Physical setting
A. Catchment Morphology
The Lockyer Valley is a roughly circular basin of approximately 2,800 km2 in southeast Queensland. The town of Gatton, population 4,600, is the largest in the valley and is 90 km west of the state capital, Brisbane.
The valley is bordered on the south and west by the Great Dividing Range which is typically 250-300 m higher than the valley floor. This range is flat-topped and covered by Tertiary age (~ 45 million years) basalt flows which weather to form dark fertile soils.
Geologically, the valley is formed of a sequence of sedimentary rocks which are dominantly sandstone. These formations consist of a sequence of Triassic and Jurassic age (~ 250-140 million years) sediments formed mainly of flat-lying fluvial (from rivers) sandstones, siltstones and shales, with minor conglomerate. These formations have different geological names.
The main stream system central to the valley is Lockyer Creek. The headwaters of the larger tributaries of the catchment abutt the basaltic ranges in the south and west. In the north, the drainage divide is less well defined and streams flowing south to Lockyer Creek are smaller and more irregular in nature.
Weathering of these sedimentary bedrocks over earlier geological time produced channels that are incised into them. Continued weathering and erosion has also provided the alluvial material that fills these channels and the adjacent valley floors.
In the larger tributaries the streams have cut into the alluvial deposits forming the existing stream channels. Under the current dry conditions, the depth of these channels can be seen.
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B.Climate
The local region is sub-topical and humid, with relatively long hot summers, and winters that are short and mild with occasional frosts.
Rainfall central to the valley has an annual average of 776 mm (1900-2004), while the annual rainfall increases on the adjacent ranges to the west and south, to around 1,000 - 1,200 mm. Within the catchment there is a marked rainfall deficit and the average annual rate of evaporation for 1995-2004 was 1,862 mm.
Stream flow for much of the drainage system is ephemeral in nature, and related to summer storms (November – March). Of importance is that over the last 15 or so years, there has been a trend to both lower rainfall, and a less regular seasonal distribution.
This sketch of the global "hydrological cycle"shows how climatic factors such as rainfall and evaporation relate to aquifer recharge. (also see Glossary, under water cycle)
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Further information
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Lockyer Valley - location [107.81 kB] - location of Lockyer Valley (Lockyer Valley Resource Atlas, Dept Natural Resources, Queensland Government) |
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Lockyer Valley - relief map [219.78 kB] - relief map (false colours) - NASA satellite image, coloured scale bar to left shows m ASL (metres above sea level) |
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Lockyer Valley - shaded relief [422.5 kB] - the topography is based on 25 m interval contour lines. Relief is highlighted by representing the "sun" at 310o, which is around mid-afternoon. |
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Lockyer Valley - satellite image [413 kB] - satellite image of the Lockyer Valley (Google Earth). Towns in valley and Toowoomba can be seen (blue-grey), and cultivated fields (green) which follow the alluvium. |
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Lockyer Valley - geological map [123.23 kB] - geological map of the area (Water Resources Commission, Queensland Government). This is a map from the 1980's but clearly shows the extent of the alluvium, and the sandstone formations (note some change of formation names) |
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Lockyer Valley - soil map [677.67 kB] - -soil distribution for Lockyer Valley taken from Atlas of Australian Soils, with a summary of soil type. It is interesting to compare soil type to the geology map. |
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Rainfall map - average for period 1900 - 2000 [101.02 kB] - rainfall distribution of Lockyer region, showing mean annual rainfall based on a 50 year period |
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Table of monthly rainfall Gatton (1990-2004) [198.85 kB] - compare these annual totals with the long term annual rainfall for Gatton (1900 to 2004) of 776 mm [data from DPI&F, Gatton] |
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