River Cam Flooding & Flood Risk

The River Cam is a placid, heavily managed lowland river, but its floodplain runs straight through the middle of Cambridge, and that is exactly why flooding from it matters. The open commons that make the city — Coe Fen, Sheep’s Green, Midsummer Common and Stourbridge Common — are also its working flood storage, holding water back from the streets behind them. When the Cam and its chalk-fed headwaters rise together after a wet spell, the low ground through Cambridge, Grantchester, Chesterton and Fen Ditton is the first to go under. Unda prepares flood risk assessments and drainage strategies for sites across the Cam floodplain. To talk a site through, call 01293 214444 or email enquiries@unda.co.uk.

The Cam’s floodplain is not a green margin at the edge of Cambridge. It is Coe Fen, Sheep’s Green and the commons in the heart of the city — land the planning system treats as functional floodplain that has to be left free to flood.

• A designated Main River. The Environment Agency regulates the River Cam and issues flood warnings for its entire length, so the 8 metre byelaw margin and Flood Risk Activity Permits apply to work near it.
• A large chalk catchment. Around 803 square kilometres drain to the Cam by Bottisham Lock, fed by the Rhee, the Granta and Bourn Brook.
• Slow to flood, slow to fall. The dominant risk is persistent out-of-bank flooding across the Cambridge commons, with a flashier chalk-stream response in the upstream Granta and Rhee arms and surface water in the city.
• Development pressure across the valley. From Grantchester and Newnham through the city to Chesterton, Fen Ditton and Waterbeach, sites in and near the floodplain almost always need a flood risk assessment for planning.

Check flood risk along the River Cam

The map below draws the River Cam in coral, from Hauxton Junction south of the city, through Cambridge, to its meeting with the Great Ouse at Pope’s Corner, with the Environment Agency’s live river gauges positioned along it — click a gauge to see its latest level and typical range. Enter a postcode near the river to zoom in and screen that location against the Environment Agency’s flood data for rivers and sea, surface water, reservoirs and the flood zones used for planning. It is an indicative screening tool, not a flood risk assessment.

Why does the River Cam flood?

The River Cam floods because a large, low-gradient chalk catchment funnels its water through a single lock-controlled channel and a floodplain that has been squeezed by centuries of building in Cambridge. Rain that falls across roughly 803 square kilometres of Hertfordshire, Essex and Cambridgeshire drains into the Rhee, the Granta and Bourn Brook, which meet just south of the city; when they arrive together, the managed channel through the Backs cannot pass the volume quickly, and it spreads sideways onto the commons.

The Cam is really three rivers. Its furthest source rises near Widdington in Essex, at about 112 metres above sea level, as the Granta, which is officially the upper course of the Cam. The Rhee begins at Ashwell Springs in Hertfordshire and gathers the River Mel at Meldreth on its way east. The two meet at Hauxton Junction, a mile south of Grantchester, and Bourn Brook joins at Byron’s Pool. Below that, the river runs through Grantchester Meadows, Newnham and the Backs into Cambridge, then north through Chesterton, Fen Ditton, Clayhithe and Upware to join the tidal Great Ouse at Pope’s Corner, south of Ely. On the lowest reaches a high Great Ouse can hold the Cam back and slow its fall after a flood. This is fluvial river flooding, made worse in the city by surface water.

• Furthest source. Near Widdington, Essex, at about 112 metres above sea level, as the Granta.
• Principal headwaters. The Rhee (from Ashwell Springs in Hertfordshire, taking the River Mel) and the Granta (from Essex); Bourn Brook joins at Byron’s Pool.
• Length and catchment. Around 69 kilometres from source to the Great Ouse, draining about 803 square kilometres by Bottisham Lock.
• A chalk river, heavily engineered. Much of the headwater flow comes from the Chalk aquifer, and Jesus Lock, Baits Bite Lock and Bottisham Lock control levels for navigation. The Conservators of the River Cam manage the river above Bottisham Lock; the Environment Agency has managed the lower river since 1995.

A history of flooding on the River Cam

The Cam is usually placid, and serious floods are infrequent rather than annual, but they are well documented. The benchmark remains the great Fen flood of March 1947, when snowmelt on frozen ground and heavy rain overwhelmed the Cam and the wider Ouse system. In living memory the worst events came in 2001, with further high water in 2014 and the wet winter of early 2024.

• March 1947. Snowmelt on frozen ground combined with heavy rain sent the Cam and the Great Ouse into the most severe flood of the twentieth century across the Fens.
• February and 22–23 October 2001. The most recent serious fluvial floods on the Cam. Around 87 millimetres of rain fell on Cambridge in a single day during the October event, roughly 94 millimetres over four days, and the commons and riverside streets went under.
• Winter 2013–14. A very wet winter brought high levels over the Christmas and New Year period.
• July 2021. An intense summer storm dropped a large amount of rain in a short time, overwhelmed the urban drainage network and flooded parts of the city centre — a reminder that the Cam corridor floods from surface water as well as from the river.
• January–February 2024. A wet winter pushed levels well above normal. Byron’s Pool ran around two feet higher than usual in early January, the Cambridge gauges recorded further peaks in February, and flood warnings were issued across Cambridgeshire.

Which areas does the River Cam put at risk?

