“Nothing beside remains”

WHITE ELEPHANT HILLS5 September, 1947

VICTORIA

STATE RIVERS AND WATER SUPPLY COMMISSION

EROSION ON THE MELTONRESERVOIR CATCHMENT

A summary of investigations to determine the causes and extentof the erosion of the catchment of the Melton Reservoir and the

consequent siltation of that storage.

ByIAN G. FORBES

ASSISTANT RESEARCH OFFICER,RIVERS AND STREAMS DIVISION

1948

STATE RIVERS AND WATER SUPPLY COMMISSION

Chairman:LEWIS R. EAST

M.C.E., M.Inst. C.E., M.ASCE, M.I.E. Aust.

Commissioners:H. W. McCAY

B.C.E., M.I.E. AUST.

HAROLD HANSLOW, J.P.

LAURENE DUGGAN, J.P.Secretary:

FOREWORD

Some 35 years ago the State Rivers and Water Supply Commission set out to establish an irrigation settlement atWerribee, and it has in the intervening years expended some ȥ300,237 on the construction of channels and otherworks which have enabled an area of 10,137 acres of dry land, then worth only about ȥ13 per acre, to be broughtunder irrigation.

Owing to the high fertility of the soil and its proximity to Melbourne and Geelong, the Werribee District isparticularly adaptable to the production of milk and vegetables and, as a result of irrigation development, the land hasincreased in value up to and over ȥ100 per acre.

To supply the settlement a reservoir was constructed on the Werribee River at Melton. This reservoir was completedin 1916 to a capacity of 17,000 acre feet, and subsequently enlarged to 19,100 acre feet in 1937. The cost of thiscompleted reservoir was ȥ103,679.

During the dry season of 1940-41 the Melton Reservoir was drawn down until towards the end of January it held onlyone or two hundred acre feet of water. At noon on the 25th January, 1941, there was sudden and heavy rain on thecatchment. By midnight on the 26th January, only 36 hours later, the reservoir was full and there was 8 feet of waterpassing over the by-wash. On the morning of the 29th January, when I visited the reservoir there was still a very largestream passing over the spillway, and the water was obviously heavily charged with silt and looked like pea soup.Heavy deposits of red silt on the rocks showed how deep the water had been and the water in the reservoir had reddiscolouration for months.

In the drought of 1944-45 the reservoir was again practically emptied and an examination showed that the reservoir'scapacity had been reduced by siltation to 15,500 acre feet, indicating a loss of over 3,500 acre feet, mainly in the eightyears prior to that time.

The number of water shortages that have occurred at Werribee have caused the irrigators to petition the Governmentto build another reservoir. The Parliamentary Public Works Committee has recommended the construction of areservoir below Melton at Cobbledick's Ford, the best site the Commission could find. The estimated cost, however,will be eight times as much as Melton, i.e., ȥ825,000 for a dam of 19,300 acre feet, only 200 acre feet larger thanMelton was. The Committee also recommended that action be taken to prevent further erosion in the catchment.

In the Commission's evidence to the Parliamentary Public Works Committee on the proposed reservoir at Cobbledick's Fordit was stated that

" If provision is not made at once to provide additional storage to replacethat already lost and to provide for further losses by siltation, and if soilconservation work is not carried out in the catchment area, then in a fewyears the storages will be so decreased that it will be impossible to supplyadequate amounts of water to Bacchus Marsh and Werribee Irrigation andWater Supply Districts, even in years of normal rainfall."

After reading Mr. I. G. Forbes' report, I personally made an inspection at the catchment. The sights can be described ashorrible. A very large portion of the catchment now is of no value, but has become a liability, and if something is not done tocorrect the soil erosion, conditions will rapidly become worse and Melton Reservoir will be valueless as a water storage.

If another water storage is to be constructed, is it not imperative that the erosion be corrected? We must also remember that,if this erosion is not checked, it will not be long before the now highly developed and valuable irrigable lands at Werribeewill only have a dry land value because the reservoir built to supply these lands will be filled by silt. washed down from thecatchment.

Mr. Forbes has presented a very valuable report which shows the urgent need for immediate action to check further erosionand to restore the catchment upon which the prosperity of the Werribee District is absolutely dependent.

H. HANSLOW,

Commissioner.

