The Swan River (Nyungar: Derbarl Yerrigan) is a major river in the southwest of Western Australia. The river runs through the metropolitan area of Perth, Western Australia's capital and largest city.

The Swan River estuary flows through the city of Perth. Its lower reaches are relatively wide and deep, with few constrictions, while the upper reaches are usually quite narrow and shallow.

The Swan River drains the Avon and coastal plain catchments, which have a total area of about 121,000 square kilometres (47,000 sq mi). It has three major tributaries, the Avon River, Canning River and Helena River. The latter two have dams (Canning Dam and Mundaring Weir) which provide a sizeable part of the potable water requirements for Perth and the surrounding regions. The Avon River contributes the majority of the freshwater flow. The climate of the catchment is Mediterranean, with mild wet winters, hot dry summers, and the associated highly seasonal rainfall and flow regime.

The Avon rises near Yealering, 221 kilometres (137 mi) southeast of Perth: it meanders north-northwest to Toodyay about 90 kilometres (56 mi) northeast of Perth, then turns southwest in Walyunga National Park – at the confluence of the Wooroloo Brook, it becomes the Swan River.

The Canning River rises from North Bannister, 100 kilometres (62 mi) southeast of Perth and joins the Swan at Applecross, opening into Melville Water. The river then narrows into Blackwall Reach, a narrow and deep stretch leading the river through Fremantle Harbour to the sea.

The estuary is subject to a microtidal regime, with a maximum tidal amplitude of about 1 metre (3 ft 3 in), although water levels are also subject to barometric pressure fluctuations.

Before the Tertiary, when the sea level was much lower than at present, the Swan River curved around to the north of Rottnest Island, and disgorged itself into the Indian Ocean slightly to the north and west of Rottnest. In doing so, it carved a gorge about the size of the Grand Canyon. Now known as Perth Canyon, this feature still exists as a submarine canyon near the edge of the continental shelf.

The Swan River drains the Swan Coastal Plain, a total catchment area of over 100,000 square kilometres (39,000 sq mi) in area. The river is located in a Mediterranean climate, with hot dry summers and cool wet winters, although this balance appears to be changing due to climate change. The Swan is located on the edge of the Darling Scarp, flowing downhill across the coastal plain to its mouth at Fremantle.

The Swan begins as the Avon River, rising near Yealering in the Darling Range, approximately 175 kilometres (109 mi) from its mouth at Fremantle. The Avon flows north, passing through the towns of Brookton, Beverley, York, Northam and Toodyay. It is joined by tributaries including the Dale River, the Mortlock River and the Brockman River. The Avon becomes the Swan as Wooroloo Brook enters the river near Walyunga National Park.

More tributaries including Ellen Brook, Jane Brook, Henley Brook, Wandoo Creek, Bennett Brook, Blackadder Creek, Limestone Creek, Susannah Brook, and the Helena River enter the river between Wooroloo Brook and Guildford; however, most of these have either dried up or become seasonally flowing due to human impacts such as land clearing and development.

Between Perth and Guildford the river goes through several loops. Originally, areas including the Maylands Peninsula, Ascot and Burswood, through Claise Brook and north of the city to Herdsman Lake were swampy wetlands. Most of the wetlands have since been reclaimed for land development. Heirisson Island, upon which The Causeway passes over, was once a collection of small islets known as the Heirisson Islands.

Perth Water, between the city and South Perth, is separated from the main estuary by the Narrows, over which the Narrows Bridge was built in 1959. The river then opens up into the large expanse of the river known as Melville Water. The Canning River enters the river at Canning Bridge in Applecross from its source 50 kilometres (31 mi) south-east of Armadale. The river is at its widest here, measuring more than 4 kilometres (2.5 mi) from north to south. Point Walter has a protruding spit that extends up to 800 metres (2,600 ft) into the river, forcing river traffic to detour around it.

The river narrows between Chidley Point and Blackwall Reach, curving around Point Roe and Preston Point before narrowing into the harbour. Stirling Bridge and the Fremantle Traffic Bridge cross the river north of the rivermouth. The Swan River empties into the Indian Ocean at Fremantle Harbour.

Plant and animal life found in or near the Swan-Canning Estuary include:

• Over 130 species of fish including bull sharks (Carcharhinus leucas), rays, cobblers (Cnidoglanis macrocephalus, also known as Swan River catfish), herring (Elops machnata), pilchard (Sardinops neopilchardus), bream (Kyphosus sydneyanus), flatheads, leatherjackets and blowfish (Tetraodontidae)
• Jellyfish including Phyllorhiza punctata and Aurelia aurita
• Bottlenose dolphins
• Crustaceans including prawns and blue manna crabs
• Amphipod Melita zeylanica kauerti described based on specimen that was collected from under Middle Swan Bridge
• Molluscs including Mytilidae, Galeommatidae
• Birds including the eponymous black swan, silver gull, cormorants (locally referred to as "shags"), twenty-eight parrots, rainbow lorikeet, kingfisher, red-tailed black cockatoo, Australian pelican, Australian magpie, heron and ducks

The river was named Swarte Swaene-Revier by Dutch explorer, Willem de Vlamingh in 1697, after the famous black swans of the area. Vlamingh sailed with a small party up the river to around Heirisson Island.

A French expedition under Nicholas Baudin also sailed up the river in 1801.

Governor Stirling's intention was that the name "Swan River" refer only to the watercourse upstream of the Heirisson Islands. All of the rest, including Perth Water, he considered estuarine and which he referred to as "Melville Water". The Government notice dated 27 July 1829 stated "... the first stone will be laid of a new town to be called 'Perth', near the entrance to the estuary of the Swan River."

