Bury’s two rivers, the Lark and the Linnet, are chalk streams that are fed from springs in the underlying (chalk) aquifer. The Linnet, 10km long to the Lark’s 50km, rises near Chedburgh, to the south west of Bury St Edmunds. The Lark rises at the 100m AOD (above Ordnance Datum) contour south of Bury, and a number of tributaries contribute to its source. It flows over the boulder clay that overlies the chalk and continues in a south east to north west direction until the chalk becomes exposed near Lackford, 10km downstream of Bury. A major tributary, the Cavenham Stream, joins the Lark 2km downstream of Lackford.
As the river flows over the chalk outcrop, the underlying aquifer provides a significant contribution to the flow. Between Mildenhall and Isleham the River Kennett and the Lea Brook join the Lark from the south. Over the final 14.5 km stretch to the junction with the Ely Ouse the low lying Fens are underlain by impervious Gault clay and at the confluence by Kimmeridge, Ampthill and Oxford clays.
Global and regional context
Throughout the world chalk streams are rare, and England, with an estimated 224, has 85% of them (O’Neill and Hughes, 2014). They follow the band of chalk that sweeps diagonally north east from the south coast under the Wash to Yorkshire. The Lark and Linnet are the two most important chalk watercourses in the southern Brecks, mirrored in the northern side of the area by the Rivers Gadder, Nar and Wissey (which, like the Lark, flow into the Ouse).
Flow and seasonality
Under natural conditions, the slow release of water from the aquifer produces a relatively stable river flow in chalk streams, with a characteristic annual cycle and constant temperature of around 11C, meaning they rarely freeze. After the onset of autumn/winter rains, the flow tends to increase in December, associated with a rainfall-induced rise in shallower sections of the aquifer, and continues to increase until March or April.
During winter, flow from springs at the perennial head increases in strength, whilst springs along the ephemeral ‘winterbourne’ section reactivate after lying dormant through the summer months. Flows then decline steadily through the summer and autumn until the shallow aquifer is again bolstered in the winter by percolating autumnal rainfall. The Linnet has an ephemeral section in the West of the town that is usually dry in the summer despite there being water in the channel both up and downstream. It is not clear from historic record whether this has always been the case.
Peak flows after heavy rain are relatively small compared to ‘flashier’ river types, and are sustained for longer periods by the high spring-fed component. In their pristine state spring-fed channels have been found to equal or exceed bank-full flows for around 30% of the time (Whiting and Starn, 1995), which produces sustained waterlogging in riparian areas, a characteristic of classic chalk river floodplains. This is not the case in Bury today, although the Linnet floodplain is waterlogged for a few days most years in recent times. 200 to 300 year-old native black poplar trees (Populus nigra var. betulifolia) on No Mans Meadow (south of the abbey) indicate the area was boggy in the past.
Management of the Lark since Roman times
Most chalk streams have changed greatly in their physical appearance over the centuries, shaped by human activities for a range of purposes. The Lark and Linnet probably originally flowed in ill-defined channels through alder (Alnus glulillosa) and willow (Salix spp.) carr (Ladle and Westlake, 1976). Inputs of woody debris would have been substantial, with frequent debris dams forming and creating diversions to flow. Larger sections of the Lark would have probably formed better-defined channels.
Canalisation, impoundment for mills, and management of the floodplains have significantly altered the ecology of the Lark and Linnet and their associated habitats over the last thousand years. It has been said that ‘more or less every metre of every chalk stream has been modified to a degree, often many times’ (O’Neill and Hughes, 2014) and Bury’s rivers are no exception. Chalk streams are low-energy systems, fed as they are from springs bubbling up from the ground (as opposed to melted snow hurtling downhill from mountain tops), and are mostly incapable of erasing a modification once it has occurred.
