As it nears the coast it passes through the Lewes and Laughton Levels, an area of flat, low-lying land that borders the river and another tributary, the Glynde Reach.

[4] The river rises near Lower Beeding in West Sussex and meanders eastwards, passing under Upper Ryelands Bridge, once the limit of navigation.

[5] Most of the tributaries are in the upper catchment that have joined it originate in the heaths and forests of the High Weald, where fast-flowing small streams cut deep valleys through woods, and flow over underlying beds of sandstones and clays.

Previously, access to the church had only been available on Sundays, when owners of Malling Deanery allowed people to use their private suspension bridge, constructed in 1934, and located a little further upstream.

[5] At Southease there is a swing bridge, designed by Henry E Wallis of Westminster in 1878 and installed two years later, to replace a previous structure slightly further upstream.

[23] In later centuries the river was draining the valley sufficiently well for some of the marshland to be reclaimed, by building embankments to create highly prized meadow land.

In 1422 a Commission of Sewers was appointed to restore the banks and drainage between the coast and Fletching, around 20 miles (32 km) inland, which may indicate that the Ouse was affected by the same storm that devastated the Netherlands in the St Elizabeth's flood of 1421.

[25] Part of the problem of flooding was caused by longshore drift creating a huge shingle bar across the river mouth, which had gradually moved eastwards to Seaford, some 2 miles (3.2 km) from its position in Roman times.

[29] At some time between 1676 and the publication of an Admiralty chart in 1698, the river flowed along the back of the shingle bar and broke through into the sea about 0.5 miles (0.8 km) further to the east, at the site of a tide mill.

In particular, the brooks of Southover, Iford, Pool Bar, and Rodmell, which he called the West Levels, were badly affected as their embankments were low and poorly maintained.

[33] He noted that the rise and fall of the tide below Piddinghoe was some 8 feet (2.4 m), but this was reduced to just 6 inches (15 cm) at the mouth of the Glynde, and was barely visible at Lewes Bridge.

He also commented on the great shingle bar crossing the mouth of the river at Newhaven, which if removed would allow the water levels to be around 6.5 feet (2.0 m) lower at low tide.

[34] The brooks above Lewes Bridge became gradually drier as he progressed towards Barcombe Mill, but with almost no fall on the river, its winding course and numerous shoals hampered the drainage of the meadows.

A second option involved raising the banks on all of the meadows, and constructing a separate sewer to carry surplus water from them to the sea, leaving the main river largely unaltered.

The third opening would be 14 feet (4.3 m) wide, with double pointed doors facing in both directions, so that it could additionally be used as a navigation lock at all states of the tide and river.

The work would involve easing sharp curves, widening the channel in places, making cuts to straighten some sections, and constructing 25 locks.

The navigation reached its greatest extent of 22.5 miles (36.2 km) in 1812, when Wells constructed a further extension to Upper Ryelands Bridge at Balcombe, with similar payment terms to his previous contract.

[48] The work on straightening and enlarging the lower river was carried out between 1791 and 1795, and Jessop's plans were overseen by a schoolmaster and civil engineer from Lewes called Cater Rand.

[49] From 1783, John Ellman, better known for his agricultural achievements, became the Expenditor for the Lewes and Laughton Levels, and as well as carrying out the tradition role of collecting the water scot tax and spending it, he worked tirelessly to organise and supervise work on the Glynde Reach and the lower Ouse, which enabled a 120-ton ship named Kitty to unload stone at Lewes Bridge in the late 1820s.

The death-knell occurred in 1858, when the Lewes to Uckfield branch line opened, running parallel to the river for much of its route, and in 1859, the company stopped keeping records.

The Sussex Ouse Conservation Society promotes awareness of the navigation, and publishes details of circular walks which include river sections.

This was replaced by a three-storey brick and timber building at some point, which was later supplemented by a four-storey square extension with a flat roof, from which the owners, the Maryon-Wilson family, could watch cricket matches at Sheffield Park.

Molineux and Johnston bought the land and built a large mill, for which the opening ceremony in July 1809 was a grand affair, according to the Sussex Weekly Advertiser.

It had been one of the biggest watermills in Sussex, but little remains apart from the four channels,[73] while a grass mound where the main building was conceals an atmospheric syphon installed in the 1960s, which is used to force water downstream when the river is in spate.

The Duke of Newcastle, Thomas Pelham, obtained an Act of Parliament to authorise construction of the mill on the foreshore of land that he owned at Bishopstone.

After 1853, the volume of flour produced gradually declined, often as a result of weather conditions, but it continued in operation until March 1883, when there was another violent storm and the new harbour at Newhaven required more water.

[77] The Sheffield and Batts Bridge tributaries have small populations of pumpkinseed, which have escaped from on-line fish ponds,[78] and since 2009 a few catfish have appeared in the river at and upstream of Lewes.

The tidal stretches contain fish that can tolerate the lower levels of salt found in brackish water, including flounder, grey mullet, bass, twait shad, and sea lampreys.

Reasons for the quality being less than good include discharges from sewage treatment works, physical modification to the channel, which prevents the free movement of fish, and runoff from agricultural land.

Like most rivers in the UK, the chemical status changed from good to fail in 2019, due to the presence of polybrominated diphenyl ethers (PBDE), perfluorooctane sulphonate (PFOS) and mercury compounds, none of which had previously been included in the assessment.