ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS

I

I I

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE lOWER DARllNG RIVER

Davia Green Mustak Shaikh

NeeraJ Malnl Hugh Cross

Jamie Slaven

AReport 10 the Murrav Darling Basin commission byt.be

Deoartmen1 of land and wa1er conservauon

centre For Natural Resources Ecosvstem Management Branch CNR98.028 Jotv1998

I I

-

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-~=- -~ - -:e.xE~1=WE-::SUM~ARY - -- --

The Department of Land and Water Conservation and the Murray-Darling Basin Commission are jointly responsible for managing the water stored in the Menindee Lakes System. Its operation has a significant impact on flows to the Lower Darling River. This project began in 1993 with the aim of maintaining or improving the instream, floodplain, and wetland habitats of the Lower Darling River by preventing and, where possible reversing, environmental deterioration resulting from the operation of the Menindee Lakes Storage Scheme. Several preliminary studies were carried out during the period 1993-1995 including investigations of blue-green algae, hydrogeology, wetland inundation and weir drown out in the Lower Darling River. The present study, carried out between 1996-98, consists of three main components, an analysis of wetland inundation by remote sensing, analysis of changes to the hydrologic regime using modelled and historical data, and an analysis of tht:i current status and flow requirements of the ecological components of the river. The character of riverine ecosystems is the result of many different interacting parameters. Physical, chemical and ecological factors operate in conjunction with one another to produce a unique system in terms of character and function. A cross tabulation approach, adapted from Thoms et al (1996) has been used to summarise the responses of each ecosystem component to specific elements of the flow regime, thus formulating a conceptual model which forms the basis for interpretation and analysis of the results. Changes to the hydrologic regime were investigated by analysis of both modelled and historical data for gauging stations at Weir 32 and Burtundy. Significant reductions have occurred in the monthly flow volumes and flow durations for all flows except the very lowest. Peak monthly flows have decreased by 30-50% in all months, while small to medium monthly flows have decreased signficantly in all months el(cept summer. Wetland inundation was investigated by the use of Landsat TM data which was obtained for four different flood events. The results from the analysis provide a predictive relationship of wetland inundation and river flows. Approximately 50% of wetlands are filled at a discharge of 13,000 MUd at Weir 32, which is considerable less than previously thought. Thirteen percent of wetlands will begin to flow during small freshes in the river (less than 7,000 MUd at Weir 32), an additional 55% of wetlands will fill.during medium sized flows (7,000-17,000 MUd), and an additional 22 percent of wetlands will fill during large floods (17-29,000 MUd). Approximately 10% of wetlands will fill only in extreme flood events su9h as the 1990 flood. The character and inundation of channel benches was investigated at 15 sites along the river. The results of the bench survey suggest that the bench features on the Lower Darling River are grouped at particular heights, that is, at particular discharges. Four significant groups of benches were identified: A) small low features flooded at flows of around 5,000 MUd at Burtundy; B) larger features, usually supporting river red gums, flooded at around 11,000 MUd at Burtundy; C) large high features supporting river red gums, flooded at around 17,000 MUd at Burtundy; and D) very high features supporting river red gums that flood at around 22,000 MUd at Burtundy. As for wetlands, a predictive relationship between discharge (at Burtundy) and the percentage of wetlands flooded has been developed to help assess the impact of a specific

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

sized flow event. A flow of around 11,000 MUd at Burtundy (15,000 MUd at Weir 32) is required to inundate 50% of the benches along the Lower Darling River. The water quality investigations were limited to desk top reviews of available information. this included comparison of turbidity, electrical conductivity (salinity) and temperature data with historical river flows. Experience by regional staff over the past three years has shown that a flow of 2,000 MUd at Weir 32 is necessary to disperse an algae bloom at Menindee in winter, and a flow of 5,000 MUd at Weir 32 is necessary to disperse a bloom in summer. Riparian vegetation on the Lower Darling River appears to be linked to floodplain features. Black box is usually found on the oldest and highest parts of the floodplain while river red gums dominate the younger more active part of the floodplai n closest to the river (the channel benches). There is about 120,000 hectares of black box woodland on the floodplain of the Lower Darting River and around 13,000 ha of river red gum woodland. Other floodplain vegetation consists of lignum, nitre goosefoot and canegrass. Literature suggests that river red gums require a minimum flooding of 3 years.in 10 {c;lverage 3.3 years) to maintain a healthy condition. Vegetation on the higher benches along the river (ie benches flooded at flows higher than 11,000 MUd at Burtundy) are currently being flooded less often than this (on average every 4·6 years). Some of the trees on these benches were found to be in poor health and this may be a sign that the reduction in flood frequency is beginning to affect the health of the riparian zone. Investigations into fish recruitment and fish passage were conducted by desktop analysis of literature combined with analysis of historical flow data. There are six known species of native fish within the Lower Darling River. The spawning requirements of these species were considered in relation to seasonality of flooding and water temperature. At least three of the species present in the river are thought to be migratory. Discharges required to provide passage over weirs below Menindee are 7,000 MUd at Weir 32, 2,000 MUd at Pooncarie and 2,300 MUd at Burtundy. It is currently thought that a minimum period of ten days is required to allow adult and young fish enough time to pass over the weirs. Macroinvertebrates were semi-quantitatively sampled for a total of four sites on the Lower Darting River. Sampling occurred only once and therefore the results can be regarded as a preliminary indication of the aquatic instream community. A total of 12 taxa were collected with insects being the most dominant group. Alt sites were dominated by collectors which suggests the importance of organic matter a s a food source within the channel of the Lower Darling River. Waterbird information was collected from a number of past surveys in the area. A total of 53 species of waterbirds have been observed in the Lower Darling region. The majority of these species breed between the months of July and January, and most species require a flood duration of at least 4.5 months to complete a successful breeding cycle. For each characteristic of the flow regime (magnitude, frequency, duration and seasonality) the requirements and current status of each ecosystem component has been summarised in matrix. The matrix allowed similarities in requirements to be identified and forms the basis for recommendations regarding ecosystem health and environmental flows. Recommendations have been proposed to cover different flow management operations such as minimum flows, releases and freshes, and floods.