Almost immediately after the Town of Perth was established, a systematic effort was underway to reshape the river. This was done for many reasons:

• to alleviate flooding in winter periods;
• improve access for boats by having deeper channels and jetties;
• removal of marshy land which created a mosquito menace;
• enlargement of dry land for agriculture and building.

Perth streets were often sandy bogs which caused Governor James Stirling in 1837 to report to the Secretary of State for Colonies:

At the present time it can scarcely be said that any roads exist, although certain lines of communication have been improved by clearing them of timber and by bridging streams and by establishing ferries in the broader parts of the Swan River ...

Parts of the river required dredging with the material dumped onto the mud flats to raise the adjoining land. An exceptionally wet winter in 1862 saw major flooding throughout the area – the effect of which was exacerbated by the extent of the reclaimed lands. The first bucket dredge in Western Australia was the Black Swan, used between 1872 and 1911 for dredging channels in the river, as well as reclamation.

A number of features of the river, particularly around the city, have reshaped its profile since European settlement in 1829:

• Claise Brook – named Clause's Brook on early maps, after Frederick Clause. This was a fresh water creek which emptied the network of natural lakes north of the city. Before an effective sewerage system was built, it became an open sewer which dumped waste directly into the river for many years during the 1800s and early 1900s. The area surrounding has been mainly industrial for most of the period of European settlement and it has a long history of neglect. Since the late 1980s, the East Perth redevelopment has dramatically tidied up the area and works include a landscaped inlet off the river large enough for boats. The area is now largely residential and the brook exists in name only with the lakes having been either removed or managed by artificial drainage systems.
• Point Fraser – early maps showed this as a major promontory on the northern side of the river west of the Causeway. It disappeared between 1921 and 1935 when land fill was added on both sides, straightening the irregular foreshore and forming the rectangular 'The Esplanade'.
• The Esplanade – the northern riverbank originally ran close to the base of the escarpment generally a single block width south of St Georges Terrace. Houses built on the southern side of St Georges Terrace included market gardens which ran to the waters edge.
• Heirisson Islands – a series of mudflats that were slightly more upstream from today's single artificial island which has deep channels on each side.
• Burswood – early in the settlement the Perth flats restricted the passage of all but flat bottom boats travelling between Perth and Guildford. It was decided that a canal be built to bypass these creating Burswood Island. In 1831 it took seven men 107 days to do the work. Once completed, it measured about 280 metres (920 ft) in length by an average top width of nearly 9 metres (30 ft) which tapered to 4 metres (13 ft) at the bottom; the depth varied between nearly 1 and 6 metres (3 ft 3 in and 19 ft 8 in). Further improvements were made in 1834. The area on the south side of the river upstream from the causeway was filled throughout the 1900s, reclaiming an area five-times the area of the Mitchell Interchange/Narrows Bridge works.
• Point Belches – later known as Mill Point, South Perth. Originally existed as a sandy promontory surrounding a deep semi-circular bay. This was later named Millers Pool and was eventually filled in and widened to become the present-day South Perth peninsula to which the Narrows Bridge and Kwinana Freeway adjoin.
• Point Lewis (also known as 'One-Tree Point' after a solitary tree that stood on the site for many years) – the northern side of the Narrows Bridge site, and now beneath the interchange.
• Mounts Bay – a modest reclamation was done between 1921 and 1935. In the 1950s works involving the Narrows Bridge started and in 1957 the bay was dramatically reduced in size with works related to the Mitchell Interchange and the northern approaches to the Narrows. An elderly Bessie Rischbieth famously protested against the project by standing in the shallows in front of the bulldozers for a whole day in 1957. She succeeded in halting progress – for that one day.
• Bazaar Terrace/Bazaar Street – in the early days of the settlement this waterfront road between William Street and Mill Street was an important commercial focus with port facilities including several jetties adjoining. It is now approximately where Mounts Bay Road is today and set well back from the foreshore. It had a prominent limestone wall and promenade built using material quarried from Mount Eliza.
• River mouth at Fremantle – the harbour was built in the 1890s and the limestone reef blocking the river was removed at the same time, after 70 years of demands. The dredging of the area to build the Harbour effectively changed the river dynamics from a winter flushing flow to a tidal flushing estuary. It was also at this time that the Helena River was dammed as part of C. Y. O'Connor's ambitious and successful plan to provide water to the Kalgoorlie Goldfields.

The river has been used for the disposal all kinds of waste. Even well into the 1970s, various local councils had rubbish tips on the mud flats along the edge of the river. Heavy industry also contributed its share of waste into the river from wool scouring plants in Fremantle to fertiliser and foundries sited in the Bayswater – Bassendean area. Remedial sites works are still ongoing in these areas to remove the toxins left to leach into the river.

During the summer months there are problems with algal blooms killing fish and caused by nutrient run-off from farming activities as well as the use of fertilisers in the catchment areas. The occasional accidental spillage of sewage and chemicals has also caused sections of the river to be closed to human access. The river has survived all this and is in relatively good condition considering on-going threats to its ecology.

In 2010 the Western Australian government imposed restrictions on phosphorus levels in fertilisers due to concerns about the health of the Swan and Canning river system.

Data collection of flood events in the estuary has been performed since European arrival in 1829. In July 1830, barely a year after the establishment of the colony, the river rose 6 metres (20 ft) above its normal level. New settlers were still arriving in steady numbers and few permanent buildings had been constructed, with most living in tents and other temporary accommodation. These included caves along the river's edge and many found their belongings washed away and livestock drowned. Other abnormal flooding events occurred in the winters of 1847 and 1860, while the most recent flooding occurred in 2017. Later events have since been assessed for probability of recurrence:

Year	1862	1872	1910	1917	1926	1930	1945	1946	1955	1958	1963	1964	1983	2017
ARI	60	100	20	20	30	15	20	10	20	20	15	10	10	TBC