The Cam puts a well-defined corridor at risk, from Grantchester and Newnham on the upstream edge of Cambridge, through the city’s commons, to Chesterton, Fen Ditton and the villages of the lower river down to Waterbeach. The Environment Agency covers this corridor with a series of named flood warning and flood alert areas, so the risk to any given site can be pinned to a specific reach. The commons through the city — Coe Fen, Sheep’s Green, Midsummer Common and Stourbridge Common — are designated functional floodplain, Flood Zone 3b, in the Greater Cambridge Strategic Flood Risk Assessment, which is precisely why they remain open. Development is steered away from that land, and sites near it face close scrutiny of finished floor levels, flood storage and safe access.

If your site sits within one of these areas, the Environment Agency’s flood map for planning is likely to place it in Flood Zone 2 or 3, which triggers the need for a site-specific flood risk assessment. You can screen any address against the live Environment Agency data using the map above, and the towns along the river are covered in more depth on our area pages for Cambridge and Cambridgeshire. Downstream, the Cam feeds the River Great Ouse.

Is the River Cam a Main River — and what does that mean for your site?

Yes. The River Cam is a designated Main River, which means the Environment Agency, not the local council, is the flood risk authority for it. If your site sits on or near the Cam, that has direct, practical consequences. Works in, over, under or within eight metres of the channel normally need a Flood Risk Activity Permit from the Environment Agency before they start, and a site in the river’s floodplain will usually need a flood risk assessment to support a planning application. Many of the smaller tributaries and the ditches feeding the Cam are ordinary watercourses instead, regulated by Cambridgeshire County Council as Lead Local Flood Authority, where the equivalent consent is Ordinary Watercourse Consent. Getting that distinction right early avoids enforcement later.

• The 8 metre byelaw margin. Building, excavation or structures near the Cam usually require a Flood Risk Activity Permit, sought alongside — not instead of — planning permission.
• Flood zones drive the assessment. A site in Flood Zone 2 or 3, or a larger site in Flood Zone 1, needs a flood risk assessment for planning, with finished floor levels set above the design flood level and the right climate change allowance applied.
• Know which watercourse you are on. The Cam is Environment Agency territory; its tributaries are ordinary watercourses under Cambridgeshire County Council as Lead Local Flood Authority. We confirm this for every site.

Flood risk and drainage services for sites near the River Cam

A development near the River Cam can call for several different reports, and it helps to be clear that they answer different questions. A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct reports. They often travel together in a planning submission, but they address separate things: whether the site is safe from flooding, whether the development should be located here, and where the surface water goes. Unda prepares each of them for sites across the Cam catchment.

• Flood risk assessment for planning. A site-specific assessment of flood risk from every source, showing the site can be made safe for its lifetime without increasing risk elsewhere. See our flood risk assessment for planning service.
• Assessments for property purchase and insurance. Independent reports for buyers, owners and lenders through our flood risk survey for property purchase and flood risk assessment for insurance.
• Drainage strategies and SuDS. Surface-water designs that manage runoff and meet the discharge limits set by the Lead Local Flood Authority — see our sustainable drainage strategy service, with detailed work to discharge a drainage condition.
• Sequential and exception tests. The locational justification the NPPF requires when a site falls in a higher-risk flood zone, through our sequential and exception test reports.
• BREEAM and groundwater. A BREEAM flood risk assessment for the Pol 03 credit, and groundwater assessed within the flood risk assessment where the chalk geology raises a high winter water table.

Where national mapping is not enough — a common situation for sites right beside a Main River — we can bring in hydraulic flood modelling to derive accurate flood levels and extents, and prepare an early preliminary drainage design before layout is fixed.

Managing flood risk on the River Cam: policy and contacts

Several bodies share responsibility for flood risk on the River Cam. The river sits within the Environment Agency’s Cam and Ely Ouse catchment, part of the wider Great Ouse system and the Anglian River Basin District. The Environment Agency regulates the Main River, publishes the flood map for planning and issues flood warnings; Cambridgeshire County Council is the Lead Local Flood Authority for surface water, groundwater and ordinary watercourses; and Cambridge City and South Cambridgeshire, working as Greater Cambridge Shared Planning, decide applications along the river against a shared Strategic Flood Risk Assessment. The Great Ouse catchment drains around 3,600 square kilometres, and the Cam is one of its four main tributaries, gathering the Granta, the South Level Lodes, the Rhee and Bourn Brook, as set out in the Environment Agency’s Cam and Ely Ouse catchment plan.

• Live river levels. Check the river’s gauges on the Environment Agency service — River Cam at Cambridge Jesus Lock (usual range 0.64–0.74 m) and Dernford on the upper river — or use our own live river levels map. Longer flow records for the catchment sit with the National River Flow Archive at Dernford.
• Screen a location. Try our free flood risk map and, for drainage design, the drainage calculators, the BRE 365 infiltration calculator and our ground conditions and soakaway map.
• Flood warnings. Sign up free for Environment Agency flood warnings for the River Cam at Cambridge and the Lower River Cam alert area, and call Floodline on 0345 988 1188.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

How Unda helps with flood risk on the River Cam

Unda understand how the River Cam behaves in flood — the slow build across the Cambridge commons, the chalk-stream response in the Granta and Rhee, and the way a high Great Ouse can hold the lower river back. We know where the floodplain bites hardest, from Grantchester Meadows to the Chesterton and Fen Ditton reaches, and what a robust, compliance-led flood risk assessment has to demonstrate to carry a development through planning. We prepare assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites across the Cam floodplain.

Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping and catchment plans, the river’s gauged levels and modelled flows, and the local planning policy that applies to the site. If you are working on a site near the River Cam, talk to one of our experienced flood risk consultants about what your application will need.

River Cam flood risk: your questions answered