a

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER OARLING RIVER

jj

ACKNOWLEDGME_Nl'~

The work reported on here is part of a five year project wholly funded by the Murray-Darling Basin Commission to improve the ecological knowledge base for the sustainable management of the Lower Darling River. The following people are thanked for their assistance in undertaking the current body of work during the period 1995-97 : Wendy Stevenson

formerly Eeological SeNices Unit

Project Scoping and review of previous work

David Ross

Ecological Services Unit

Macro·invertebrate Jdentificalion

Jenny Woods

Waler Quality Unit

P1ovtslon of water quality data

.lenila Acaba

Water Qua~ty Untt

Plotting of waler quality dala

Tony Dunphy

GlS and Remote Sensing Unh

Remote sensing ac!Vlce

Julio Briones

GIS and Remote Sensing Unit

Wetland inundation maps and GIS work

Andy Close

MurTay Daring Basin Commission

Provision of monlhly simulated ftow data

The following people contributed to previous work funded by this project : David Hamss

Murray Region

Algae investlgallons

OonReld

Far West Region

Algae investigations

Sonya Ardill

forTRorly Eoological Setvlces Unft

Welland cornmeno.1o-ftow investigations

Tm Cooney

formerly Ecological Sorvk:es Unil

Wetland COtMlenc&-lo-flow Investigations

The following people provided valuable comments on the drafts of this report : Peter Terrill

forTRe~y

Paul Naninga

Murray-Darling Basin Commission

LyndSayWhlte

MurTay-OarUng Basin Commission

Trevor Jaa>lls

Murray-Oaring Basin Commission

Bryan Haipe • To"'4>fltalure effects (eg. algae)

•

• Establishment

• persistence of perennial species

•

•

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G.eneral

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1 1 t dl Macrophytes

• Relative communey Slabili1y • Pools important refuges • Zero flow = clarity • Rare h•b"at • Disturbance effects • Resetting of nutrient availabilily and opecies OOITIJ)OS!lion Of communities

• Long term svnrilral

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• Dependant on subsurface groundwater

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(de$1ocalion and cracking) • Low flow morphology

availab~ey

(presence or absence of species requiring specific oon~itlonsl

ASSESSW.ENT Of ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

18

Table 1b

Ecosystem responses to a flow event

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Fish i

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• Extent of inundatiOtl and proportion Of weUancl habitat avaii.l>le

• Copth to waler table

• Araa of benthic h abitat

•

-

'·Chang~& ' •. ! '

moOstulelnublent

• Life-cycle aspects • Resetting mechanism (by death of es!allished plants In channel) • Geimlnatlon and es1abllshment in weU3nds

•

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• Extent of Inundation (affectln~ spread of seed and soil

11

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events • Stranding • Species composition

recharge)

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• Balance between nallve and exotic species • Nativesrespond to small level changes

• •

. • Migme "'°"'"""""'

ASSESSMENT OF ENVIRONMENTAi. FLOW NEEDS FOR THE LOWER CARLING RIVER

sources Use of habitat Sttandil>g Tracking of food sources Use of hab~at

• • Deposition of organic matter for food and habitat • Food utilisation • Completion of role cyclos • Use of l\abi!at

,, Watiirt>)Wls • Breeding stimulation

'Wate~ ;tiuan.Y 1

I', iG&'l>iTiorph:

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• Dilution •Dispersal

• Sttanding Ofjuveniles • Predation

• Tult>idity levels (highe< if rapid due to bank slumplng)

• Breeding success • Oevelopment of food organisms (bentllic algae, plants•

• Bani< stailiily • Cross··S&Ctiona1 channel shape • Erosion of trees into channel

• Cross-section an

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Table 19. Results of cluster analysis on cha nnel bent:hes

all sites

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2, 5, 6b, 7a, Sb, 15b,

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* Shaded area is the level at which the results are considered relevant given the error margin

Figure 17. Relationship between discharge at Burtundy and percentage of benches inundated

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ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