TABLE OF CONTENTS

FOREWORD ........................................................................................................................................................ 4

THE MELTON RESERVOIR CATCHMENT ................................................................................................. 6

GENERAL DESCRIPTION ................................................................................................................................ 7

PHYSIOGRAPHY AND GEOLOGY................................................................................................................. 8

Brief Geological History..................................................................................................................................... 8Notes On The Distribution, Nature And Erodibility Of The Various Rock Types. ............................................ 9Geology of Certain Severely Eroded Areas. ..................................................................................................... 12Notes on the Geological Map. .......................................................................................................................... 15

THE MELTON RESERVOIR CATCHMENT

This report constitutes a study of the various factors, natural and artificial, which influence the erosion of the catchment ofthe Werribee River and its tributaries insofar as they drain the country above Melton Dam. It does not deal with any terraindraining into the Werribee below that point.

No recommendations are made, the information being solely tended as basic data for subsequent detailed investigation andthe establishment of works designed to alleviate erosion in the catchment's lands and watercourses and so to arrest thesiltation of the Melton Reservoir.

To this end, information concerning the geological structure of the area has been summarized and mapped, using the mostrecent data in all cases. Soil characteristics, using this geological data, have been detailed and give the probable scale oferodibility. The vegetation of the area is discussed, indicating the types found in the various areas and to some extent thereason. Rainfall, on which the vegetation types and degrees of erosion of the soil largely depend, is set out, as are also theforms of land use, variation in land values and the incidence of rabbits as a major contributary factor. Works already done orin progress are also mentioned.

It was originally intended to attempt a determination of the areas and types of erosion, both of streams and land, by the use ofair photographs. On investigation, however, it was found that the coverage in the catchment was not adequate. Suchphotographs as are available were taken in 1931, under such conditions of altitude, &c., as to render the scale too small foraccurate interpretation and estimation.

This investigation has involved the co-operation of specialized personnel who have contributed much of the data from whichthe individual sections are written, and credit is given to these informants for their ready co-operation. In this regard, specialthanks are due to Mr. C. S. Gloe, B.Sc., Geologist of this Commission, for his enthusiastic co-operation in preparing thesection dealing with the geology of the catchment (about which the whole question really revolves) and for his help,otherwise, regarding sources of information.

Regarding the information, mention must be made of the data concerning erosion incidence, pasture types and land usage,and referred to as being as at 1941.

Early in that year the Soil Conservation Board, under its Act, decided to carry out a survey to ascertain the nature and extentof soil erosion throughout the State and, to facilitate the gathering of the required data, authorized one of its part-timemembers, Mr. C. T. Clark, L.S., M.V.I.S. I (now Deputy Surveyor-General) to evolve a scheme for such a survey. It is fromsuggestions in his comprehensive handbook, "A Survey of Soil Erosion," that certain phases of this report have beenprepared.

To initiate that survey, the appropriate departments were asked for their co-operation in obtaining information which, oncollation, would enable a summary of conditions to be made, and, later, arrangements were made with the field officers,inspectors, &c., of those departments, to gather this information and forward it to the Board.

Unfortunately, however, the war situation at that time did not allow of this work being done, and it was only from the thenLands Department Inspector at Bacchus Marsh that information was received as requested. Fortunately, the material soavailable deals with the catchment under review and it is from his maps that the data detailed under the above-mentionedheadings was prepared.

As a result, it must be appreciated that, when other catchments are similarly dealt with, it will not be possible to set out thistype of information in such detail or so conveniently unless further cooperation of departmental personnel is arranged, onsimilar lines, for the gathering of the required data.

The Lands Department officer in question took about six weeks to cover his district and, when forwarding the last of hisplans, commented as follows:

“….. I found it necessary to visit most parts of the district for itis easy to overlook erosion when not previously conscious of itin the district. I have completed the survey more rapidly thanwill be done by most inspectors In order to assist you in gettingthe scheme started but I see no reason where it should cause anyhardship to inspectors if they always take their plans with them,and mark them up daily……”

It is felt that some stress must be placed on the defeatism which was found now and then when talking over the matter withpersons who know this area. It is of course obvious, even to a casual observer, that there is here an extraordinary erosion andsiltation problem, and that, because of the peculiar geological and other circumstances, complete cessation of the siltation isnot possible. Nevertheless, it is definitely felt, in the light of reclamation works done elsewhere, that it should be quitefeasible to reduce this trouble to a reasonable minimum.

