area of . They range from sometimes raging torrents, damaging bridges and flooding city areas, to trickles and completely dry in summer. Winter and spring flooding has prompted the construction of flood reduction works. A constructed sea outlet, landscaped linear parks and three holding reservoirs contain peak flow. Physiography The River Torrens runs largely westward from the Adelaide Hills, through the centre of Adelaide to the Gulf St Vincent. It originates close to the eastern fault scarp of the Mount Lofty Ranges, near Mount Pleasant, approximately above sea level. It runs predominantly along faulted north-south ground structures, which were formed over 250 million years ago during the Paleozoic era then further dislocated during the Cretaceous and earliest Tertiary. There is a subsidence along the Para Fault which also affects the rivers flow. This subsidence was formed in the last two million years, after the Pliocene era. From its origin to Birdwood the river follows rolling, relatively level country before entering a hilly section that continues to Gumeracha. The river then follows sedimentary rock strata before entering a gorge after Cudlee Creek. It flows through the gorge to Athelstone, passing over the Eden Fault Zone of the Adelaide Hills face and associated escarpment. After the scarp it flows over sedimentary rocks of varying resistance to erosion, which has led to interspersed narrows and broad basins. From the base of the Adelaide Hills to Adelaide's central business district it runs in a shallow valley with a terraced floor, then down the slope of its own alluvial fan. The structure of this fan shows that the river formerly entered Gulf St Vincent via the Port River. Over time the Torrens deposited sediment, choking its own outflow; becoming locked behind coastal sand dunes and forming the swampy areas of the Cowandilla Plains and The Reedbeds. Tributaries The Torrens is fed by numerous seasonal creeks, which are dry for most of the year in their lower reaches, but prone to occasional flooding during the winter and spring. There are five main creeks that join it from the southern side as it crosses the Adelaide Plains east of Adelaide, and at least five more in its path through the Adelaide Hills. The plains tributaries, known as First to Fifth Creeks, lie to the east of the city, with First being the most southerly and the rest numbered consecutively northward. They were once named Greenhill, Hallett, Todd,