River Tamar Flooding & Flood Risk

The River Tamar has two flood personalities, and they meet in the middle. Its upper catchment is a responsive rural landscape that sheds heavy Atlantic rain quickly, sending fast fluvial peaks down towards Gunnislake; below Gunnislake weir the river turns tidal, widening into the estuary that feeds Plymouth Sound, where spring tides and storm surge take over. Calstock sits almost exactly where the two collide, which is why it has flooded again and again. Unda prepares flood risk assessments, drainage strategies and related reports for sites across the Tamar floodplain, from the Cornish headwaters to the Hamoaze. To talk it through, call 01293 214444 or email enquiries@unda.co.uk.

In the early hours of 28 December 1979 the Tamar reached 4.47 metres at the Gunnislake gauge, its highest recorded level, as the river carried a peak flow of 484 cubic metres per second, more than twenty times its long-term average.

• A river of two halves. The Tamar runs fast and fluvial above Gunnislake and turns tidal below it, so a site’s flood risk depends heavily on which side of the weir it sits.
• A designated Main River. The Environment Agency regulates the Tamar, so the byelaw margin along its banks and Flood Risk Activity Permits apply to work near it: 16 metres on the tidal reaches, not the usual 8.
• Six live Environment Agency gauges. From Bridge Rule and Tamarstone in the headwaters down to Gunnislake at the tidal limit, the river is well monitored, and every gauge appears on the map below.
• The Devon–Cornwall border. The Tamar divides the two counties for almost its entire length, so a scheme beside it can straddle two lead local flood authorities.

Check flood risk along the River Tamar

The map below draws the River Tamar in coral, with the Environment Agency’s live river gauges positioned along its course. Click any gauge to see its latest level, its typical range and its record peak. 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 Tamar flood?

The Tamar floods for two quite different reasons along its length, and working out which one applies to a site is the first thing any assessment has to establish. In the upper and middle catchment the cause is runoff. The river rises on Woolley Moor near Bude, less than four miles from the north Cornish coast, then runs south for about 61 miles to Plymouth Sound, draining a catchment of roughly 920 square kilometres by the time it reaches Gunnislake. That catchment is rural and moderately steep, underlain by Carboniferous slates, shales and grits that hold little water, so prolonged Atlantic rain runs off the fields and into the channel fairly quickly. The National River Flow Archive record for the Tamar at Gunnislake describes a fairly responsive catchment of moderate relief, and its base flow index of 0.47 confirms that a good share of the river’s flow arrives as quick runoff rather than steady groundwater.

Rain is not in short supply. The average annual rainfall over the catchment is around 1,300 millimetres, well above the England average, and the higher ground on the fringes of Bodmin Moor and Dartmoor wrings still more out of incoming weather systems. The Tamar also gathers a string of substantial tributaries that each add their own pulse of water: the Inny, Ottery and Kensey on the Cornish side, and the Deer, Lyd and Tavy on the Devon side, with the Lynher joining the tidal estuary near Saltash. When several of these respond to the same storm, the flows combine downstream.

Below Gunnislake the story changes. Gunnislake weir marks the tidal limit, and everything downstream of it is estuary: the tidal Tamar, joined by the tidal Tavy and Lynher, opening into the Hamoaze and Plymouth Sound. Here flood risk is driven by the sea, as high spring tides, low pressure and onshore gales raise water levels regardless of how much rain has fallen inland. The most difficult conditions of all occur when a wet catchment sends a fluvial peak down to Gunnislake at the same time as a high tide holds the estuary up, leaving the floodwater nowhere to go. That combination is what a flood risk assessment near the lower Tamar has to model.

The River Tamar’s history of flooding

The Tamar’s flood record is long and well documented, and the benchmark event remains the flood of late December 1979. Over 27 and 28 December the river rose to 4.47 metres at Gunnislake, its highest recorded level, on a peak flow of 484 cubic metres per second, a figure the gauge has not matched in the decades since. Calstock, on the tidal river a few miles downstream, has a longer folk memory of flooding still: the village was badly flooded in the 1980s, after which the Environment Agency’s predecessor built the earth embankment that has protected it ever since.

That embankment has needed repeated attention. In the winter of 2012 to 2013 the Environment Agency carried out emergency repairs to just over 100 metres of the Calstock bank at a cost of more than £300,000, and in 2021 it completed a larger scheme that set a new flood embankment back from the river, created intertidal habitat on the old floodplain and removed the ageing bank in November of that year. The tidal Tamar’s defences are actively managed, not permanent.

More recent winters have kept the river in the news. During Storm Bert on 24 November 2024 the Environment Agency issued flood alerts along the Lower Tamar and the Tamar Bridge was closed to high-sided vehicles, caravans and motorcycles as gusts and heavy rain swept the estuary. In late January 2026 Storm Chandra brought another round of widespread flooding across Devon and Cornwall, with dozens of flood warnings and alerts in force across the two counties at the same time. Each of these events is part of the evidence base any assessment for a nearby site has to weigh.