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12.2

River Flows and Water Quality

Plots of historical flows at Weir 32 and Burtundy, and data on EC, turbidity and temperature are shown in Figures 20 and 21. Electrical conductivity The impact of flows on electrical conductivity is as expected. Significant decreases in electical conductiv'ity at both Burtundy and Weir 32 correlate with the occurence of flood peaks in 1977, 1978, 1983, 1988 and 1990. The impact of a period of prolonged flows on electrical conductivity is shown clearly during the period 1983 to 1988. Following the flood peak in 1983/84 flows remained at minimum levels for three years. During this time electrical conductivity increased steadily, from around 200 US/cm up to 900 US/cm at Weir 32, with similar readings for Burtundy. In December 1989 electrical conductivity at both sites exceeded ANZECC guidelines (for protection of aquatic ecosystems) with a reading of about 1150 US/cm. In all of the above flood events the drop in electrical conductivity occured on the first sharp rise of the hydrograph, in most cases before the flood peak. The peak of the event and the duration for which the event is sustained however, do not appear to have any influence on maintaining low electrical conductivity values in the river. This is illustrated for Weir 32 in Figure 22 - electrical conductivity falls sharply on the rising limb of the flood, but continues to rise steadily throughout 1984, even though high flows were maintained through most of that year. This is also the case in 1990 (Figure 23) when high flows prevailed for approximately 8 months, yet electrical conductivity began to increase again even before the flood peak had been reached. The small flood peak in December 1991 (Figure 23) illustrates that even small flows can have some impact on reducing electrical conductivity. In this case EC was reduced from around 450 US/cm to around 380 US/cm, a decrease of around 15%. This compares with a decrease in EC of about 71% for both the June 1983 and May 1990 flood events. It appears, therefore, that the decrease in electrical conductivity which could be expected from a flood peak is proportional to the size of the flood peak. The impact of the smaller peak, however, is short -lived, as EC levels continue to rise very quickly afterwards. Turbidity Turbidity appears to be directly related to flows, but in the opposite way to electrical conductivity. Peaks in turbidity correlate with flood peaks occurlng in 1983, 1988, 1989, 1990 and 1995. The peak turbidity however is not necessarily relative to the size of the flow. For example, during the period 1988-1990 (Figure 24), the peak turbidity values recorded during the 1988 event were higher than in 1989 and 1990, even though the 1988 event was about half the size of the other two. Similarly the two highest turbidity values correlate to very small flow events in early 1995 and 1996. In this case it appears that considerable inputs of sediment were derived from local runoff due to high intensity rainfall. The opposite relationship between turbidity and flows is also shown in the period 1983 to 1988 when turbidity levels drop steadily with the declining river flows and then remain low from 1985-88 when river levels remained stable (Figure 25). As with electrical conductivity, both Figures 20 and 21 show that it is the initial pulse of water which results in the highest turbidities, not necessarily the flood peak itself.

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

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ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR TiiE LOWER DARLING RIVER

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12.4

Water Quality and Menindee Lakes Operation

Flows to the Darling River from the Menindee Lakes Storage may be released via Menindee Main Weir, Lake Wetherell Outlet, Lake Pamamaroo Outlet and Lake Menindee Outlet, either singularly or in combination (refer to Figure 2). A preliminary investigation of water quality at Weir 32 and corresponding Lakes operations has been undertaken. Due to time constraints only a few examples were able to be investigated, however the results suggest that wsi\er S!:!_~lity !!l,!~ri'l.eJ.Jlli:lYJle..affe.cte.d..by~§W9e..OPeration&. ie. water released from different outlets may have different impacts on river water quality. Further work is required to investigate a greater number of events, and analyse these relationships further.

1

The following examples (Figures 30 and 31) suggest a relationship between water quality and storage operation.

1991 During the period January to April 1991, flows were being released simultaneously from Wetherell, Pamamaroo and Menindee Lakes outlets (Figure 30). Temperature • 19 Jan - 9 Feb

Temperature decreases slighily as flows from Pamamaroo and Wetherell cease. Menindee flows are also decreasing. • 9 Feb - 16 Feb Temperature jumps 10°C when flows from Pamamaroo and Wetherell resume; Releases from Menindee Lake remain constant and low. • 16 Feb· 25 Feb Temperature drops to "normal" when flows from Pamamaroo and Wetherall reduce again (Wetherell ceases flowing); • 25 Feb - 2 Mar Temperature rises again as flow from Wetherell resumes, Menindee increases slightly and Pamamaroo rises slightly; Temperature and flows from each outlet stable. • 2 Mar 30 Mar In this example the peak in temperature on the 12th February and the elevated temperatures in March would appear to be related to releases from Wetherell and/or Pamamaroo. Electrical Conductivity While electrical conductivity rose to correspond with the flow peak in mid-February from Pamamaroo and Wetherell, the second peak on the 11th March occured when flows from all outlets were stable. The results are therefore inconclusive. Turbidity Turbidity levels fluctuate throughout the period from January to April, and do not appear to correspond with any changes to storage operation.

1980 During the period October to December 1980 releases to the Lower Darling River were occuring from Pamamaroo and Menindee Lake outlets (Figure 31 ). Temperature • 1 Oct-5 Nov • 5 Nov-19 Nov

•

19Nov-17Dec

•

17 Dec- 31 Dec

Both temperature and flows remain stable Over a period of about 9 days, temperature rises by 14°C, corresponding to a significant Increase in flows from Menindee Lake. Temperature then falls rapidly by about 6-7°C when releases from Pamamaroo begin. Temperature remains elevated with flows from both Menindee Lake and Pamamaroo continuing. Temperatures appear to rise with increasing flows from Pamamaroo; Ftows from Menindee Lake still decreasing.