GENERAL DESCRIPTION

The following is a brief resume of some of the major details which determine the conditions giving rise to theerosion and consequent siltation of streams and storages in the Melton Reservoir catchment.

This catchment, having an area of 424 square miles, lies 24-40 miles west-north-west of Melbourne, is about 24miles north-south and is 20 miles east-west.

Geologically, the main features are Ordovician sandstones and shales in the north, tertiary clays, sands, andgravels and basalts on the southern areas, and a complex of both the foregoing together with granite, glacialtillites and further basalts in the central areas.

The topography of this area is the result, to a major degree, of extensive faulting which has given rise towell-defined areas of uplift and sinking. Five main topographic "blocks" are defined by extensive major faults,and, from the block diagram (Fig. 4), it is obvious that this complex is the principal factor in determining thelarge-scale erosion which has given rise to the gorges of the Werribee and Lerderderg Rivers, and to theformation of the extensive Parwan Valley. Just as a sudden change of grade in a creek or gully will tend to give.rise to headward erosion, so these large-scale changes of grade, with the streams dropping to the lower Sunklandsover the fault lines, have caused the extensive cutting back which has formed the narrow gorges and the widevalleys of the catchment, the differences in shape, area, and depth being due to the variation in hardness orerodibility of the formations through which the streams flow.

It is this natural, large-scale erosion which has formed the Bacchus Marsh flats and has placed the good soils onthe Werribee Plains. It should be apparent, therefore, that such workings cannot be stopped. What can and mustbe attended to, if the unnatural siltation is to be slowed down, are the artificial processes by which the naturalchanges are so greatly accelerated.

The Werribee River, on which Melton Reservoir lies, approximately skirts the western and southern sections ofthe watershed and receives its major tributary flows from the waters of the Djerriwarrh Creek, Coimadai (Pyrete)Creek, the Lerderderg River, Korkuperrimul Creek, Myrniong Creek and Pyke's Creek. On its southern side,Parwan Creek, with its tributaries, Spring Creek, Yaloak Creek and Whitehorse Creek, is the only contributor ofconsequence.

The rainfall of the area varies generally from about 40-42 inches along the Divide, on the north, down to 17-20inches in the southern areas, and is fairly evenly distributed, between these figures, according to location andelevation. Because of the comparative smallness of the area and the somewhat scattered stations, records are notas conclusive as they might otherwise be. Again, topographical influences determining sheltering effects havenever been fully investigated.

The vegetation of this catchment lies in two major divisions. Across the north, and containing about a third of thetotal area, is a considerable extent of State Forest in which lie the headwaters of the Werribee River and itssouth-flowing tributaries. This is a well-tended mixed-forest area containing much rough and precipitous country.The chief species of timber are Messmate, Red Stringybark and Candlebark Gum. The southern half totwo-thirds is alienated country, almost entirely given over to grazing and pastoral pursuits. About Bacchus Marshis an Irrigation District of some 6,700 acres in which are found, in order of extent, sown pasture, lucerne, cerealsand market gardens, and orchards.

In the State Forest section the production is that of construction timbers and firewood with some small miningareas. The alienated lands outside the Irrigation District are fairly mixed in their main usage of cattle and sheepraising. Cereals are prominent in the southern and eastern areas with scattered locales elsewhere.

A comparison of the Geological map (Fig. 1) with the Vegetation and Erosion Distribution maps (Figs. 7 and 8)will clearly show the inter-connection between these phases. The probable soil types and their characteristics, canalso be best followed by consulting the Geological map.

Certain works have been carried out from time to time to correct badly eroding streams, gullies, hillsides, &c.Much of this has been done by grants from the Rivers and Streams Fund with contributions from the interestedparties. In this regard, works costing approximately ȥ3,000 have been carried out, the Fund contribution beingabout ȥ2,200. Besides these works, several areas have, with the advice and assistance of the Soil ConservationBoard, been brought under control by contour furrowing and similar practices. In some cases the local authoritiesand some landholders have done works at their own individual expense. It must be appreciated that the totaleffect of these activities is negligible in retarding the over-all effect of the erosion. They are purely emergencymeasures and only serve to indicate what can be done.

