The Cam Valley
The Cam Valley lies to the south of the county and comprises the Holocene floodplain, peat and adjacent Quaternary river terrace deposits of the River Granta, River Cam and River Rhee and their tributaries. It is underlain by Cretaceous Gault clay and the lower part of the Chalk. It is located between the Western Plateau and Swavesey Apron to the west, the Heydon Chalk Uplands and Whaddon Apron to the south and the Balsham Chalk Uplands and Lode Apron to the east. The northern part of the valley is also confined by the North Cambridge Terraces and empties into the Fen Edge south of Upware. The Cam Valley has both poorly drained alluvial and peat soils and well drained soils on terrace gravels. The elevation ranges from 5 – 40m OD.
The River Cam ‘system’
The ‘River Cam’ flows through the south of the county towards the north, merging with the River Great Ouse at Pope’s Corner, two miles south of Ely. The various names used for the River Cam and its tributaries are confusing, with two different channels still referred to by the same name today. It used to be called the Granta in old English, hence the village Grantchester and the previous name of Cambridge as Grantebrycge. When the city changed its name to Cambridge, the name of the lower reaches of the river changed to match. The lower (northern) part of the river is now called the Cam up to Hauxton Junction (1 mile/1.5 km south of Granchester). At Hauxton Junction the two main tributaries meet. The first, referred to as the Cam by the Environment Agency and National Rivers Authority, but often referred to by many local people as the Granta, rises south of Debden (112 m/ 367 ft altitude) near Widdington in Essex. It flows through the Cam Valley Geodiversity Landscape to join with the second, the Rhee, which rises at Ashwell springs (49 m/161 ft altitude) in Hertfordshire and flows through the Whaddon Apron Geodiversity Landscape to the Cam Valley. The EA and NRA reserve the name Granta for the third main branch that rises south of Castle Camps in the far south east of Cambridgeshire and flows through the Cam Valley Geodiversity Landscape to join the Cam near the Shelfords, southeast of Hauxton Junction.
The landscape we see today was extensively modified by the Anglian Glaciation, ending c.425,000 years ago when ice covered the whole county, and by subsequent cold periglacial periods. The Tottenhill Glaciation c.160,000 years ago may also have modified the lower Cam Valley in the Fen Basin as a tongue of ice possibly extended south nearly as far as Cambridge. Between and since these glaciations, the Cam Valley has had a dynamic history and this has left a legacy of a complex pattern of river terraces and glacial deposits. At present, four Cam River Terraces have been identified (with some research suggesting the gravel deposits known generally as ‘the Observatory Gravels’, in the north west of Cambridge, represent a 5th). The terraces formed at different times as the river changed its course and, therefore, represent stages in the development of the valley. Each terrace contains deposits of various ages whose relationship to each other are often complex. Occasionally, material has been reworked from earlier deposits making dating difficult in some places. Individual deposits have been named according to a key area where they have been found e.g. Huntingdon Road Member (4th Terrace), Histon Road Member (3rd Terrace), Sidgwick Road Member (2nd Terrace) and Barnwell Station Road Member (1st Terrace).
Navigation
The Cam is navigable by normal river traffic from its junction with the Ouse up to Jesus Lock to the north of Cambridge city centre. Punts and canoes can reach further upstream to Grantchester and Byron’s Pool. The stretch between Jesus Lock and Bates Bite Lock is used for rowing and the unique Cambridge Bumps races are held here every year.
Chalk springs and streams
Many chalk springs form ecologically important chalk streams that flow into the Cam, although low water flows, due to high abstraction rates, have meant that a supplementary supply has had to be provided for some. CGS, in partnership with other members of the County Sites Panel including the Wildlife Trust, are currently researching the chalk streams in the county and identifying the springs that give rise to them.
There are numerous small chalk streams that feed into the River Rhee as it flows east to join the Cam near Hauxton. Notable springs are at Fowlmere Nature Reserve and LGS, lying in the Heydon Uplands Geodiversity Landscape and supplying the River Shep and then the Rhee. The River Mel flows from springs at Meldreth Bury.
Nine Wells and Giant’s Grave (Cherry Hinton) lie at the base of the Balsham Uplands, their water flowing into the Cam Valley. Nine Wells is the source of Hobson’s Brook, which was converted into Hobson’s Conduit, an important water resource for the people of Cambridge. The ecology of Nine Wells has undergone considerable study and there are now efforts to restore some of its ecological value. It is a Local Nature Reserve and a Local Geological Site. Giant’s Grave (also a Local Geological Site) is the source of Cherry Hinton Brook. There are also chalk springs along the south east fen edge, to the east of the Cam Valley. These flow from the Balsham Uplands, across the Lode Apron Geodiversity Landscape to join the Cam where it flows into and through the Fen Edge. Castle Spring at Burwell is a Local Geological Site due to its chalk spring.
Most of the chalk springs arise from the hard band of the Totternhoe Stone, lying at he base of the Zig Zag Chalk, but the picture is often more complex e.g. at Fowlmere where water from the Melbourn Rock, another hard band, higher in the sequence between the top of the Zig Zag Chalk and the base of the Holywell Nodular Rock, possibly supplements water arising under pressure from the Totternhoe Stone band below.
For more about chalk springs and streams see the Hertfordshire Geological Society website and the The Wildlife Trust’s website.
Other springs and streams
The Bourn Brook, rises in the higher land of the Western Plateau Geodiversity Landscape, flows across the Swavesey Apron Geodiversity Landscape and joins the Cam at Byron’s Pool LNR in the Cam Valley. It is fed by a number of springs, several of which arise from water that percolates down through the Woburn Sands until it reaches an impermeable layer of clay (the Kimmeridge or Gault clays). It plays an important role in the area’s biodiversity and is particularly important for water voles.
The River Cam system has five sections:
1 The upper part of the River Cam rises near Debden in the Chalk hills of Essex and flows via Great Chesterford to Hauxton Junction. The settlements tend to be near the river and in the steep sided valley bottom. The valley is an important north south route.
2 The River Granta flows from the Chalk ridge near Castle Camps to join the Cam before its junction with the Rhee. The Granta drains the higher land of the Balsham Chalk Uplands.
3 The River Rhee which rises at Ashwell in Hertfordshire and flows through a wide, low-lying area, the Whaddon Apron, via Barrington to Hauxton Junction. The source is Ashwell spring, one of the ecologically valuable ‘chalk springs’ found in the area. Some still contain a particular species of flatworm which which has survived from colder times – the characteristic temperature of the spring water emerging from the Chalk, at 10.4C, being cool enough for it. The settlements and roads are built away from the river on slightly higher ground.
4 The ‘Cambridge Gap‘ from Hauxton Junction, about 1.5 km /1 m south of Grantchester, to Cambridge. The Gog Magog Hills, to the east of the ‘gap’ in the Balsham Chalk Uplands, rise to 68 m above the river, whilst to the west, the Madingley Ridge, part of the Western Plateau, rises to 54 m above the river.
5 The lower stretch flows from Cambridge in the Cam valley to the Fen Edge south of Upware. From here it heads north to meet with the River Great Ouse at Popes Corner, not far south of Ely. The river at Cambridge is of particular historic interest, being a river crossing point since early times due to its location on the southern edge of what would have been extensive marshes for thousands of years. It runs through the world-famous college ‘Backs’ with the famous view across the water to King’s College Chapel and then passes under Magdalene Bridge, overlooked by Castle Hill to the north, with its Norman earth castle mound.
The ‘River Cam’ in relation to the contours and broader geology.