Mary Catchment Story

Transcript

The Great Sandy Strait catchment is located north of the Sunshine Coast between Maleny and Burnett Heads.

The sub-catchments that make up the broader Great Sandy Strait catchment include the Mary River, Fraser Island and a number of smaller coastal catchments running from Tin Can Bay to Hervey Bay.

The Great Sandy Strait has been identified under the ‘international Ramsar convention for wetlands of significance’ as a sand passage estuary of outstanding quality. It is also part of the Great Sandy Marine Park. Other significant features in the catchment are the World Heritage listed Fraser Island, and the sensitive and valuable estuarine and freshwater systems of the Mary River. The area has significant ecological values, including rare species and threatened ecosystems. Other values of the area are the significant agricultural heritage of the Mary River Basin, and the tourism industry on Fraser Island, which receives between 350-500,000 visitors per year.

To effectively manage this beautiful and internationally recognised area it’s important to understand how water flows in the catchment and how it can be affected by land use and management practices.

Prior to development, eucalypt woodland dominated most of the mainland catchment areas. Due to variability of climate and topography in the catchment, the upland areas had significant patches of rainforest while mangroves and heath grew on the coastal regions and sand islands of the Great Sandy Strait. This vegetation slowed water coming off the land, retaining it longer in the landscape.

Today about a third of the catchment has been cleared with the main land uses being forestry, grazing, and agriculture such as beans, sugar cane, pineapples and macadamias. The area is also home to about 140,000 people, most living in the major town centres of Gympie, Maryborough and Hervey Bay. Residential and industrial developments as well as other land clearing can significantly modify water flow. These developments change the shape of the landscape and add impermeable surfaces, such as roads, which lead to increases in run-off.

While roads, railways and creek crossings are essential, they modify the flow of water and may act as barriers, redirecting water through piped channels or culverts. There are also a number of dams, weirs and barrages throughout the area that can change the flow paths across the landscape. The two largest dams serve a multiple purposes, servicing agriculture and providing drinking water to the South East Queensland Water Grid and towns outside of the catchment.

The geology of the catchment is complex and varied and can significantly affect the flow paths of water falling on the catchment. Through most of the Mary catchment, the primary flow path is across the surface of the land into local streams, and eventually into the Great Sandy Strait. In the coastal areas associated with sandy landscapes, like Fraser Island, water primarily travels through subsurface pathways to the ocean. This groundwater supports a variety of terrestrial and aquatic groundwater-dependant ecosystems.

The upland areas of the Mary catchment are different in terms of topography and rainfall. The southern areas of the catchment are characterised by impermeable geologies and the steep slopes of the Blackall range. These areas receive the most rainfall and consequently have the potential for rapid run-off and flash flooding in areas with restricted channels and steep gullies. There are a number of dams in the catchment that capture rainfall which generally results in less flows being available downstream when conditions are dry. However these dams do allow scouring flows from high rainfall events. The Munna Creek sub-catchment, in the far North West has a totally different character. It is a flatter arid landscape, with prolonged dry periods, slower moving flows, and less regular floods.

The mid-systems of the Mary River receive flows from the upland areas. The River itself is comprised of a series of large, diverse habitat pools linked by reaches of riffles and runs. It is surrounded by a broad well developed alluvium which historically was full of floodplain billabongs, only a few of these still remain.

The main changes to the area have been a reduction in the low flows, due to the dams in the upper reaches, and the influence of the Mary Barrage. The barrage prevents salt water from moving up the system, as it would otherwise do, and so changes the animal and plant species that are able to survive in this habitat. It also causes freshwater to stop flowing through the system, causing it to pond as far up as Tiaro. Downstream of the barrage the ecosystems in the mouth of the Mary River have also been changed significantly by the reduction in freshwater flows. These ecosystems now have different mangrove communities that reflect the saltier environment.

The small series of coastal catchments directly bordering the Great Sandy Strait are varied, and differ greatly to the sub-catchments of the Mary. The northern most of these catchments sit on weathered sandstone, with low undulating terrain. These landscapes are quite permeable to water and contain significant wetlands and groundwater dependant ecosystems. In contrast, the Cooloola and Fraser Island sand masses are highly permeable. The flow through these systems is strongly tied to the amount of water in the groundwater aquifer. They demonstrate a variety of surface expressions of groundwater from lakes that are formed from impermeable surfaces at higher elevations, to those which are surface expressions of the regional groundwater table, to intermittent and perennial creek systems. The landscape through all of these sand areas can change dramatically and quickly when heavy rain falls on them, rapidly scouring out sand channels, and exposing the bedrock substrate.

All of the freshwater systems mentioned flow into the Great Sandy Strait, where they mix with the saltwater marine systems. The tide rises faster in the southern reach of the strait, where the channel is more direct. The northern flows must pass around Fraser Island and through the eddies of Hervey Bay before entering the strait. Just south of the mouth of the Mary is an area where tidal flows are limited due to the tides from the north and south meeting and reducing the flow strength. This area of low flow acts as a point where sediments drop out of the water column, forming a shallow area with a region of sand banks. These interchanges between the flows from the freshwater and the tidal action of the saltwater systems can create strikingly different ecosystems depending on specific local conditions. More freshwater mangroves, such as those found around Tin Can Bay Inlet and the Cooloola Sand Masses, can dominate coastal areas with strong groundwater connection.

Due to the low lying topography and diverse geologies there are over 600km² of wetlands that vary from saltmarsh mangroves and freshwater springs to rare ecosystems like the patterned fens of the Cooloola sand masses. These varied ecosystems support a significant number of beautiful, important and endangered species like the Giant Barred frog, the critically endangered Mary River cod, dugongs, whales, and many species of migratory and native shorebirds.

Last updated: 20 December 2017