The Thames Estuary is a paradise for bird-watchers. Every winter, large flocks of brent geese arrive from northern Russia, to feed in tightly packed groups on eel-grass. They can be recognised by their black necks marked with white flashes, dark bodies, and deep nasal call - a rumbling "rronk-rronk", which echoes across the marshes.

Another winter visitor, from the Arctic, is the knot, with its grey back and white belly. Like starlings, knots gather in vast flocks, and form spectacular whirling shapes in the late afternoon skies.

On the mudflats, look out for oystercatchers, with their white bellies, black backs and long bright red chisel-shaped beaks, used to prise open cockles and mussels. Their cry is a high sharp "kleep!". Scurrying along by the water's edge, there are brownish-grey ringed plovers, hunting shrimp. Their call is a rising "too-li!"

Sailing upriver

Joseph Conrad, the seafarer turned novelist, wrote a vivid description of a journey up the Estuary in his autobiographical work, The Mirror Of The Sea (1909):

For a long time the feeling of the open water remains with the ship steering to the westward... There are no features to this land, no conspicuous, far-famed landmarks for the eye; there is nothing so far down to tell you of the greatest agglomeration of mankind on earth dwelling no more than five and twenty miles away...

The first place a ship passes is the Nore, a wide sandbank on the south side of the estuary. A historic Naval anchorage, this was the site, in 1797, of the worst mutiny in British history. The sailors, protesting at their terrible conditions, seized all 21 ships of the fleet, and refused to obey orders for a month. The mutineers were eventually starved into submission, and 29 ringleaders were hanged from their ships' yardarms.

Dangerous wreck

South of the Nore, you can see the masts of the USS Richard Montgomery, rising above the water. This was a munitions ship, packed with bombs, which sank here in 1944. It is carefully monitered, though it has been judged too dangerous to move. It has been calculated that, if the ship ever exploded, it would throw up a 1,000ft-wide column of water and wreckage 10,000 feet in the air, and generate a 16ft-high wave. Every window in the neighbouring town of Sheerness would be shattered.

Thames sailing barges

Beyond the Nore lies the River Medway, where, wrote Conrad, the "famous Thames barges sit in brown clusters upon the water with an effect of birds floating upon a pond". These were the great working boats of the estuary, built to carry bricks and cement from Kent up to London. Their flat bottoms allowed them to go far upstream, and sit safely on the mud when the tide went out. To make up for the lack of a keel, there were two side "larboards", lowered to stabilise them in windy weather. The tall masts could be quickly taken down to pass under bridges. In 1863, Henry Dodd began annual Sailing Barge Races, which still take place today, although the numbers competing have now fallen from more than 70 to just 16.

Find out about the Thames Sailing Barge Race here

On the north side of the Estuary, we pass Southend, with its 1.34-mile-long pier, still standing despite four fires. The longest pier in the world, it was built to allow people to catch pleasure boats at low tide.

Leigh-on-Sea

Just past Southend is Leigh-on-Sea, with its fishing quarter, where the white beach is made from the crushed shells of millions of cockles, caught by generations of fishermen. Every day, the cockle boats bring in their catch, which is boiled in the sheds on the beach. There are stalls here selling cockles, which are eaten soaked with vinegar and washed down with pints of bitter from the seafront pubs.

Hadleigh Castle

Beyond Leigh stands one of the few raised landmarks of the estuary, the ruined keep of Hadleigh Castle. Built in the 13th century from Kentish ragstone, the castle looks down from a hill onto the marshes. In 1814, John Constable came here to sketch the scene, yet it was not until his wife died, in 1828, that he made a large-scale painting of the castle. The flat, bleak landscape and tempestuous sky captured his own sense of desolation. See Constable's painting of Hadleigh Castle here

Click here to discover more about John Constable and another of his paintings, The Hay Wain

Canvey Island

From Hadleigh, we pass low-lying Canvey Island, enclosed by a 14-mile long sea wall. The first wall was built in the 1620s by the Dutch engineer, Cornelius Vermuyden, and you can still see 17th century Dutch cottages, with thatched roofs, on the island. In the late 19th century, a local businessman called Frederick Hester bought up much of Canvey. He divided it into plots to sell cheaply to East Enders, for holiday homes.

Canvey was the site of a disastrous flood, in 1953, when the sea wall was breached and 58 people drowned. The wall was strengthened and raised after the flood, and islanders began to build "upside down houses", with living rooms on the top floors giving them views over the marshes. Find out more about the flood, and other English weather disasters, here

In 1954, a strange creature was washed up, dead, on Canvey's shore. It looked like a fish, but appeared to have two hind legs. When a second similar creature was discovered in 1955, it was named the "Canvey Island Monster". Read about the Canvey Island Monster here

Looming over Canvey is the Shell Haven Oil Refinery, with its white cylindrical storage tanks, and flare towers, whose flames light up the night sky. Canvey Islander Wilko Johnson, the songwriter and guitarist of Dr Feelgood, immortalised this view in one of his first songs, All through the City, which begins, "Stand and watch the towers burning at the break of day."

The song appeared on Dr Feelgood's 1974 debut album, Down By The Jetty, whose cover had a black-and-white photo of the band hunched against the wind on the Canvey foreshore. The picture reflected their stripped-down blues music, deliberately recorded in mono. In his songs, Johnson mythologised his home as the "Oil City" and he would compare the Thames with the Mississippi deltas - both birthplaces of the Blues.

See Dr Feelgood posing Down by the Jetty

Beyond Canvey, the Estuary narrows, and in Conrad's words, "the salt, acrid flavour is gone out of the air, together with a sense of unlimited space opening free." We pass Tilbury and Gravesend, on our approach to London.