| River Derwent | |
| Name Native: | (Mouheneenner language) |
| Map Size: | 280 |
| Pushpin Map: | Australia Tasmania |
| Pushpin Map Size: | 280 |
| Pushpin Map Caption: | Location of the river mouth in Tasmania |
| Subdivision Type1: | Country |
| Subdivision Name1: | Australia |
| Subdivision Type2: | State |
| Subdivision Name2: | Tasmania |
| Subdivision Type5: | Cities |
| Subdivision Name5: | Derwent Bridge, New Norfolk, Hobart |
| Length: | 239km (149miles) |
| Discharge1 Location: | Storm Bay |
| Discharge1 Min: | 50m3/s |
| Discharge1 Avg: | 90m3/s |
| Discharge1 Max: | 140m3/s |
| Source1: | Lake St Clair |
| Source1 Location: | Central Highlands |
| Source Confluence Location: | Cradle Mountain-Lake St Clair National Park |
| Source Confluence Coordinates: | -42.12°N 146.2103°W |
| Source Confluence Elevation: | 738m (2,421feet) |
| Mouth: | Storm Bay |
| Mouth Location: | Hobart |
| Mouth Coordinates: | -43.0508°N 147.3772°W |
| Mouth Elevation: | 0m (00feet) |
| Basin Size: | 9832km2 |
| Tributaries Left: | Nive River, Dee River, River Ouse, Clyde River, Jordan River |
| Tributaries Right: | Repulse River, Tyenna River, Styx River, Plenty River, Lachlan River |
| Custom Label: | Natural lakes |
| Custom Data: | Saint Clair Lagoon; Lake St Clair |
| Extra: | [1] [2] |
The River Derwent, also known as timtumili minanya in palawa kani,[3] is a significant river and tidal estuary in Tasmania, Australia. It begins its journey as a freshwater river in the Central Highlands at Lake St Clair, descending over across a distance of more than . At the settlement of New Norfolk in the Derwent Valley its waters become brackish, flowing through Hobart, the capital city of Tasmania, its seawater estuary eventually empties into Storm Bay and the Tasman Sea.
Historically, the banks of the Derwent were covered by forests and frequented by Aboriginal Tasmanians. With the arrival of European settlers, the area saw agricultural development and the construction of dams for hydro-electricity generation during the 20th century.
Today, the Derwent's catchment area is characterised by agriculture, forestry, and hydropower generation. It serves as a vital source of water for irrigation and urban supply, notably providing a significant portion of Hobart's water needs.[4] The river is also crucial for recreation, supporting activities such as boating, fishing, and leisure along its banks. Additionally, it facilitates marine transportation and various industrial activities, playing a central role in Tasmania's economy and lifestyle.[2]
The upper part of the river was named after the River Derwent, Cumbria by British Commodore John Hayes who explored it in 1793. The name is Brythonic Celtic for "valley thick with oaks".[5] [6]
Matthew Flinders placed the name "Derwent River" on all of the river.[7]
The name "River Derwent" was officially endorsed on 20 May 1959.[8]
The River Derwent valley was inhabited by the Mouheneener people for at least 8,000 years before British settlement.[9] Evidence of their occupation is found in many middens along the banks of the river. The first European to chart the river was Bruni d'Entrecasteaux, who named it the Rivière du Nord in 1793.[10] Later that same year, John Hayes explored the river and named it after the River Derwent, which runs past his birthplace of Bridekirk, Cumberland.
When first explored by Europeans, the lower parts of the valley were clad in thick she-oak forests, remnants of which remain in various parts of the lower foreshore.
There was a thriving whaling industry until the 1840s when the industry rapidly declined due to over-exploitation.[11]
Formed by the confluence of the Narcissus and Cuvier rivers within Lake St Clair, the Derwent flows generally southeast over a distance of to New Norfolk and the estuary portion extends a further out to the Tasman Sea. Flows average in range from 50to and the mean annual flow is .
