River Mersey Flooding & Flood Risk

River Mersey flooding is a different problem on either side of the Manchester Ship Canal. Above the canal the Mersey is a Pennine river held between embankments through Stockport, Didsbury, Northenden and Sale, and on 1 January 2025 it reached the highest levels ever recorded at Stockport, Didsbury and Northenden. Below Howley Weir in Warrington it is tidal. The dates that matter there are two high tides, in February 1990 and December 2013. Our consultants assess flood risk and drainage for sites on both.

On New Year's Day 2025 the Mersey set records at four gauges between Stockport and Urmston, 346 cubic metres a second went past Brinksway, and the embankments at Didsbury and Northenden gave way in four places.

To talk a site through, ring 01293 214444 or email enquiries@unda.co.uk.

• Source to sea. The Mersey begins where the Tame and the Goyt meet in the middle of Stockport and ends in Liverpool Bay, a course usually put at about 70 miles (110 km). Older maps start it higher up, where the Etherow joins the Goyt near Marple.
• A river that borrows a canal. Below Flixton it runs into the Manchester Ship Canal near Irlam and shares that channel for about 6 km before leaving it again at Rixton, above Warrington.
• Where the tide stops. Howley Weir in Warrington is the tidal limit, and everything below it, through the Runcorn Gap and out between Liverpool and the Wirral, is the Mersey Estuary.
• Regulator. A Main River, regulated by the Environment Agency, from Stockport to the Ship Canal. Below the canal the Agency's map does not show it as one, which changes the consents a site needs.
• Warnings. The Agency runs 13 flood warning areas on the river between Stockport and Flixton and 15 tidal ones from Woolston to the Wirral, under five wider alert areas.

Why does the River Mersey flood? Three reaches, three causes

Because three different things happen along it. From Stockport to Flixton, rain falling on the Pennines arrives within hours and is kept off south Manchester by 22 km of earth embankment and two flood storage basins. Through the Ship Canal, sluice gates set the level. Below Howley Weir the tide decides. The highest levels on record there came with storm surges, not rain.

• Steep, wet headwaters. The Goyt, the Tame and the Etherow drain gritstone moor that climbs to 636 m and takes about 1,130 mm of rain a year. The National River Flow Archive calls the river responsive even though reservoirs, the Longdendale chain among them, control nearly a quarter of the catchment above Ashton Weir.
• A built-up lower catchment. More than a quarter of that catchment is urban. When the river runs high, the outfalls draining Stockport and south Manchester cannot empty into it. Stockport Council's report on the 2025 flood found that sewers flooded mainly because high river levels surcharged their outfalls, not for want of maintenance.
• Embankments in place of floodplain. Between Heaton Mersey and Ashton-on-Mersey the river runs between raised banks. In a flood the Agency opens gates into basins at Didsbury and Sale Water Park to take the top off the peak, which is why the flow archive describes flood flows at Ashton Weir as truncated.
• The Ship Canal. Since 1894 the Mersey has joined the canalised Irwell just below Irlam Locks; our River Irwell page covers that river from Bacup to Salford. Through Carrington, Cadishead and Partington the water level belongs to the canal and its sluices.
• Tide. At Liverpool the tide ranges from about 4 m on neaps to 10 m on springs, among the largest in Britain, and a surge on top of a spring tide pushes up the estuary to Warrington. Our guide to tidal flooding explains how surge, tide and river combine.

The record is moving. At Ashton Weir, gauged since 1958, the three largest peak flows have all come since January 2021: 248 cubic metres a second in Storm Christoph, 242 in February 2022 and 284 on New Year's Day 2025. A site assessment that still treats 2002 or 2011 as the benchmark flood is three events behind.

River Mersey map and gauge records, Stockport to Fiddlers Ferry

The map draws the Mersey in coral from the confluence in Stockport to Liverpool Bay and pins the Environment Agency's live gauges along it; type in a postcode and it screens that spot against the Agency's flood data. Our live river levels map shows the same gauges next to those on the Goyt, the Tame, the Irwell and the Bollin.

