Hydrology

2.4.1 The Lake Eyre Basin

The Lake Eyre Basin drainage division, occupying an area of 1,140,000 km², represents one of the largest endorheic or internal drainage systems in the world (Kotwicki, 1986), covering approximately 17% of the Australian continent (Nanson et al., 1998). The basin, stretching from the tropics in the north to the temperate zone in the south, also contains the driest region in Australia and some of the most hydrologically variable arid zone rivers in the world which maintain relatively high levels of ecological health (Kotwicki, 1986, Puckridge, 1998, Puckridge et al., 1998, McMahon et al., 2008b). Parts of Lake Eyre, within the basin, lie at 15.2m below sea level, the lowest areas on the continent (Kotwicki and Isdale, 1991).

The Lake Eyre Basin is comprised of the catchments of five main river systems. Along with the Diamantina and Georgina Rivers, the inaptly named Cooper „Creek‟, is one of the three major rivers of the Lake Eyre Basin (Figure 1-2). Almost half the basin area receives less than 200 mm of rainfall annually. Those areas receiving higher annual rainfall of around 400-500 mm occur in the north and north-east of the basin, regions influenced by the southern edges of the summer monsoon (Kotwicki and Allan, 1998). More than 95% of the inflows to Lake Eyre are generated during the months of the summer monsoon. These inflows are fed by the north- eastern tributaries; the Cooper Creek and Diamantina and Georgina Rivers. Of these, the Diamantina and Georgina Rivers are deemed to be the major contributors of flows to the Lake, supplying approximately 65% of the total volume entering the Lake (Kotwicki and Isdale, 1991). However it must be emphasized that typically only a small proportion of flows from these tributaries ever reach the Lake due to massive transmission losses en route associated with extensive cracking clays soils and high evapo-transpiration losses from the floodplain during the summer (Knighton and Nanson, 1994a). Extreme variability in discharge and flow duration along the tributaries is the norm. Major floods are the exception. Lake Eyre is usually empty. The major flow events reaching Lake Eyre are most typically associated with La Niña phases of ENSO which enhance the summer monsoon. Significantly these Lake Eyre Basin rivers remain some of the few unregulated inland river systems both nationally and internationally.

2.4.2 The Cooper Creek Catchment

The Cooper Creek catchment, comprises a total area of approximately 296,000 km², extending across sections of Queensland, New South Wales and South Australia (Figure 1-3). The Cooper Creek catchment comprises almost one quarter of the Lake Eyre Basin, taking in parts of western Queensland, north-east South Australia and a small area of far northwest New South Wales. Approximately 82% of the total Cooper Creek catchment area is found within Queensland. Cooper Creek is considered to be one of the largest remaining wild rivers on a global scale (Brock et al., 2006). Kingsford et al (1999) consider the Cooper as being the

longest and probably most important dryland river in Australia. This importance is attributed to the extent and biological significance of the extensive and associated wetlands system across the floodplain (ibid). Most of the significant wetland habitat on Cooper Creek is located within South Australia (Jenkins et al., 2005).

Within the Lake Eyre Basin the Cooper arises in the uplands of north-eastern Australia near Hughenden. Here the Great Divide turns a network of drainage systems south and inland (Kotwicki, 1986, Rust and Nanson, 1986, Nanson et al., 1988) (Figure 1-3). These systems eventually coalesce to become the Thomson River which joins the Barcoo River near Windorah to become the Cooper proper, giving full expression, via an enormous system of braided and anastomosing channels, to the parochial and iconic term „The Channel Country‟. The catchment is also fed by run-off from the Grey, Gowan and Warrego Ranges. Thus the Cooper, whilst arising in monsoonal climes, wends for most of its length through an environment of increasing aridity until finally terminating in Lake Eyre. A significant characteristic, with major influence upon geomorphological and soil characteristics, is the low gradient of the Cooper system. To the east of Innamincka the gradient averages a fall of 3 to 4 cm per kilometre; to the west of Innamincka the gradient decreases to less than 0.5 cm per kilometre (Wopfner, 1970). The water of the Cooper is considered to be predominantly fresh. However in places, particularly in the lower reaches within South Australia, the Cooper does pass though areas where saline groundwater intersects the channel.