The stretch of the Lark downstream of Bury and through the Brecks to the Ouse suffers from canalisation – the straightening and widening of the channel, and the installation of sluices and locks to make it navigable by working boats. The phases of this process have been summarised by local historian David Addy, dating back to the Roman Emperors Hadrian and Antony (AD 120 to 140).
|Roman times||an early canal from Isleham to Prickwillow. This was probably to ship out clunch (hardened chalk also known as Tottenhoe Stone) as a building material from Clunch Pits at Isleham.|
|17th Century||attempts halted by civil war|
|1699 to 1715||Henry Ashley’s successful construction of the Lark Navigation|
|1715 to 1815||Navigation in operation|
|1820 to 1846||Thomas Gery Cullum, senior and junior, made improvements until halted by the impact of railways|
|1890s||Lord Bristol’s Improvements until bankruptcy in 1905|
|After 1905||decline into a ‘drainage river’|
A flood relief scheme for Bury St Edmunds that included channel-straightening and concrete flood walls and banks for both rivers was built following a disastrous flood in 1968. A weir was installed downstream of Eastgate Bridge (just outside the abbey precinct) on the Lark. More channel-straightening befell the Lark when Tesco was built in 1996, compelling the author Roger Deakin to observe:
I stood outside the Bury St Edmunds Tesco. Here the Lark had been treated with something less than reverence as it flowed through the forecourt carpark. Pure Spring Water may be highly valued on the shelves inside, but outside, the real thing was ignored. The hapless Lark, which once meandered gently through water meadows here, had been neatly packaged in an outsized concrete canyon. No water vole would dream of venturing here, nor otter, purple loosestrife or figwort.”Roger Deakin, Waterlog: A swimmer’s journey through Britain, 2014
The geomorphology of a river – its shape, form and connectivity – is intrinsic to its value as a habitat. The meanders, riffles, pools and connected floodplains of a natural chalk stream provide the variety of conditions needed for rich biodiversity. It is also better able to cope with pollution and flooding than a heavily modified one (O’Neill and Hughes, 2014). Man made structures, such as weirs and sluices prevent re-colonisation of wildlife after extreme events, and prevent inappropriate sediment being removed from the river.
In addition to physical modification of the channel, abstraction has also had a major impact on the Lark. Today, 17000 million cubic litres are abstracted from the Lark catchment for public water supply each year (the equivalent of 18.7 Olympic sized swimming pools every day), leaving little for the river itself but surface run-off and outflow from sewage treatment works. What water there is left in the river suffers pollution from farming and sewage. In 2019 the Lark and Linnet’s ecological condition was assessed as poor (Environment Agency 2020).
While there are no species that are endemic to chalk streams, these rivers provide optimal habitat for a number of England’s iconic and indicator species. In their healthy state, chalk stream flows are excellent for trout and salmon, invertebrates, fish fry and macrophytes such as water crowfoot (O’Neill and Hughes, 2014). The fish fry and invertebrates are then food sources for insectivorous and fish-eating birds, mammals and amphibians. The chalk also acts as a buffer against floods and droughts, which means that they provide good refuge for flow-vulnerable species such as water vole.
The pressures on the ecology of the Lark and Linnet in Bury, and their associated habitats are of course wider than the issues of abstraction and physical modification. Agricultural practices, climate change, urbanisation, pollution, invasive non-native species and woodland management all play a part too. It is widely accepted that biodiversity across the UK has been massively depleted by centuries of habitat loss, management changes, development and persecution (State of Nature Partnership, 2019). Bury St Edmunds is no exception.
At the time of the foundation of the Abbey one thousand years ago, the Lark and Linnet in Bury St Edmunds would have been teeming with life and biodiversity. We know that during the 14th Century the waters at Tay Fen and Babwell Fen supported large fish populations and Bury St Edmunds had a flourishing fishing industry (Addy, 2012). Until the early 20th Century stickleback, barbot, trout, pike, eel, gudgeon, minnow, rudd and stone loach (Linnet) were present in Bury (Collings, 1936). Of these, barbot has not been seen in the UK for decades, and whilst wild brown trout is still present on the mid/lower Lark, redds (spawning sites) have not been recorded in Bury in recent years (Glenn Smithson 2020).
The history of the ecology of the Lark and Linnet at Bury would repay further research, and it is hoped the biodiversity survey work we have recently undertaken will provide a useful benchmark of what has been lost.
Libby Ranzetta, December 2020