ASSESSMENT OF ENVIRONMENTAL FLOW NEEOS FOR THE LOWER DARLING RIVER

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ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

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This example clearly demonstrates the effect of summer stratification within the Lakes Scheme, Minimum outflows from Menindee, an undershot , gated weir, result in temperatures averaging 18-19°C. When the outlet is opened further to release flows of 2,500 MUd, water is released from a greater range of depths and surface water is entrained, resulting in higher temperatures downstream. As noted earlier in this report, suitable temperatures during the summer period are critical to provide opportunities for breeding by fish. The release of cold water from the bottom of the storage will not meet the requirements of most native fish species. Electrical conductivity Electrical conductivity remains relatively stable during the entire period despite significant increases in releases from Menindee and Pamamaroo Lakes. A slight upward trend as Pamamaroo flows increase may indicate higher salinities in that lake. Turbidity During this period, turbidity appears to be directly related to flows from Menindee Lake. Turbidity increases significantly as releases from Menindee are increased to 2,50.0 MUd. Turbidity decreases gradually as releases from Menindee are reduced and replaced by increasing flows from Pamamaroo from the beginning of December. Turbidity levels may have been higher in Menindee Lake than Pamamaroo Lake at this time. In the previous pages, turbidity has been shown to be directly related to increases in peak flow (which is well illustrated here). This example suggests that there may be a relationship between turbidity and the source of the water (le which lake the water is released from). however this is difficult to conclude given that only two outlets were operating at this time.

ASSESSMENT OF ENVIRONMENTAL FLOW Neeos FOR THE LOWER DARLING RIVER

97

Conclusions And Flow Requirements For Water Quality Algae blooms: 1. Experience has shown that a flow of 2,000 MUd at Weir 32 is required to disperse an algae bloom at Menindee in winter, and a flow of 5000 MUd is required to disperse a bloom in summer (D. Harriss, pers. comm.).

2. For algae blooms at Pooncarie, similar flows at Weir 32 are required, but flows must be maintained for several days (D. Harriss, pers. comm.).

3. Further research should be conducted to determine the flow regime (if any) that will prevent a bloom from occurring in the first place. Electrical Conductivity:

4. Large flood events correlate with significant decreases in conductivity within the Lower Darling River. The drop in electrical conductivity occurs on the rising hydrograph, in most cases before the flood peak.

5. The passing of the flood peak and the duration of the event do not appear to have any effect in maintaining low electrical conductivity values in the river.

6. The decrease in electrical conductivity which could be expected from a flood peak appears to be proportional to the size of the flood peak.

7. Mean and median electrical conductivity (1963-1996) is higher in the Lower Darling River than upstream at Wilcannia. Turbidity:

8. Peaks in turbidity relate to large flood peaks in the Lower Darling River, although peak turbidity levels are not neccesarily relative to the size of the flood peak.

9. As with electrical conductivity, ii is the initial pulse of water which causes the turbidity peak, not the flood peak itself. Even when high flows are sustained for long periods, turbidity will begin to fall after the Initial flood pulse.

10. Turbidity in the Lower Darling River is generally lower than in the Darling River upstream of the Menindee Lakes. Temperature:

11 . Temperatures within the Lower Darling River fluctuate seasonal ly, generally ranging from 8 - 28 -$C. 12. The general flow regime does not appear to exert any influence on temperature trends. 13. Mean and median temperatures decrease downstream along the Darling River. Temperatures at Burtundy are up to 2.5il!C cooler than in the Darling River upstream of the Menindee Lakes.

A SSESSMENT OF ENVlRONMEITTAL FLOW NEEOS FOR THE LOWER OARUHG RIVER

98

Storage Operations: 14. It is possible that water quality in the Lower Darling River may be affected by storage operations, especially temperature and turbidity. 15. Further work is required to investigate a greater number of events and analyse the relationship between water quality and storage operations.

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

99

13.1

Description of the Riparian Vegetation

As on the Darling River upstream of Menindee (Roberts 1996), the distribution of floodplain trees on the Lower Darling River appears to be linked to geomorphic features. Black box is usually found on the older, higher floodplain of the river while river red gums dominate the younger part of the floodplain beside and within the active channel of the river (ie the channel benches). Areas of the dominant perrenial riparian vegetation associated with the Lower Darling River are shown in Table 26. These species are indicative of the long term water regime of the river and its floodplain. Common vegetation species found at the bench sites surveyed in the field are listed In Appendix H. A list of water requirements has been compiled for those species which are typically dependant on flooding (Table 27).

Table 26. Riparian Vegetation of the Lower Darling River (Source: King and Green 1993) veg~~tlon Jiipe

black box

·- -Area (l)i) 101 ,320

canegrass

340

cumbungi

40

lignum

2,720

river red gum

5,590

river red gum I lignum

1,200

river red gum I nitre goosefoot

60

black box I lignum

9,010

black box / lignum I nitre goosefoot

320

black box I nilre goosefoot

8,800

black box I river red gum

6,300

cane grass I nitre goosefoot

90 2,670

open water habilat Total

138,480

ASSESSMENT OF ENVIRONMENTAL FLOW NEEOS FOR THE LOWER DARLING RIVER

100

Table 27. Water requirements of riparian vegetation of the Lower Darling River. (Sources: Leitch 1989. Cross and Keenan 1988, Rankine and Hill 1979. Bren and Gibbs 1988). Vegetation

Minimum duration

Maximum duration

Minimum frequency

Best time for effective flooding

2weeks 1 month

18 months 18 months

3 years in 10 7 years in 10

June to November (for regeneration)

Black box

o months

1 month

Oyears

any tlme

Lignum

3-5 months

3 years(?)