The summer monsoon rains to the north of the Cooper Creek catchment margins account for flood pulse events of varying intensity and duration down the Cooper (Fagan and Nanson, 2004). Flows along Cooper Creek, on average, reach Lake Eyre once in six years. Most flows terminate in the Coongie Lakes system in the study area. Since a gauging station was established in the vicinity of Innamincka in 1973, Coongie Lake has received flows every year. During this period the Lake has only completely dried on two occasions (Costelloe, 2004). A review conducted by Puckridge et al. (1998) of global dryland river systems found Cooper Creek to exhibit one of the most hydrologically variable flow regimes in the world. An example of the extreme variability of these pulse events is provided by the floods of 1974 which resulted in a peak flow at Innamincka of 6400 m3/sec, over one hundred times greater than the average flow of 63 m3/sec recorded for the period 1973-1993 (ibid 2004) (Appendix 1: Satellite images). The major flow events down the Cooper, driving the ecology of the whole system, are driven by climatic events distant in genesis, which are manifestations of global circulation patterns. The extreme rainfall and flood variability within the study area is thus directly related to climatic variability operating at a range of temporal and spatial scales driven remotely by interactions between teleconnected climate systems (e.g. ENSO, Indian Ocean Dipole, Australian Summer Monsoon, East Asian Monsoon).

As a consequence of this highly stochastic rainfall regime and its transmission through the most arid region of Australia, the Cooper epitomizes the characteristics of a dryland river system.

The surface hydrology of the study area is driven primarily by rainfall events in the north- eastern quarter of the Cooper Creek catchment in Queensland. Mollenmans et al (1984) proposed a useful conceptual model of hydrology in the study region, defining four classes of flood based on the nature of rainfall in the upper catchment. The classes are as follows:

1. Average Flows – generally occur annually between April and July: water flows to about as far as Cuttapirie Corner Waterhole on the main channel and Coongie Lake on the North West Branch; the location of last remnants of the northern river red gum Eucalyptus camaldulensis var. obtusa indicates the outer limits of influence of these flow events;

2. Flows Moderately Above Average (Level 1 Flood) – occur irregularly in response to greater than average rainfall; water flows to Lake Goyder on the North West Branch and may reach Lake Hope on the Main Branch; thought to occur once every three or four years;

3. Flows Well Above Average (Level 2 Flood) – occur rarely in response to much greater than average rainfall; water covers much of the Cooper Creek floodplain and fills Lake Hope to capacity (maximum depth of 10 m); floodwaters may reach Lake Eyre; once every 6-10 years on average; and,

4. Extreme Flows (Level 3 Flood) – extremely rare events as a result of heavy rainfall over an extended period; floodwaters enter the Strzelecki Creek and flow along the Cooper to Lake Eyre in substantial quantities; such events may occur every 40 to 60 years on average.

Costelloe (1998) utilized and refined the Mollenmans flood classes whilst developing a hydrological model for flooding in the Coongie Lakes Wetlands.

Table 2-1 Flood classes

Flow Class Daily Flow Volume Frequency Extent

Class 1 300-5,000 Megalitres (Ml) Annual Coongie Lake to northern Lakes, Apanburra

Class 2 5,000-35,000Ml 2 years Cooroomunchena Water Hole, Lake Lady Blanche, Apanburra, Christmas Creek

Class 3 35,000-100,000Ml 5 years Lake Hope, Wilpinnie Creek, Northern Overflow

Class 4 100,000-150,000Ml 10 years Strzelecki Creek, Toolerinna Swamp Class 5 150,000-450,000Ml 30 years Lake Lady Blanche, Lake Eyre North Class 6 >450,000Ml 100 years Lake Callabonna –Gregory, Lake Eyre

Costelloe derived his classification of flood pulses from daily flow volume data recorded at Cullyamurra (often spelt Callamurra) water hole gauging station, a short distance upstream from Innamincka; and the associated observed area and pattern of inundation within the Coongie Lakes district from satellite imagery (Costelloe, 1998). The effects of a range of Cooper Creek flood pulses over the period 1988-1990 were modelled and the importance of prior hydrological conditions upon the effective transmission of flood waters down the system was established. Costelloe found that the combination of pre-existing volume of water in the system together with the total volume of an individual flood pulse were important determinants of the longitudinal and lateral transmission and duration of floodwaters. However Costelloe did later emphasize that;

“The requirements of ecological models and water resource plans are driving demand for hydrological models of the rivers of the arid zone. Knowledge of the hydrology of Australia‟s arid zone is poor, yet is critical in understanding the ecology of the region” (Costelloe, 2004)pi

Puckridge et al. (2000) analysed hydrograph records over a 48 year period and determined that the major floods were influenced by ENSO conditions in the upper catchment. These floods coincided with clusters of La Niña episodes. Puckridge et al. also determined that clusters of flows/flood pulses over a number of years play a vital role in the recruitment of biota. These clusters are closely associated with La Niña episodes (Puckridge and Walker, 2000). Flood events are a crucial driving force or natural disturbance factor in the structure and functioning of the wetland and floodplain ecosystems of dryland river systems such as the Cooper. These episodic events, distant in genesis, are vital for the support and functioning of floodplain biota given that rainfall in this region is highly variable spatially, temporally, in duration and intensity even in comparison with other global arid regions (Stafford Smith and Morton, 1990).