Each gauge measures from its own datum, so 7.69 m at Didsbury and 3.76 m at Northenden, about a mile downstream, are one flood seen from two places. Compare the dates instead. Four of the six gauges above the Ship Canal set their record on the same day in 2025, while the two at Warrington still hold records from the tides of 1990 and 2013.

Peak flows put the floods in the same order, with a longer record behind them. The flow archive's series at Brinksway starts in 1956, and the 346 cubic metres a second measured there on 1 January 2025 beat the previous highest, 299 in July 1973, by about a sixth. The Environment Agency described the river that morning as at its highest in 66 years.

River Mersey flooding since 1990: the dates behind the records

Every record in the table belongs to one of four dates, and two more floods since 2021 came close to joining them. Two of the six are tides at Warrington. Four are winter storms over the Pennines. The last three fell inside four years.

1. 27 February 1990. On a very high tide the Mersey came over its banks in Warrington and flooded Knutsford Road, Latchford, Howley, Arpley Bridge and Lower Walton. The Environment Agency's account on The Flood Hub has the water stopping inches short of 1,500 terraced houses. Westy's 5.54 m still stands.
2. February 2002. Sale Ees Gates reached 5.43 m on 11 February and Flixton Bridge 4.92 m on the 20th. Both remain the highest on the Agency's pages for those gauges.
3. 5 December 2013. The storm surge that hit the North Sea coast that day ran up the Irish Sea as well. Fiddlers Ferry recorded 7.37 m, its highest, and Westy came within 3 cm of 1990. The first phase of Warrington's new defences had just been finished, and the Agency credits it with keeping some 200 properties dry.
4. 20 and 21 January 2021. Storm Christoph. Greater Manchester declared a major incident and about 2,000 properties along the river in south Manchester were evacuated in the middle of a national lockdown. The Agency used the Didsbury basin, and its own account of the night put the margin between safety and a threat to life at "a matter of centimetres".
5. 20 and 21 February 2022. Storms Eunice and Franklin. For a second winter running the river reached record levels, severe flood warnings went out for parts of Northenden and West and East Didsbury, both basins were used, and the Agency spent about £2.5 million on repairs afterwards.
6. 31 December 2024 and 1 January 2025. About 70 mm of rain fell in 18 hours on ground that was already wet. A major incident was declared at 4 am. Records fell at Brinksway, Didsbury, Northenden and Ashton Weir. The river overtopped its embankments and broke through them at Didsbury Sports Ground, twice at Northenden Golf Club and behind the Britannia Hotel. In Stockport it flooded ten or more units at Heapriding Business Park on Ford Street; across Manchester, the council counted 60 homes and 29 businesses flooded inside from all sources.

The Agency plugged the breaches by helicopter in January 2025 and finished permanent repairs that September, restoring protection to 316 homes and businesses. The banks are back to their original standard. That standard has been exceeded once already. The councils' accounts of the night are Section 19 reports, and our explainer on Section 19 flood investigations sets out what they can and cannot tell a buyer or a developer.

Places at risk along the River Mersey, from Stockport to the Wirral

The Mersey threatens three kinds of place: riverside industrial land in Stockport, the suburbs and sports grounds behind the embankments from Didsbury to Flixton, and the low streets, docks and works that face the tide from Warrington to Birkenhead. The table runs downstream and gives the Environment Agency warning areas for each.

Above these sit the alert areas: the Mersey Uplands for the Goyt, Tame and Etherow, the Middle River Mersey from Stockport to Urmston, and two tidal alerts, for Warrington and for Liverpool and the Wirral up to Widnes. The boroughs on the estuary are covered on our Merseyside and Cheshire pages, and any address can be checked against the flood zones on our flood risk map.

Where does the River Mersey become tidal?

At Howley Weir, just east of Warrington town centre. Below the weir the Environment Agency treats the river as estuary: every warning area from Howley to Fiddlers Ferry is named "Mersey Estuary at" somewhere, and the gauge at Fiddlers Ferry moves through more than three metres on an ordinary tide. The highest spring tides come over the weir itself. That is how the gauge at Westy, upstream of it, came to set its record on a tide.