PHYSIOGRAPHY AND GEOLOGY

(Compiled by C. S. GLOE, B.Sc., Geologist, S.R. & W.S.C.)

The area drained by the Werribee River and its tributaries upstream from the Melton Dam may be divided intofive main topographic "blocks". In early Tertiary times this area formed part of the great peneplain of EasternAustralia which was subsequently destroyed by differential uplift along several well-defined faults (see Figs. 4and 5).

The upthrown blocks A, C and D, which are known as the Lerderderg (Blackwood) Ranges, Brisbane Rangesand Gisborne Highlands respectively, are geologically relatively simple. Each is made up of rocks of Ordovicianage covered by several long tongues of Newer Basalt. A number of small outliers of Permian glacial beds arealso found in Block A.

The downthrown blocks B and E are known as the Ballan Sunkland and Werribee Plains respectively, the latterforming portion of the Port Phillip Sunkland. The wide variety of rocks and geological structures found in theBallan Sunkland have aroused the interest of geologists ever since the area was first surveyed over 80 years ago.The Werribee Plains is largely volcanic but a variety of older rocks are exposed in the north-western corner ofthat area.

Brief Geological History.

In Lower Ordovician times considerable thicknesses of muds and sands were deposited in the marine seas whichcovered the area under review. The seas gradually receded to the east where Upper Ordovician sediments werelaid down. Uplift of the area was associated with, and followed by, folding which produced great ranges ofmountains running north and south, and the long period of erosion to which much of this area has been subjectedwas initiated.

In Devonian times these sediments were intruded by a granitic magma with its accompanying acid dykes. Thegranitic rocks were exposed by the denudation which continued until Permian times when the temperaturebecame much colder and glaciers moved over the land. The glacial period was itself an erosive period, but itscompletion left great masses of morainic material in the old valleys. During recession of the glaciersfluvio-glacial conglomerates and sandstones were deposited in glacial rivers and glacial lakes. Triassicsandstones were laid down, apparently conformably, on the underlying glacial beds.

The "great erosion period" continued. It is possible that some earlier movements along the Greendale and SpringCreek Faults took place at this time, letting down the Ballan Sunkland and so protecting the glacial beds fromcomplete removal by erosion. By Early Tertiary times the area had been reduced to a peneplain but this was thenpartly destroyed by the outpourings of flows of Older Basalt. Differential uplift along the Greendale, SpringCreek, Gisborne and Coimadai Faults followed which produced the raised blocks of the Lerderderg Ranges, theBrisbane Ranges and the Gisborne Highlands, together with the Sunklands of Ballan and the Werribee Plains.

The raised blocks were then dissected. Coarse fault aprons were deposited near the scarps, while up to 1,000 feetof finer sediments were laid down at some distance from the fault lines. This latter series include lignites, which,in the south-east corner of the area, are interbedded with Miocene marine clays and limestones, and sands andironstones with leaf impressions, again indicating a Miocene age.

Very extensive Newer Basaltic eruptions then took place, filling valleys in the uplifted blocks and practicallycovering the Sunklands. Movement along the Rowsley Fault was associated with this period, and causedincreased dissection of the Lerderderg and Brisbane blocks, bringing them to their present condition of deeplydissected peneplains. Dissection of the Ballan Sunkland was also initiated and led to the formation of theWerribee Gorge and Parwan Basin. A, widespread but relatively thin fault apron was deposited along the base ofthe Rowsley Scarp as was alluvium along the Parwan and Werribee just above the fault. The recent alluvium ofthe Bacchus Marsh Flats was also formed at this time.

Notes On The Distribution, Nature And Erodibility Of The Various Rock Types.

Ordovician Sediments.-These rocks, which form the bed-rock of the area, are found in each of the five blocks.They occupy practically the whole of the northern half of the area where, with the exception of Parwan Creek, allthe main tributaries of the Werribee rise. Outcrops of a few square miles in extent are also found in the WerribeeGorge area, in that portion of the Brisbane Ranges which falls within the Melton Catchment and in the valleys of'he Coimadai and Djerriwarrh Creeks, north of the Melbourne-Ballarat road. The rocks consist of hard slates andsandstones which have been indurated, in the Werribee Gorge area, by the granitic intrusions. The general highresistance to erosion of the Ordovician sediments can be seen in the Lerderderg Gorge which has cliff-like sidesof up to nearly 1,000 feet in height.

