The Ouse ( ) is a long river in the English counties of West and East Sussex. It rises near Lower Beeding in West Sussex, and flows eastwards and then southwards to reach the sea at Newhaven. It skirts Haywards Heath and passes through Lewes. It forms the main spine of an extensive network of smaller streams, of which the River Uck is the main tributary. 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. It was a large tidal inlet at the time of the Domesday Book in 1086, but over the following centuries, some attempts were made to reclaim some of the valley floor for agriculture, by building embankments, but the drainage was hampered by the buildup of a large shingle bar which formed across the mouth of the river by longshore drift.
In 1539, a new channel for the entrance to the river was cut through the shingle bar, and meadows flourished for a time, but flooding returned and meadows reverted to marshland. The engineer John Smeaton proposed a solution for the drainage of the valley in 1767, but it was only partly implemented. William Jessop surveyed the river in 1788, and produced proposals to canalise the upper river above Lewes, and to radically improve the lower river. The Proprietors of the River Ouse Navigation were created by act of Parliament, the River Ouse Navigation Act 1790 (30 Geo. 3. c. 52), and eventually built 19 locks, to enable boats to reach Upper Ryelands Bridge at Balcombe. Trustees and the Commissioners of the Lewes and Laughton Levels jointly managed the work on the lower river, and the agriculturalist John Ellman continued the progress while he was Expenditor for the Commissioners, which enabled 120-ton ships to reach Lewes by 1829. Navigation on the upper river could not compete with the railways, and all traffic had ceased by 1868.
On the lower river, Newhaven became an important port and barge traffic continued using the river up to Lewes until the 1950s. Cross-Channel ferries still sail from the port. The river provides habitat for many varieties of fish, including unusually large sea trout that swim up the river to spawn in the higher tributaries. The Lewes Brooks area of the levels is a Site of Special Scientific Interest because of its wide variety of invertebrates. Walkers can follow the course of the river by using the Sussex Ouse Valley Way long-distance footpath, and the Sussex Ouse Conservation Society promotes awareness of the navigation by publishing details of shorter walks. The Sussex Ouse Restoration Trust is hoping to see navigation restored to the upper river, but this is not universally popular, as the Ouse and Adur Rivers Trust is opposed to the idea.
Ecology and wildlife The River Ouse catchment plays a critical role in supporting regional biodiversity, notably providing habitat for the Sussex sea trout (Salmo trutta). As an apex predator and key indicator species, sea trout spend up to two years in nursery streams before migrating into the English Channel and returning to their natal spawning grounds. The chalk-fed North End Stream, alongside the Bevern Stream, forms a vital part of this ecological system. The Wild Trout Trust identifies the North End Stream as a nationally and locally significant spawning ground and nursery for the species. The sea trout holds protected status as a Species of Principal Importance under Section 41 of the Natural Environment and Rural Communities Act 2006, is listed under the Environment Targets (Biodiversity) Regulations 2023, and is prioritized in the draft East Sussex Local Nature Recovery Strategy.
Despite ongoing restoration work across the catchment, local populations face severe pressure from habitat degradation, climate change, and pollution. Emerging threats include proposed housing developments adjacent to the North End Stream, which risk damaging spawning beds through siltation, toxic runoff, and increased sewage discharge. To mitigate these risks, local community groups and conservation organisations, including the Sussex Wildlife Trust have partnered to improve habitat connectivity. This collaborative effort led to the formation of the Charter of Rights for the River Ouse, a community-led initiative officially supported by Lewes District Council, Lewes Town Council, and Hamsey Parish Council to establish legal and practical protections for the river and its tributaries.
There are also populations of pike and more recently carp, both of which can exceed in weight. Other species include barbel, roach, dace, rudd, perch, chub, bream and tench. The Sheffield and Batts Bridge tributaries have small populations of pumpkinseed, which have escaped from on-line fish ponds, and since 2009 a few Wels catfish have appeared in the river at and upstream of Lewes. Rarer breeds which inhabit the river include grayling, stone loach, river lampreys, and bullhead, which are a species under threat on an international scale. 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.
Lewes Brooks is a Site of Special Scientific Interest on the levels to the west of the river below Lewes. It covers an area of , and is noted for its wide diversity of water beetles, rare snails, flies and moths. The habitat is enhanced by a gradation in the water, which varies from fresh in the west to brackish in the east. Several studies into the presence and effects of organic and inorganic pollutants in the Sussex Ouse have been carried out, including one on the effects of estrogens entering the river from Sewage Treatment Works outfalls on the reproductive physiology of fish.
Water quality The Environment Agency measure the water quality of the river systems in England. Each is given an overall ecological status, which may be one of five levels: high, good, moderate, poor and bad. There are several components that are used to determine this, including biological status, which looks at the quantity and varieties of invertebrates, angiosperms and fish. Chemical status, which compares the concentrations of various chemicals against known safe concentrations, is rated good or fail.
Apart from the initial section from its source to Ardingly, the channel is designated as heavily modified, because of the canalisation works. The water quality of the River Ouse system was as follows in 2019. Section Ecological Status Chemical Status Length Catchment Channel heavily modified heavily modified heavily modified heavily modified 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.
Course The River Ouse, together with its main tributary the River Uck, form the central spines of a network of streams and rivers which drain over of Sussex. There are around of streams in total, of which some are designated as main rivers, on which the Environment Agency has a responsibility for maintenance.
The river rises near Lower Beeding in West Sussex and meanders eastwards, passing under Upper Ryelands Bridge, once the limit of navigation. After it has been joined by the outflow from Ardingly Reservoir, created by building a dam across the valley of a tributary, which has resulted in the partial flooding of two river valleys, it turns to the south east, passing to the north of Lindfield and Haywards Heath. It passes into East Sussex just before reaching Sheffield Park railway station on the preserved Bluebell Railway. After skirting around Newick, it turns to the south and is joined by its main tributary, the River Uck, flowing in from the north east, before reaching Isfield.Ordnance Survey, 1:25,000 map and 1:50,000 map. 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.
Below Isfield, the narrow valley widens out into a broad flood plain, and the river is joined by the Longford Stream, the Iron River and the Bevern Stream, before it reaches Barcombe Mills. There is a network of channels at Barcombe, which once supplied water for at least two watermills, but the last one was destroyed by fire in 1938. On the east bank of the river is Barcombe Reservoir and Water Treatment Works. Both are owned by South East Water, who extract water from the river and depending on the quality of the water, store it in the reservoir or treat it immediately and pump it into the public water supply. The works was constructed in two stages, in 1962 and 1977 and can deliver up to 75 Megalitres per day (Mld) for public consumption. Just north of this on the west bank of the river is the Anchor Inn, dating from 1790, which has boats for hire, restaurants and bars, and is licensed for civil weddings.
Some of the tributaries on this middle section are similar to those on the upper section, but others are lowland streams, where the underlying geology is alluvium and clays, and which flow more slowly. The Bevern Stream and Northend Stream both originate in the chalk uplands of the South Downs, but traverse greensand and clay before they reach the Ouse. Below the weirs of Barcombe, the river is partially tidal, and forms large meanders, with numerous ox-bow lakes. At Hamsey, a long lock cut crosses the neck of a large meander creating Hamsey Island, home to St Peter's Church, which is situated on a mount. Much of it dates from the 12th century, with 14th and 15th century additions, and the structure is Grade I listed. Flow on the river above is modulated by a half-weir, which prevented a serious ecological disaster spreading further downstream, when a spillage of pesticide near Newick in 2001 killed the insect populations and more than 500 fish on a stretch of the river.