• What floods Warrington. The tide, above all a spring tide with a surge behind it. The town's two record levels, at Westy in 1990 and at Fiddlers Ferry in 2013, both came that way.
• What the canal does. How the Ship Canal's sluice gates are worked changes the river flood picture in the town. Warrington's 2011 strategic flood risk assessment points out that the national flood map assumes the gates stay shut, which draws a wider flood zone through Warrington than the council's own hazard mapping, run with the canal operating.
• What was built. After 1990 came £34 million of defences, paid for by government, the council and Scottish Power and finished in 2018: 5 km of flood wall and 2.2 km of embankment. The North West flood risk management plan counts 1,995 homes and 161 businesses at lower risk because of them.

Downstream of Warrington the question changes from today's tide to the tide a century on. A scheme on the Liverpool waterfront or the Wirral docks is tested against sea level rise over its whole life. How each council's own modelling treats that is the subject of our Liverpool and Wirral pages.

Is the River Mersey a Main River? Only as far as the Ship Canal

From the confluence in Stockport down to the Manchester Ship Canal below Flixton, yes: the Environment Agency's Statutory Main River Map draws that whole length as Main River. When we checked the map on 10 November 2026 it did not draw the Mersey below that point, through Woolston, Warrington or the estuary, as Main River. The consents a site needs change with the reach.

• Stockport to Flixton. Building, digging or placing anything within 8 metres of the bank or of a flood embankment may need a flood risk activity permit from the Environment Agency, on top of planning permission. With 22 km of embankment lining this reach, the margin covers a lot of ground that is nowhere near the water's edge.
• The Ship Canal. A working harbour owned by the Manchester Ship Canal Company, part of Peel Ports. Works that touch it, a new outfall included, need the company's agreement.
• Warrington and the estuary. The permit rules set 16 metres from a tidal Main River and 16 metres from a sea defence. Anyone planning work near Warrington's flood walls or an estuary embankment should ask the Agency which applies, and work in tidal water may also need a marine licence.
• Brooks and drains. Micker Brook, Chorlton Brook and several other tributaries are Main Rivers too, with their own flood warnings. Smaller streams and ditches are ordinary watercourses, where the council consents works and the riparian owner maintains the channel.
• Planning. Under chapter 18 of the National Planning Policy Framework, development in Flood Zone 2 or 3, and any site of a hectare or more in Flood Zone 1, needs a site-specific flood risk assessment. On this river that takes in most of the land behind the embankments.

Flood basins, embankments and walls: what the Mersey's defences change

They lower the chance of flooding behind them. They leave the flood zone exactly where it was. The Flood Map for Planning is drawn as if defences were absent, so a house behind the Didsbury embankment or a unit behind Warrington's walls usually still sits in Flood Zone 3. January 2025 showed the other half of the problem. An embankment can be overtopped, and then it can fail.

• Didsbury basin. Filled through gates from the river when levels threaten East and West Didsbury. The Agency says it protects around 3,000 properties and holds the equivalent of 310 Olympic swimming pools. On 1 January 2025 it was opened at 9.30 am.
• Sale Ees. The second basin, on the Trafford bank where Sale Water Park lies. Both basins, like most of the embankments, date from the 1960s and 1970s.
• The embankments. Raised earth banks through Stockport, Manchester and Trafford, with weirs at Brinksway, Heaton Mersey, Northenden, Sale Ees and Ashton. The Agency says many of these assets are near the end of their design life, and its review of how the reach should be managed for the next hundred years is expected to finish in autumn 2027 if funding allows.
• Warrington's walls. Built between 2013 and 2018 to keep out a flood with a 1 in 100 chance in any year. They shield the Howley Lane substation as well as homes.

For a planning application this turns into two questions the map alone cannot answer: how deep and how fast water would arrive if the nearest bank were breached or overtopped, and whether people could get out. Our articles on residual flood risk and on why land stays in a flood zone after defences are built cover both.

