The Lark's Four Types of River
The Lark is distinctive not only because its central section is a chalk stream, but because it comprises different types of river formation in one stream. If we are to restore it effectively, each must be approached differently.
Type 1 : Fen
Between Mildenhall and where the Lark joins the Great Ouse downstream of Prickwillow lie the fens. This is now an entirely engineered landscape, much of it below sea level. It is characterised by black peat soils, and the waving plumes of reeds picking out the straight lines of drainage ditches.
This part of the catchment is managed by an IDB (Internal Drainage Board), which is responsible for pumping excess water from the ditches up into the river which is raised high above the surrounding land in an embanked canal.
Type 2 : Chalkstream
The Lark and its tributaries between Bury St Edmunds and Mildenhall are chalk streams. Chalk streams have close connectivity with water in the underlying chalk aquifer.
The water in a healthy chalk stream, as in this picture of the Cavenham stream, is beautifully clear and of a fairly constant temperature. Natural processes in the river keep the gravel bed largely free of silt, enabling fish and invertebrates to lay their eggs.
Such streams support an abundance of wildlife.
Unfortunately, not all the Lark's chalk streams are healthy.
Type 3 : 'Beads on a String'
Like Ely in the Fens, the town of Bury St Edmunds is built on a hill rising out of a relatively flat and naturally watery landscape. Here the rivers flow gently. They are fed by springs from ‘perched water’ buried in the clay, and naturally form meres and wetlands along their length. This river morphology is evocatively termed ‘beads on a string’. This 'beads' landscape stretches from southwest of Bury St Edmunds round to the East. It includes the iconic water meadows that ring the town centre, and includes the catchments of the Black Bourn (otherwise known as the Sapiston River) and the Pakenham Stream, both tributaries of the Little Ouse. The meres have given rise to placenames such as Sicklesmere and Livermere, but this landscape is now heavily drained.
Type 4 : The Headwaters
The Lark's headwater streams are technically not chalk streams, although they too have gravel beds, and are just as remarkable.
Most of them start at about 100 m above sea level on the eastern end of the Newmarket ridge, south of Bury St Edmunds, where the chalk is overlain with a thick layer of boulder clay. They can flow with extreme force, descending about 50m over the course of no more than a mile. Since the banks are resistant to erosion, the force of the water in these tiny streams in full flood means that they have carved themselves into deep ravines.
Where they have been protected from straightening and dredging by steep sided valleys and woodland, they form beautiful meanders.