1 year in 10

winter - spring

can tolerate prolonged inundation

periodic

River red gum low quality high quality

Nitre goosefoot

Splkerushes (Eleocharis

2 years in 3

winter - spring

3 years in 4

any time

annually

summer

spp.)

Rushes (Juncus spp.)

2 months

30 months

Common reed Cumbungl

6 months

permanent inundation

annuauy

summer

Water couch

4weeks

8weeks

annually

summer

13.2

Seasonality

Durations and frequencies vary considerably between species, however in terms of seasonality Table 27 shows that winter or spring is considered the most effective lime for flooding and will satisfy the seasonal requirements of most species. Those which require water to remain over summer, such as common reed and cumbungi are located along the channel banks where there is access to a more permanent water supply. These were mostly observed at the southern end of the Lower Darling River where the weir pool on the Murray River creates a stable water level, and hence they are not generally Indicative of the natural flow regime of the river. In general summer flooding of trees such as river red gum is tolerated for shorter periods than winter flooding when cooler temperatures lead to lower root oxygen demand. Seasonality of inundation for riparian vegetation along the Lower Darling River can be inferred from the results of the channel bench investigations. River red gum communities were found on the Level B, c and D bench formations. Inundation of these benches may occur at any time of the year, however there are some months for which there has been a slightly higher frequency of flooding. Prior to regulation by the Menindee Lakes, Level B benches had a higher frequency of inundation during mid-summer and autumn (January to

ASSESSMENT OF ENVIRONMeNTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

101

May). Since regulation, flooding has occured with slightly higher frequency during the winter and early spring months (May to September). Pre-Menindee lakes data is insufficient to determine seasonal trends for Level C and D benches, however since regulation in 1960 level C benches have been flooded more frequently in late autumn/winter (April to July) and September, while Level D benches have been flooded more frequently in autumn/winter (April to June). While these trends are not necessarily consistent with the optimum periods for flooding of river red gum communities identified in Table 27 (winter/spring), It should be noted that these requirements have been based on work for the Murray River In most cases. The higher frequency of flooding in the Lower Darling during autumn Is a consequence of its catchment, which lies In that part of the state with a predominance of summer rainfall. Whilst winter/spring may be considered the optimum time for flooding to boost spring/summer growth, it is likely that communities In the more arid areas of the state would respond well to flooding at any time of the year.

13.3

Frequency of Inundation

The minimum frequency of inundation for river red gum is 3 years in 10 (average 'or once every 3.3 years). or up to 7 years in 10 (average of once every 1.4 years) for high quality river red gum forest (Table 27). These figures, however, are derived from studies undertaken on the forests of the River Murray, and hence the lower frequency of 3 years in 10 is likely to be more applicable to the arid environment of the Lower Darling River. River red gum woodland is found on bench features B, C and D (discussed in the chapter on channel geomorphology). The approximate discharges at which these features are flooded at Burtundy are 11,000 MUd (15,000 MUd at Weir 32), 17,000 MUd (29,000 MUd at Weir 32). and 22,000 MUd (unknown at Weir 32) respectively. It was determined from historical hydrologlc records (Appendix G and Table 21) that prior to regulation level 8 benches flooded on average every 1.1 years, but this has been reduced under current conditions to once every 2 years. There is inadequte record to know the inundation frequency of the higher benches prior to regulation, however since 1960 Level C benches have flooded 6 times (average frequency of 6.1 years) and Level D benches 4 times times (average of every 9.3 years). II would therefore appear that the current flood frequency on level B benches, although reduced compared to pre-Menindee conditions, is still within the requirements for river red gums. However river red gum on benches C and D are being flooded less frequently than is required to maintain good health.

13.4

Duration of Inundation

The duration of inundation of riparian river red gum communites along the Lower Darling River can also be assessed by refering to the bench investigations in Section 11. For Level B benches (the lowest on which river red gums occur) the median duration of flooding has increased from 43 days (pre-Menindee Lakes) to 50 days (post·Menindee Lakes). Since regulation, the range of flood durations has been from 4 to 322 days on these benches. There is insufficient data to properly assess changes in duration for Level C and D benches, however Appendix G shows that the range of durations which have been experienced since regulation is 1 to 119 days for Level C benches and 7-56 days for Level D benches. Table 27 Indicates that the minimum duration of flooding for river red gum is 2-4 weeks which Is well within the range of durations currently being experienced for each of the benches. It should be noted that this assessment of frequency and duration of flooding for riparian vegetation is subject to the same limitations as the bench investigations (ie the short

ASSESSMENT OF ENVIRONMENTAL FLCJW NEEDS FOR THE LOWER DARLING RIVER

102

periods of record used for the analysis). Based on this preliminary assessment, however, it would appear that the current duration of flooding of river red gum communities is consistent with the requirements of this species, and has not changed significantly or adversely since regulation.

