Derwent River (Tasmania)

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For other rivers called Derwent, see River Derwent.

The Derwent is a river in Tasmania, Australia.

It was named after the River Derwent, Cumbria by British Commodore John Hayes who explored it in 1794. The name is Brythonic Celtic for "valley thick with oaks"^[1]^[2]^[3]. When first explored by Europeans, the lower parts of the picturesque valley were clad in thick she-oak forests, remnants of which remain in various parts of the lower foreshore. ^[4]

The river originates at Lake St Clair and flows south over a distance of 187 km to New Norfolk and the estuary portion extends a further 52 km out to sea. Flows average in range from 50 to 140 cubic metres per second. Mean annual flow is 90 cubic metres per second. ^[4]

The large estuary forms the Port of the City of Hobart – often claimed to be the deepest sheltered harbour in the Southern Hemisphere; some past guests of the port include the USS Enterprise and USS Missouri. The largest vessel to ever travel the Derwent is the 113,000 tonne, 61 metre high, ocean liner 'Diamond Princess', which made its first visit in January 2006. At points in its lower reaches the river is nearly three kilometres wide, and as such is the widest river in Tasmania. This is a great contrast to the upper reaches of the Derwent, near the town of Derwent Bridge, where, in summer, the river often slows to a trickle and can be easily crossed with one step.^{[citation needed]}

Until the construction of several hydroelectric dams between 1934 and 1968, it was prone to frequent flooding. Seven lakes have been formed by damming the Derwent and Nive tributary for hydroelectric purposes: Lakes Meadowbank, Cluny, Repulse, Catagunya, Wayatinah, Liapootah and King William. Further other dams and diversions add to this complex scheme. A substantial part of the dam construction work was performed by Polish and British migrants after World War II. The State of Tasmania is now attempting to diversify its over reliance on power from hydroelectricity, specially considering the low storage capacity of the overall Hydro system. ^[5]

Several bridges connect the western shore (the more heavily populated side of the river) to the eastern shore of Hobart – in the greater Hobart area, these include the five lane Tasman Bridge, near the CBD, just north of the port; the four lane Bowen Bridge; and the two lane Bridgewater Bridge and Causeway. Until 1964 the Derwent was crossed by the unique Hobart Bridge, a floating concrete structure just upstream from where the Tasman Bridge now stands. ^[6]

There was a thriving whaling industry until the 1840s when the industry rapidly declined due to over-exploitation. ^[7]

The Derwent suffers from extremely high levels of heavy metal contamination in sediments. The State Government-backed Derwent Estuary Program points out in particular that levels of mercury, lead, zinc and cadmium exceed national guidelines. They also recommend against consuming shellfish and caution against consuming fish in general. ^[4] A large proportion of the heavy metal contamination comes from major industries that discharge into the river: an electrolytic zinc smelter at Lutana established in 1917, and a paper mill at Boyer which opened in 1941. ^[4]