Devonian Granite-Outcrops of this rock, which is better described as adamellite, are confined to the BallanSunkland where a few square miles occur in the Werribee Gorge near Ingliston. A bore put down some 5 milesfurther south on Parwan Creek struck granite at a depth of nearly 500 feet beneath the surface. Although formingwell rounded hills these rocks can be considered as fairly resistant to erosion.

Permian Glacial Beds.-Numerous outcrops of rocks of this series, none of which is more than a few square milesin extent, are exposed in the central portion of the area, i.e. the Ballan Sunkland and northwest corner of theWerribee Plains. Large numbers of small outcrops, many of which have not been mapped, are also found in thesouthern portion of the Lerderderg Ranges. These rocks, which consist of true tillites together withconglomerates and sandstones, present some differences in their powers of resisting erosion. Generally speakingthey are much less resistant than the Ordovician rocks, and have been preserved only where they have beenprotected in the sunklands.

Triassic Sandstones.-A limited outcrop (one of the few known occurrences of rocks of this age in Victoria)occurs on the eastern slopes of Bald Hill near Bacchus Marsh.

The Older Volcanic Series.-These rocks are found only in the Ballan Sunkland and except for several small pipesin the Parwan Creek valley are confined to the area between the Greendale Fault and the Werribee River. Severaltypes of basalts and ultra basic flows are represented in this series. All types are generally fairly decomposed andconsequently are liable to rapid erosion. However, as pointed out in the section dealing with soil types, theserocks form very rich soils which themselves are not easily erodable.

Miocene Sediments.-Included in this series are a wide variety of marine and fluviatile sediments. However, themarine clays and limestones do not appear at the surface, and hence need not be mentioned further. The fluviatilesediments, which are also known as the Tertiary Leaf Beds, are confined to the sunken blocks of Ballan and theWerribee Plains. The rocks vary greatly in composition, and include incoherent boulder deposits, sands andclays, ferruginous sandstones and mudstones, and almost pure deposits of limonite. Beds of brown coal are foundclose to the surface near Bacchus Marsh. Most of these rocks are very poorly cemented so that their resistance toerosion is feeble.

In the vicinity of Coimadai several small remnants of a fresh-water limestone deposit containing Pleistocenefossils occur. These beds are associated with the Miocene gravels, &c., so that it is probable that the latter seriesextend from Miocene to Post Pleistocene.

The Newer Volcanic Series.-The Newer Basalt covers large portions of the Werribee Plains and BallanSunklands while tongues of basalt are found on each of the uplifted blocks. The rock which is generally denseand well crystallized may be classed as very resistant to erosion.

Post Newer Volcanic Gravels, Sands, Clays And Alluvium.-The gravels and sands are found principally along thewestern edge of the Werribee Plain Sunkland, and constitute the Rowsley Fault Apron. Alluvium has beendeposited along the courses of streams such as the Parwan and Werribee, above the Rowsley Scarp. The streamsbelow the scarp have recently cut, or are in the process of cutting, their courses through the basalt, whereas,above the fault-line, they have long since cut through the hard basalt cap and into the softer underlying rocks.This has enabled them to reach grade and to, deposit alluvium along their valleys. Below the Fault scarp recentalluvium is found in the fertile and extensive Bacchus Marsh flats. These rocks are naturally very looselycompacted, and present slight resistance to erosion.

Figure 1

Geology of Certain Severely Eroded Areas.

In this section, the words "severely eroded" are applied to areas where the rate of erosion has been vastlyaccelerated since the advent of our civilization, and which to-day present serious problems to the landholders andauthorities concerned with the utilization of such areas.

The Parwan Basin.-The extent of this basin, which is very great considering the size of Parwan Creek, is largelydue to two factors:

(i) The unconsolidated Miocene sediments which underlie the relatively thin cover of Newer Basalt, and(ii) The differential uplift along the Rowsley Fault which increased the "head" of Parwan Creek by possibly,

500 feet.