13.5

Vegetation Condition

At most sites the canopy condition of the river red gums were assessed as healthy (Table 28). Infestations of mistletoe were noted at two sites (12 and 13) and the presence of stressed trees at sites 2, 4 and 11 could indicate that water stress is beginning to occur (most cit the higher benches were last flooded in 1990). The sites at which poor trees were noted are located on bench levels B (site 2), C (site 4), D (11 and 12) and the very highest bench In the study (site 13). Apart from site 2, this would appear to support the hydrologic evidence above that level C and D benches are being flooded less frequently than is required to maintain a healthy riparian community. Regeneration was noted at approximately 50% of sites visited (Table 28). Saplings appeared to range in age from 5 years (1990) to 20 years (mid-70's). Regeneration was present over the whole range of bench levels. Three of the sites with saplings present were Level B benches (sites 2,5,7), one was a Level C bench (9), two were Level D benches (11, 14) and one was the highest bench in the study (13). Groundcover species were generally sparse or almost non-existant, due to the prevailing dry conditions preceding the field inspections. Common species included chenopod shrubs, small daisies, darling pea, stinkwort, saltbush, bluebush and tussock grasses. Dense deposits of leaf litter and bark may also have inhibited the growth of groundcover species on some benches. A list of vegetation species found on channel benches is given in Appendix

H.

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARL.ING RIVER

103

j

Table 28. Vegetation condition at bench sites on the Lower Darling River (May 1996). 'Other comments

Site

:RiY.er·red.!ium .c-onditton

1

Large trees by the water suffering from water stress

2

Most trees healthy, a few with dead limbs and sparse crowns. Some stands of saplings present.

3·

Most trees healthy, a few with dead limbs and sparse crowns.

4

Healthy

Groundcover almost non-existant

5

Saplings present on the floodplain above the bench.

Groundcover very sparse

6

Healthy

Lignum very poor quality

7

One dead tree, most healthy. A band of saplings present at upper edge of bench B.

Lignum poor quality

8

One dead tree, most healthy.

9

Saplings present at back of bench with lignum.

10

Healthy

Groundcover sparse and mostly dead.

11

Some trees stressed (probably lack of water). Some saplings present

Groundcover very sparse

12

Healthy

River coobas (bench C) in extremely poor condition infested by mistletoe. lignum (bench C) in poor condition.

13

4 trees in very poor condition , infested with mistletoe. Lots of young saplings present.

14

Stand of saplings present on bench B.

15

Healthy

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

104

Conclusions And Flow Requirements For Riparian Vegetation The following conclusions may be drawn regarding riparian vegetation: 1. literature indicates that the best time for flooding of most riparian vegetation is winter through spring. 2. River red gum requires flooding a minimum of 3 years out of 10 (average of once every 3.3 years). Benches supporting river red gum which are inundated at discharges of 11,000 MUd or more (Benches C and D) are currently being inundated at frequencies much lower than this (average of once every 6-9 years). 3. The minimum duration of flooding for river red gum is 2-4 weeks which is within the range of durations currently being experienced for each of the benches. 4. The presence of stressed trees on level C and D benches (and higher) may be e.vidence that the current waler regime (most likely frequency) is inadequate to sustain a healthy riparian community at these levels. 5. The presence of common reed and cumbungi in the lower part of the river (affected by Wentworth Weir pool) is indicative of elevated, relatively stable, summer water levels.

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

105

FISH:

A total of eight fish species are known to inhabit the Lower Darling River (Table 29). This list was derived from recent surveys by NSW Fisheries. Of these eight species, two, the goldfish and common carp, are introduced . At least some of the native fish species that inhabit the Lower Darling River have migratory and spawning requirements which must be met in order to sustain a viable population (Cooney, 1994). A summary of the spawning requirements is shown in Table 30.

14.1

Flow Requirements for Spawning

A rise in water level or an increase in flow volume are specific requirements for many native fish species to induce spawning. Three of the species in Table 30 require a rise In water level to inundate the floodplain. This inundation provides an increased habitat and food supply which is critical for fry survival (Wager and Jackson, 1993).

14.2

Temperature and Seasonality Requirements for Spawning

Native fish have specific temperature requirements to stimulate spawning and this requirement varies significantly for each species. The majority of species occurring in the Lower Darling River require rising water temperatures, between 18°C and 26°C, hence spawning predominantly occurs during mid to late spring and early summer (Figure 32). Therefore floods which inundate wetlands on the floodplain are useful for fish recruitment only when they occur during this spring - summer period. The Australian smelt requires a lower water temperature to spawn compared with other native fish species while the crimson spotted rainbow fish requires relatively high water temperatures.

Table 29. Fish species found in the Lower Darling River. Source: NSW Fisheries, 1996.

-Goldfish* Common Carp • Western Carp Gudgeon Golden Perch Elony Bream , Murray Cod-_ ' Cjfl'.1s·o n Spotted Rainbow Fish / ,Australian Smelt

-

f!ooncarie ·

Do.w nham Farm (19kin d/s··aurtundy Weir)

x x x x

x

x x x x x x x

x

x

--------·-·-

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

106

Table 30. Spawning requirements for native fish of the Lower Darling River. Source: Cooney (1994), Brady (1991) and Lake (1978) Species

Flow Req.

