanketed with sand renders the method less useful in application than in theory.

At a broad scale this method may be used quantitatively by considering the total proportion of channel segments or channel lengths missing from the theoretical network. In this way (Fig. 9-1) it can be used to compare catchments on different morphostructural divisions to determine relative degrees of disorganisation. Although bifurcation ratios, and hence theoretical numbers of stream segments in the network, Fig. 9-1 Graph of the percentage of channel segment lengths of each stream order missing from the theoretical network, for the three sample catchments Percentage of channel lengths missing from theoretical o. — network

Stream order 125.

vary with catchment shape, the proportion of channel lengths missing

from the network is based on the bifurcation ratio appropriate to the

local catchment, allowing comparison of catchments of different forms.

Factors Determining the Completeness of a Channel Network

As this measure incorporates aspects of the two previous measures,

the factors which determine the level of disorganisation by this

measure are a combination of those influencing the other two. A

high proportion of channel segments of low order missing from the

network correlates with a low Ratio of Effective Catchment, and a

high proportion of higher order segments missing from the theoretical

network relates to a low level of connectivity.

Rainfall is a factor influencing this measure, in that it is necessary

to generate and maintain a connected network. It can be seen from

Table 9-1 that the Georgina-Diamantina catchment, with higher mean annual rainfall throughout, has a lower proportion of channel lengths missing from its theoretical network than either the Finke-Macumba

or Lake Way-Lake Cowan catchments with somewhat lower rainfalls.

Slope determines the parts of the catchment from which the network

is derived, and continuity of slope is necessary to maintain a connected system. Networks such as those of the Finke and Sandover have channel lengths missing from throughout the system, since large parts of their subcatchments are flat-lying surfaces. Within the

Lake Way-Lake Cowan catchment the long valley slopes are not suf­ ficient to maintain the connected network, and the high proportion of channel segments missing reflect breaks in the trunk channels. 126.

Surficial material also determines areas of generation of drainage,

and maintenance of the network. Erosional surfaces throughout most

of the Georgina, Diamantina and Macumba subcatchments generate runoff

and support connected channel networks. In contrast channels fail on

sandy surfaces in the Finke and Sandover subcatchments, and no net­

works are generated on the sandy margins of the Lake Way-Lake Cowan

catchment.

Catchment shape and relief distribution are factors influencing

connectivity and hence the failure of higher order segments in the

network. Widely spaced uplands and long catchments result in failure

in the lower channels of the Finke and Sandover systems. Elongated

low-gradient catchments result in breaks in the trunk channels of

the Lake Way-Lake Cowan system, and a more elongated catchment makes the Georgina system more vulnerable to breaks in the network than occur within the Diamantina system.

In general, for catchments in the arid zone, a -pattern drainage network with high bifurcation ratios at low stream orders, is more vulnerable to the loss of channel segment lengths than a dendritic system. Within the trellis drainage pattern of the

Yilgarn Plateau, where a large number of lateral streams are tri­ butary to a single trunk channel, there is a higher level of dis­ organisation of drainage than occurs in the central Interior Lowlands, with its dendritic drainage pattern. The central Australian drainage, with elements of each pattern, is disorganised to an intermediate extent. Such drainage patterns are controlled by the form of the catchment, which is in turn dependent on structure and geomorphic history. 127.

This measure can be used in any catchment, and is most successfully

used when the theoretical network can be reliably reconstructed.

Application of the Three Measures

The Way in which the Measures May he Used

Three criteria will determine the use of the three measures of

drainage disorganisation: the pappose of the study, the scale

of the study, and the specific area of the study.

^t-r/Jose. '■ Each of the measures has different relative advantages

and hence, if used in combination can highlight all of the aspects

of disorganisation, effecAiUe \s u.s

Scale. The scale of the study, and consequently the evidence available, may determine the choice of a measure. For mapped net­ works with no other information available, only the numbering of the stage of channel breakdown may be possible; for a detailed framework for studies of water balance, or rainfall-runoff relationships, the

Ratio of Effective Catchment is the most appropriate measure available. In considering processes within a channel system, the measures of connectivity and completeness are most appropriate, as is the case for studies based on available hydrological data.