Although the basalt forms a protective cover over the sediments, the easy erodibility of the latter allows theundercutting of the basalt. The abundance of landslips along the steeper cliff sections below the basalt flows aremarked on the Geological Plan of Yaloak published by & Mines Department, and can be seen on Figs. 2, 9 and51. It can be expected that the headwaters of the Yaloak and Parwan creeks will continue to enlarge the basin tothe west and north-and at a much faster rate than similarly sized creeks in the Lerderderg Ranges would erodetheir basins. The basalt gorge below the Rowsley Scarp has provided a temporary base level of erosion at thispoint, and until this is removed, much of the sediment derived by the enlargement of the basin will be depositedas alluvial flats along the main creeks. These flats themselves are liable to a constant process of removal andsubsequent redeposition at some lower level.

The Table Top Area.-This is a badly eroded area lying west and north of the junction of the Werribee River andKorkuperrimul Creek (Figs. 2 and 8). South of the Werribee River, conditions are very similar to those found inthe Parwan Basin. Between the Werribee and Korkuperrimul Creek the rocks exposed are mainly PermianGlacial beds and Miocene fluviatile sediments with small patches of Older Basalt and a small remnant of NewerBasalt on the top of Table Top or Trig Hill. Further north, along Korkuperrimul Creek, are found considerableexposures of Older Basalt. None of the three main rock types offers strong resistance to erosion, and, with theirlocation just on the uplift side of the Rowsley Fault movement, it is not surprising to find this area todaysuffering from severe sheet erosion and containing numerous deep gullies.

One particularly bad portion of this area is generally known as Bald Hill, and is located on the ridge between theBacchus Marsh Flats near Darley and Korkuperrimul Creek. This area is practically defined by the Rowsley andBald Hill Faults. On the eastern slopes of the ridge the Triassic sandstones, &c., are fairly well grassed, but onthe steeper western slopes, where Permian fluvio-glacial sandstones, &c., crop out, severe erosion has takenplace. The latter rocks no doubt formed rather poor soils which, with the steep slopes having a westerly aspect,render them to be very prone to wind and water erosion.

Goodman's Creek Area.-This area is bounded by the Lerderderg River, the lower reaches of Goodman's Creek,and the Coimadai Fault. Apart from alluvial flats along the river valleys, the only rocks exposed are PermianGlacial beds which once again show their general weakness in resisting erosion.

Coimadai Creek Area.-This is an area of a few square miles on Coimadai Creek, north of Coimadai Township.Apart from about one square mile of Permian Glacial beds, only Ordovician slates and sandstones are exposed.In their natural state the latter rocks are very resistant to erosion, although in lower rainfall areas they tend toproduce rather poor class soils. It is the latter fact which has probably been the main cause of the severe erosionfound in this area. Further severe erosion, found downstream along the banks of Coimadai Creek, is probably dueto very steep slopes in poor class soils.

Figu

re 2

Notes on the Geological Map.

The Geological map (Fig. 1) has been compiled from all available information, both published and unpublished,held by the Mines Department of Victoria, and from plans published by the Royal Society of Victoria. A largepart of the area has been surveyed in detail, and apart from some slight confusion over certain rock types and alittle doubt about several boundaries, the compilation. can be considered as fairly accurate. Certain minor detailshave been omitted.

All major and a few minor Faults have been marked with those actually mapped in the field distinguished fromthose which are only projected. It will be noticed that the Greendale Fault is not a simple fault as shown on theblock diagrams drawn by C. Fenner (1918)-Fig. 4-and it appears likely that more than one fault exists. As statedpreviously the Gisborne Highlands constitute a topographic block and this appears to be the main evidence forthe existence of the Gisborne Fault.

The Djerriwarrh Fault (Harris and Crawford, 1921) which separates Lower Ordovician sediments on the westfrom Upper Ordovician' sediments on the east, is thought to be an old fault which has contributed little to thevery important Tertiary history of the area.

The following is a list of the plans used in the compilation of the geological maps (Figs. 1 and 2).

(i) Mines Department-Quarter Sheets 6 S.E., 7 N.W., 11 S.E. (unpublished), 12 N.E.

(ii) Mines Department-Parish PlansYaloak (and portion of Bungeeltap).Gorong (and portions of Blackwood and Myrniong)Blackwood.Moorarbool West.Moorarbool East (unpublished). Bullarto (unpublished).Trentham (unpublished).

(iii) Coimadai District (A. L. Coulson, 1924).

(iv) Korkuperrimul Creek Area (R. Jacobson and T. R. Scott, 1937).