Western Carp Gudgeon

Req.d Spawning Temp -::Season (oC)

Spawning Habitat

Spawning Requirements and - Behaviour

Aquatic plants

Spawn following a rise in river level and may occur repeatedly at intervals of a few weeks. Spawn in still water near the surface.

rise

18-23

Dec-Feb

rise

23-26

Oct-Mar or Floo(lplain April if rise gravel or rook

normal

20-23

Oct-Dec

Floodplain

In shallow backWaters during Hoods.

pelagic

rise

15-25

Oct-Dec

Clay banks and submerged logs

adhesiVe, demersal

normal

26-27

Oct-Dec

Aquatic plants

A flood not essential to induce spawning however does enhance survival by providing space/food. Induced by rapid warming of floodwater.

adhesive, threadbearing

normal

>15

Sept-Nov

Aquatic plants and ravel

Success independent of flooding.

adhesiVe, demersal

{Hypse/eolris

klut1zlngen)

Golden Perch (Mscquatia ambigua)

Bony Bream (Nematotosa erebl)

Murray Cod (M~Ua pee/J)

Crimson Spotted Rainbow Fish (Melanotaenia RuviBY/is)

Australian smelt (RB/ropinna S9fll0tll)

Egg Type: Adhesive

Egg Type

adhesive, demersal

nonadhesive, pelagic

fish eggs Which cootaln a sticl7000 ML/d at Weir 32 (historical flows 1960 -1996) Shaded events are those which lasted less than 1o days..

Qver Over Over over

9'J.er over Over Over Over Over Over Over Over

.

2/Q~/,gQ"

~W,:1L6.Q_=:

18/01/60 20/02/60 11/05162 20/06/62 -~1_0109JB5 - _-_~{W~_ "-1610.r16a ·· · 23IDjf63

1102163

16/04/63

.,__--

-.

·- 6/o~isi _:-. ·-

-

7 -33 40 .- ~_.6 -_ -. - -

7 21

130 33 15101/64 27102164 43 5108164 19/09/64 45 24/10/64 22111/64 29 12101/71 27/05/71 135 9/09/71 29/10/71 50 7 ',~~v "..!! '."'' r c ~~q.i!.:rrt1'Jr7'> ~.-.om-~~ J:t. - "" "'~lo'.~: -~-~~!(.~ .....:=:-i':.8•: Over 20103173 7104173 18 Over 30/08/73 8/09/73 9 Over 13/10/73 20/06/74 250 Over 23/08/74 13/09174 21 Over 14/10/74 1/11/74 18 over 4/04/75 21/05/75 47 Over 19/01176 9/07176 172 Over 28/11/76 6/01/77 39 Over 12/03m 20/07177 130 Missing 20107177 28/08m Over 28108177 14/09/77 17 Over 15/07178 29/01179 198 9 QY.~ ·21.L®lif3 =- "" -3.0ff:%/.83. Missing 30io6i83 - ·11/00184 Over 7108184 29/11/84 114 Over 12112184 6/01/85 25 Over 10/06/88 26/06/88 16 Over 6107188 6/09/88 62 Over 17/10/88 3/11/88 17 Over 18/05/89 3110/89 13.8 Over 1/12189 28/01/90 58 Over 28/04/90 12/01/91 259 Over 27/11191 20/12/91 23 over 4/04/96 :&®.f.9.6"'- .-.r Over 20/06/96 10/07/96 20 20109163

24/08163 23/10/63

TOTAL %TIME MEAN MEDIAN

MAX MIN

733 5.40% 61 33 259 1

:,,.-

•9

811 102 ·102:-

-:.-

-.9..7.57 12514 14557

-

--= '7438_

_· _1g . -- . :;_~1 ~ 41 27 84 160 35 2242 105 119

-~~:--=145 35 64

31 154 243 142

65

=-

-~

15805 17615 10780 17361 13530 12179 72800 13574 >'

~= :.·

11731 8161 87736 8843 8489 22841 126126 10694 20638

304 1604

12032 19749 ·18_8 ®:

13 1251 10 41 196 59 9.0 319 1·567 7{f:

17727 14171 9817 16729 8732 28937 9344 44178 7240

7lfo3 17007

313 102 2242 9

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

114

Conclusions And Flow Requirements For Fish The following conclusions are made regarding native fish in the Lower Darling River: 1.

At least 3 species known to occur in the Darling River below Menindee require a rise in river levels to initiate spawning.

2.

Species which rely on a rise in flows spawn during the months of October to March, and hence floods are required during this period to maintain fish recruitment.

3.

At least two of the species in the Lower Darling (golden perch and bony bream) specifically require floodplain habitat for spawning.

4.

It is generally accepted that the floodplain or its wetlands need to remain inundated for at least 4 months to allow spawning and movement of young fry back into the main · channel.

5.

There have been 9 events since regulation where connection be.tween wetlands and the river was maintained for 3-4 months. Of these, seven occurred wholly or partly within the fish breeding season (October to March).

6. At least three of the species occurring in the Lower Darling River are recognised as migratory (golden perch, Murray cod and Australian smelt). 7.

Discharges required to provide passage over weirs below Menindee are 7000 MUd at Weir 32, 2000 MUd at Pooncarie and 2300 MUd at Burtundy. Satisfying fish passage requirements at Weir 32 will automatically satisfy fish passage requirements at Burtundy and Pooncarie.