Area. The specific conditions applying to the area being studied will also influence the choice of a measure. If drainage is well-organised throughout the area, the measure of the proportion of effective 128.

catchment does not discriminate within the system, and the other two measures will focus attention on the form of breakdown of the network. In contrast, in areas of small generating uplands and extensive lowlands, the Ratio of Effective Catchment, and the mapping of areas of network generation is most appropriate to categories drainage failure.

Their Application in Association

Each of the three measures emphasises somewhat different aspects of disorganisation, and factors influencing the failure of drainage.

Each is therefore more appropriate to a particular pattern of dis­ organisation: the Ratio of Effective Catchment to distinguish areas of runoff response within a catchment, the stage of breakdown of the channel network to identify the form of breakdown within the trunk system, and the comparison of actual and potential channel lengths to identify the part of the network in which failure occurs, and to allow comparison between systems in different morphostructural regions.

The measures are also partly complementary, and in combination allow a comparison, not only of the levels of disorganisation, but also of the form this takes, and the role of various factors.

In combination these measures clearly show that although the amount and distribution of rainfall, and the size of the catchment do have some effect on the degree of disorganisation, by far the most important in this respect are factors of terrain or geomorphology.

The factors of landsurface are most important in determining the proportion of a catchment which generates runoff. Sloping surfaces 129.

on rocky or stony ground do generate channel networks, while flatter

areas of sand accumulation either in the low-lying parts of the

catchment or on the margins, are the surfaces which do not give

rise to drainage networks.

Relief, gradient, and the distribution and spacing of uplands are

the geomorphic factors which control the development of integrated

channel systems, and determine their likelihood of reaching potential

baselevel. The lack of upland sources of drainage, or the excessive

distance between upland source and drainage artery will result in a

disconnected and disorganised drainage system, as will long flat axial

systems developed on surfaces of low gradient.

Drainage disorganisation is not simply related to aridity. Indeed, as Hills (1953) indicated, climatic contrasts within the Australian arid zone are inadequate to explain the degree of disorganisation of drainage, which is more a function of physiographic controls such as differences in relief, and the nature of surface deposits, themselves consequent upon the geologic setting and geomorphic history of the continent. 130,

APPENDIX 1

Method of Mechanical Analysis

A modified Buoyocos (1928) mechanical analysis was used to separate

sand from silt + clay sized particles.

Samples were oven dried, and two 100 gm subsamples ground lightly

with a mortar and pestle, and passed through a 2.00 mm (-1 0) sieve,

to assess the amount of gravel present.

50 ml of 50 gm/1 "Calgon" (sodium hexametaphosphate) solution was

used as a dispersant, and the sieved sediment fractions agitated

in distilled water for five minutes on an electrical mixer.

After dispersal the suspension was allowed to stand for five minutes

for sand sized (larger than 3.5 0) particles to settle, and the

supernatant suspension containing silt + clay sized particles or colloids was discarded. Repeated washings were continued until the

supernatant liquid remained clear.

The sand was oven dried and weighed to determine the proportion of

silt + clay discarded during decantation.

As the proportions of silt + clay were high in many samples, it appeared probable that finer particles would be adhering to the sand grains. Accordingly the duplicate sand samples were then dry sieved for 20 minutes through a series of ten standard eight inch "Endecott" test sieves, graded at 0.5 0 units, and all material passing through the 4.0 0 sieve collected and weighed. This weight 131.

was added to the weight of silt + clay discarded during decantation, and the total proportion of silt + clay determined.

Modal sand classes, and the proportions of sand and gravel in each sample were also noted, to describe the sediments of the channel perimeter. 132.

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