Which areas does the River Tamar put at risk?

Flood risk along the Tamar is concentrated on the valley floor and the estuary shoreline, and the Environment Agency runs a formal warning service for the communities most exposed. A catchment-based flood alert area, “Lower River Tamar”, covers the reach around Gunnislake, Calstock, Cargreen, Bere Alston, Morwellham and Cotehele, and within it two more precise flood warning areas allow the Environment Agency to warn individual communities: one for the River Tamar at Calstock, and one for the reach from Greystone Bridge to Cotehele. Downstream, the tidal shoreline around Saltash, Cargreen and Millbrook falls within the large “Plymouth Sound, Wembury Bay and the tidal estuaries” warning area. The table below sets out the main settlements at risk from source to mouth.

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 areas along the river are covered in more depth on our local pages for Cornwall, Devon and Plymouth.

Is the River Tamar a Main River, and what that means for your site

Yes. The Environment Agency has designated the Tamar a Main River along its length, which means the Agency, not the local council, regulates the channel and its margins. For anyone developing nearby, that has practical consequences. Works close to the bank generally need a Flood Risk Activity Permit from the Environment Agency (a flood risk activity environmental permit) before they start, and the byelaw margin is wider on the tidal river than on the fluvial one: 16 metres from the bank on the tidal reaches below Gunnislake, against 8 metres on the freshwater river above it. A site in the floodplain will also normally need a flood risk assessment to support a planning application. The many smaller streams, brooks and ditches feeding the Tamar are ordinary watercourses rather than main river, regulated by the relevant county or unitary council as lead local flood authority and by the riparian owners whose land they cross, and confirming which regime applies to a site at the outset saves trouble later.

• The byelaw margin. Building, excavation or structures near the Tamar usually require a Flood Risk Activity Permit, sought alongside planning permission rather than instead of it, and the margin is 16 metres on the tidal river, 8 metres above Gunnislake.
• 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.
• Know which watercourse you are on. The Tamar itself is Environment Agency territory; each feeder ditch or stream is an ordinary watercourse under the lead local flood authority. We confirm this for every site.

Flood risk and drainage services for sites near the River Tamar

A development near the Tamar 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 Tamar catchment.

• Flood risk assessments for planning. Site-specific assessments dealing directly with the Environment Agency’s requirements for development in the river’s flood zones. 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, through our sustainable drainage strategy service.
• Sequential and exception tests. Locational justification where planning policy requires it, through our sequential and exception test reports.
• BREEAM and groundwater. A BREEAM flood risk assessment for the Pol 03 credit, and a groundwater flood risk assessment where a high water table is in play.

Where national mapping is not enough, as is common for a site right beside a tidal main river, we can bring in hydraulic flood modelling to derive accurate flood levels and extents, including the combined fluvial and tidal case, and prepare the detailed scheme needed to discharge a drainage condition. Everything we do is framed by the NPPF and its planning practice guidance.

Managing flood risk on the River Tamar: policy and contacts

Several bodies share responsibility for flood risk on the Tamar, and it helps to know who does what. The river sits within the South West River Basin District, and the Environment Agency’s catchment-scale approach is set out in its Tamar Catchment Flood Management Plan and its Tamar management catchment data, which favour managing runoff at source in the rural catchment and maintaining defences at the places most at risk, such as Calstock. The Environment Agency regulates the main river, publishes the flood map for planning and issues flood warnings; Cornwall Council, Devon County Council and Plymouth City Council act as lead local flood authorities on their respective sides of the river for surface water, groundwater and ordinary watercourses, each with its own strategic flood risk assessment. South West Water is responsible for the public sewer network where sewer flooding is in play.

• Live river levels. Check the river’s gauges on the Environment Agency service, where the Tamar at Gunnislake is the key tidal-limit gauge, or use our own live river levels map.
• Screen a location. Try our free flood risk map, and for drainage design the drainage calculators and the geology and ground conditions map.
• Flood warnings. Sign up for Environment Agency flood warnings 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 Tamar

Unda understand how the Tamar behaves in flood: how quickly its rural catchment responds to Atlantic rain, where the tidal reach below Gunnislake takes over, and how the two can coincide to produce the worst conditions of all. We know what the planning system and the Environment Agency expect from the reports that follow, and we prepare assessments and drainage strategies for developers, planning consultants, architects, agents and homeowners with sites the length of the valley, on both the Cornish and Devon banks.

Every assessment rests on the same evidence the regulators use: the Environment Agency’s flood mapping, 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 Tamar, talk to one of our experienced flood risk consultants about what your application will need. You can also read how we approach flood risk assessments for planning and see the other rivers we cover on our rivers hub, including the cross-border River Severn. For buyers and owners, our guides to understanding flood risk before a purchase and flood risk and mortgage lending are a useful start.

River Tamar flood risk: your questions answered