8.

Events for which the discharge at Weir 32 equalled or exceeded 7000 MUd have occured 37 times in 37 years. 81 % of these events had a duration of 10 days or more. The duration of current events is therefore consistent with requirements for fish passage, and als.o with the duration of events prior to regulation.

9.

The average period between fish passage opportunities has Increased significantly since regulation , from 126 days to 318 days, although the median has remained unchanged (93 days). The maximum period between fish passage events has increased from 1 year 5 months prior to regulation, to 6 years 1 month since regulation.

)

rOJO. Q

./

~

Flows suitable for fish passage occured in all of the 30 years prior to regulation and in 51% of years since regulation. Prior to regulation these flows rarely occurred more than twice within the fish breeding/migration season (October to March). In most years since regulation fish passage has occurred as a single event.

11. Flows providing fish passage in the Lower Darling River may occur at any time of the year althougth prior to regulation there was a slightly higher frequency during the months of March-April and September-October.

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER OARLING RIVER

115

MACROINVERTEBRATES 15.1

Sampling Results Macroinvertebrates were semi-quantitatively sampled from a total of four sites on the Lower Darling River. Samples were taken at bench sites 1, 5, 9 and 13. The aim of the sampling was to gain a preliminary snapshot of the common taxa present within the channel envlroment of the Lower Darling River. All samples were taken from 10m sections of channel edge, however sites were chosen so as to encompass the range of channel edge conditions present eg. vegetated, bare bank, snags etc. (Table 34 ). A total of 12 macroinvertebrate taxa were collected from the four sites (Table 35). The characteristics of the taxa sampled are found in Appendix I. Insects were the dominant group comprising 71% of all taxa, with mayflies and chironomids being the most common insects in the samples. Crustacea were the next most dominant group in terms of taxa, with fresh-water shrimps being common at all sites sampled. This is consistent with Sheldon (1996) who found that channel habitats on the Darling River above Menindee were typified by shrimps and various chironomids. The macroinvertebrate assemblages of specific micro-habitats often reflect structural habitat complexity (Boulton and Lloyd 1991) with species diversity and abundance greater on complex surfaces (Minshall 1984; Cyr and Downig 1988). Snags within the river form one of the most important types of microhabltat and can cover a range of complexity from smooth cylindrical surfaces to rough, highly grooved and dissected surfaces (O'Connor 1992). The difference in taxa observed between site 2 (dense river red gum roots) and site 3 (sparse timber) appears to reflect this difference in microhabitat complexity. The greatest number of taxa were , however, observed at site 4 which is the least complex of the sites sampled. However this could be due to factors other than habitat complexity. As samples in each micro-habitat were not replicated (the aim of the survey being to obtain a species list, not to explore microhabitats) no conclusions can therefore be drawn regarding differences between community diversity with regard to microhabitats.

Table 34. Description of sites for macroi nvertebrate sampling

Site

Reach

1

5

Channel edge with dense fringe of common reed

5

3

Channel edge with dense overhanging red gum roots

13

2

Channel edge with sparse overhanging and submerged timber

9

1

Channel edge. unvegetated

Description

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

116

Table 35. Macroinvertebrate data from 4 sites on the Lower Darling River.

•

Phylum

Family

..

Genus

common

Name

Feeding Category

' ,site 1.

Mollusca

Ancylidae

Ferrissia spp.

Freshwater limpet

Scraper

Crustacea

Cirolanidae

Austroargathona spp.

lsopod

Collector

x

Palaemonidae

Macrobrachium spp.

Freshwater shrimp

Collector

x

Parastacidae

Cherax spp.

Yabby

Collector

Caenidae

Tasmanocoenls spp.

Mayfly

Collector

Micronacta spp.

"True bugs•

Collector

Non-biting midge

Collector

Site 2

x

Slte3

Slte'4

x

x x

x x

x

x

lnsecta Ephemeroptera Hemiptera

Corixidae Diptera Chironomidae (SubF.Chironomlnae) Chironomidae (SubF. unidentified) Chironomidae (SubF, Orthocladiinae) Chironomidae (SubF. Tanypodinae) Ecnomidae

Non-biting midge

x

x

x

x

x

x

Collector

Non-biting midge

Predator

Ecnomus spp.

Caddis fly

Predator

x

Trip/ectides spp.

Caddisfly

Collector

x

Odonata unidentified

Damsel fly

Predator

x

unidentified

Damsel fly

Predator

x

Total No. of taxa

ASSESSMENT OF ENVIRONMENTAL FLOW NEEDS FOR THE LOWER DARLING RIVER

x

x

Non-biting midge

Leptoceridae

x

x

5

9

x x

x

x

5

9

117

15.2

Functional Feeding Groups

As per Thoms et al. (1996), the taxa have been divided into three broad functional feeding groups - collectors, predators and scrapers (Table 35). Functional feeding groups can be used to suggest the significance of different food resources in sampled habitats. Functional groups are divided by feeding mechanism rather than food source as a given feeding mode may intake all food categories (Cummins 1974). All sites in the Lower Darling were dominated by collectors. Collectors are groups of macroinvertebrates which utilise fine detrital particles (