Flood and drainage reports for land beside the Mersey and its estuary

A scheme near the Mersey can need up to three separate reports. They are not versions of one another. One tests whether the development is safe from the river or the tide and leaves nobody else worse off, another asks whether a site at lower risk was reasonably available, and the third settles where the rain that lands on the finished site will go. Unda Consulting Limited is a CIWEM Business Partner, and our chartered consultants prepare all three for sites from Portwood to the Wirral docks.

• Sites in the Mersey's flood zones. A flood risk assessment that starts from the nearest gauge record, the reach the site is on and the defences in front of it.
• Planning applications. A flood risk assessment for planning that answers the Environment Agency and the council on floor levels, safe access and the effect of the scheme on its neighbours.
• Buying near the river. A flood risk survey for property purchase for a house in Didsbury, Northenden or Latchford, reporting on that one building before exchange.
• New homes in Flood Zone 3. A sequential and exception test showing why the site was chosen and, where it applies, how the scheme stays safe for its lifetime.
• Surface water. A drainage strategy for schemes whose runoff ends up in the Mersey, a brook or a combined sewer.
• Outfalls the river can lock. A surface water drainage strategy that sets discharge rates and storage for an outfall which, as in January 2025, may be under water when it is needed most.
• After permission. Discharge of drainage conditions once approval arrives with a drainage condition to clear before work starts on site.
• Behind an embankment or a tidal wall. Flood modelling where the national mapping gives no level for the site, or where a breach of the Mersey embankments or Warrington's walls has to be tested.
• Getting out before the gates open. A flood warning and evacuation plan for sites in Didsbury and Northenden, where streets were evacuated in 2021, 2022 and 2025, and for the tidal warning areas in Warrington.
• Homes that flooded in 2025. A flood risk assessment for insurance setting out what the river, the repaired embankments and the basin mean for one address in south Manchester today.
• Units on the valley floor. A business continuity flood risk assessment for premises such as those at Ford Street in Stockport, flooded in 2025, or at Bank Quay beside the tidal river.
• Dock and estuary land. Flood risk due diligence for buyers and lenders on industrial frontage at Widnes, Runcorn, Ellesmere Port or Birkenhead, where the tide over the life of the asset is what needs pricing.

Much of the lower catchment lies on boulder clay over sandstone, so whether a soakaway will work changes from one plot to the next. Our ground conditions and soakaway checker gives a first view before anyone pays for infiltration testing.

Who deals with flooding on the Mersey

Eleven councils border the Mersey between Stockport and the sea: Stockport, Manchester, Trafford and Salford share the river above and along the Ship Canal; Warrington, Halton and Cheshire West and Chester hold the tidal river and the south shore; Liverpool, Knowsley, Sefton and Wirral face the outer estuary. Each is the lead local flood authority for its own stretch, so the drainage rules a scheme meets in Stockport are not the ones it meets in Sale or Warrington.

• Environment Agency. Its Greater Manchester, Merseyside and Cheshire area looks after the Main River reach, the basins, the embankments, Warrington's walls and every flood warning on this page.
• Catchment plan. The Mersey sits in the North West river basin district, whose flood risk management plan for 2021 to 2027 lists Warrington as a flood risk area for rivers and the sea. The North West Regional Flood and Coastal Committee approves the region's schemes.
• Council policy. Each council sets its own drainage and flood rules; our page on Manchester City Council's requirements shows how far they can go beyond national policy.
• Warnings. The Agency's free flood warnings cover every area named on this page, and Floodline answers on 0345 988 1188.
• Emergency planning. The Greater Manchester Resilience Forum co-ordinates a major flood from Stockport to Irlam, the Cheshire Resilience Forum covers Warrington and Halton, and the Merseyside Resilience Forum the outer estuary.
• Sewers. United Utilities deals with sewer flooding the length of the river.
• Emergencies. Ring 999 if anyone is in danger, and 105 for a power cut.

The Mersey is one of the rivers on our river flood pages, which cover watercourses across England and the Welsh border in the same way.

Questions about River Mersey flooding

For a site anywhere on the Mersey, from Portwood to New Brighton, ring 01293 214444 or email enquiries@unda.co.uk. Give us the address and what is planned. Our chartered consultants will come back with the reports the site is likely to need and a fixed fee for preparing them.