The large estuary forms the Port of the City of Hobart – the deepest sheltered harbour in the Southern Hemisphere.[12] Some past guests of the port include in February 1836, carrying Charles Darwin; the ; and . The largest vessel to ever travel the Derwent is the 113000t, 61m (200feet) high, ocean liner Diamond Princess, which made her first visit in January 2006.[13]
At points in its lower reaches the river is nearly wide, and as such is the widest river in Tasmania.
The Derwent estuary contains dozens of white sandy beaches, many of which are staples of local activity within Hobart suburbs and include Bellerive Beach, Blackmans Bay Beach, Howrah Beach, Nutgrove Beach, Lords Beach, Long Beach, Taroona Beach, Hinsby Beach, Kingston Beach and Windermere Beach.
Until the construction of several hydro-electric dams between 1934 and 1968, the river was prone to flooding. Now there are more than twenty dams and reservoirs used for the generation of hydro-electricity on the Derwent and its tributaries, including the Clyde, Dee, Jordan, Nive, Ouse, Plenty and Styx rivers. Seven lakes have been formed by damming the Derwent and the Nive rivers for hydroelectric purposes and include the Meadowbank, Cluny, Repulse, Catagunya, Wayatinah, Liapootah and King William lakes or lagoons.
The Upper Derwent is affected by agricultural run-off, particularly from land clearing and forestry. The Lower Derwent suffers from high levels of toxic heavy metal contamination in sediments. The Tasmanian Government-backed Derwent Estuary Program has commented that the levels of mercury, lead, zinc and cadmium in the river exceed national guidelines. In 2015 the program recommended against consuming shellfish and cautioned against consuming fish in general. Nutrient levels in the Derwent between 2010 and 2015 increased in the upper estuary (between Bridgewater and New Norfolk) where there had been algal blooms.[14]
A large proportion of toxic heavy metal contamination stems from legacy pollution caused by major industries that discharge into the river including Nyrstar Hobart, a historic smelter establish at Lutana in 1916,[15] and the Norske Skog Boyer paper mill at Boyer which opened in 1941.
The Derwent adjoins or flows through the Pittwater–Orielton Lagoon, Interlaken Lakeside Reserve and Goulds Lagoon, all wetlands of significance protected under the Ramsar Convention.[2]
In recent years, southern right whales finally started making appearance in the river during months in winter and spring when their migration takes place. Some females even started using calm waters of the river as a safe ground for giving birth to their calves and would stay over following weeks after disappearance of almost 200 years due to being wiped out by intense whaling activities. In the winter months of 2014, humpback whales and a minke whale (being the first confirmed record of this species in the river) have been recorded feeding in the River Derwent for the first time since the whaling days of the 1800s.[16]
The rare spotted handfish (Brachionichthys hirsutus), whose only habitat is in the Derwent estuary and surrounds, was the first marine fish to be listed as critically endangered in the IUCN Red List, in 1996.[17] [18] The fish is threatened by the Northern Pacific seastar's invasion into southern Australian waters. The Northern Pacific seastar (Asterias amurensis), now firmly established in the Derwent,[19] preys on not only the fish eggs,[20] but also on the sea squirts (ascidians)[21] that help to form the substrate that the fish spawn on.[22]
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.[23]
Travelling further north from the Bridgewater crossing, the next crossing point is the Blair Street Bridge at New Norfolk, slightly north of the point where the Derwent reverts from seawater to fresh water, Bushy Park, Upper Meadowbank Lake, Lake Repulse Road, Wayatinah, and the most northerly crossing is at Derwent Bridge, before the river reaches its source of Lake St Clair. At the Derwent Bridge crossing, the flow of the river is generally narrow enough to be stepped across.
The river is the subject of the multimedia performance "Falling Mountain" (2005 Mountain Festival), a reference to the mountain in the Cradle Mountain-Lake St Clair National Park from which the river rises.
The Derwent is mentioned in the song, Mt Wellington Reverie by Australian band, Augie March.[24] Hobart is located in the foothills of Mount Wellington.