The Misbourne is a chalk stream

CHALK streams are special !

We commonly refer to our Chiltern rivers as ‘chalk streams’ and in speech we may utter the words with a certain reverence, as though we regard them as something special.  Indeed, they are; but it is certainly not their length or volume, so what is it that makes them special ?

How special ?

 

     For a start, they are rather rare.  Their name derives from the fact that they occur in landscapes that overlie a predominantly chalk geology, they are therefore confined mainly to France, New Zealand and England south of a line from Lyme Bay to the Humber,.  This global rarity justifies their listing in Annex 1 of the European Habitats Directive, and the UK Biodiversity Steering Group accordingly identifies chalk streams as a key priority habitat for which it has produced a separate action plan.

A different provenance

     All river water derives originally from rainfall, but chalk streams differ from other rivers in the way that this rainwater finds its way into the river channel.  In most other catchments, water runs across the surface from higher ground, forming streams which combine to produce ever larger watercourses, and eventually a main channel.  The result is a tree-like (‘dendritic’) branching pattern of waterways. (e.g. as in Aylesbury Vale)

     By contrast, on chalk only a small proportion of rainfall reaches streams directly as surface runoff; most is absorbed and either taken up by plant roots or passed down into porous and permeable chalk where it is stored as ‘groundwater’. The upper surface of this stored groundwater is the ‘water table’; this rises and falls according to the seasons, being highest after winter rains and lowest following summer, when most rainfall either evaporates or is taken up by vegetation.

     Chalk streams appear only when and where the water table rises to intersect valley bottoms.  Thus, water reaches the river channel from below, as ‘baseflow’, and stream patterns tend to be simple with few, if any, tributaries.

A different flow regime

 

     Because the height of the water table varies with the seasons, its point of intersection with the valley bottom also changes.  The head of a chalk stream will migrate up-valley during the winter as the water table rises, and down-valley through the summer as it falls.

     The upper reach of a chalk stream, where flow may occur over winter but which dries out through the summer months, is called the ‘winterbourne’ and the highest point of flow the ‘winterbourne head’.  The point at which flow normally occurs throughout the year is often termed the ‘perennial head’ although no part of a chalk stream’s upper reaches can truly be described as perennial; in severe droughts (e.g. 1997 in the Chilterns) some streams dried out almost completely.

     Chalk streams have a characteristic annual cycle with maximum discharge around April and minimum around October ahead of the onset of autumn rains.  Because baseflow reaches stream channels only after slow percolation through the chalk, the effects of heavy rain events tend to be attenuated; streamflows are more stable and less ‘flashy’ than in rivers which receive direct rain runoff

Different chemistry  – different physics

     Slow percolation of baseflow through the chalk leads to a stable chemistry with characteristically high alkalinity, due mostly to uptake of calcium bicarbonate.  pH values are typically within the range 7.4 to 8.0 .

     Long residence times within the chalk aquifer stabilise water temperature, minimising the effect of seasonal extremes.  In southern England, water entering stream systems from springs will usually have a temperature not far removed from 11 ^oC, whatever the season.  Exposure to the atmosphere, mixing with the receiving waters and distance from the spring will modify the temperature at any point within the river but even so, a typical temperature range for a sizeable chalk stream is around 5 – 17 ^oC.

     Filtration through the chalk ensures that baseflow waters are naturally low in suspended solids; clear waters and low fine-sediment loads are typical attributes of natural chalk streams.

Different wildlife

     Unique physical and chemical properties result in characteristic biological communities, including some rare or endangered species which display strong preferences for chalk streams.

     Classic chalk stream plants include Ranunculus spp., (varieties of water-crowfoot), starwort and water-cress; it is no accident that water-cress farming was once so widespread within the Chilterns valleys.

     Chalk streams are host to many invertebrates identified by legislation or national review as high priority species.  These include species of mayfly, caddis, beetles, snails and a mussel, but perhaps the most notable entry (at least for the Chilterns) is the white-clawed crayfish.  This animal is now feared to be locally extinct, having succumbed to the larger, more aggressive and plague-carrying signal crayfish, introduced from America.

     Larger animals include the water vole, Britain’s fastest declining mammal.  Once common, it is now restricted to but a few Chiltern locations, its decline attributed (partly at least) to predation by an another introduced American species – the mink.  Loss of habitat due to riverside developments is an additional reason for the water vole’s decline.

Different threats –  (a) to quantity

     Seasonal drying of upstream winterbourne sections is a normal chalk stream characteristic; many invertebrate species have adapted to this regime, developing ways for surviving periods of drought and thereby avoiding competition from other species that lack such strategies.

     However, lowering of the water table by over-abstraction of groundwater to satisfy consumer demands can dramatically extend the period of winterbourne exposure, putting the survival of vulnerable invertebrate populations at risk.  Drying extending over several seasons, as in the mid-1990s, can seriously alter the entire riverside habitat, leading to the replacement of characteristic chalk stream flora and fauna by terrestrial species.

     ‘Restoring Sustainable Abstraction Programmes (RSAP)’, (formerly called Alleviation of Low Flow (ALF) programmes) are now in place on the Ver and Misbourne to curb water company over-abstraction, thereby allowing a return to more natural flow regimes in these streams.  Similar schemes are being implemented on the Wye and Bulbourne and studies are in progress to assess the need for RSA programmes on the Gade and Mimram.

and                              (b) to quality

     All rivers are subject to degradation of water quality by human actions but the effects upon chalk streams can be particularly devastating for they have few tributaries in which mobile animals can seek refuge or from which plants and animals may re-colonise the main channel once acceptable water quality is restored.

     Urban development poses the risk of contamination by accidental (sometimes deliberate?) discharge of pollutants.  Toxic substances as well as hydrocarbon and heavy metals from road runoff, may devastate aquatic communities (e.g. fish kills). Agricultural pesticides may be equally damaging.  Increased suspended solids clog animal feeding systems and restrict the light penetration needed by photosynthesising in-stream vegetation.  Settlement of silt smothers the characteristic gravel bed, so important to fish spawning and home to many invertebrates.

     Breakdown of organic pollution from sewage effluent discharge and runoff from livestock farms depletes oxygen; less tolerant species may be eliminated.  Nutrient enrichment, especially phosphate and nitrate from agricultural runoff, can produce algal blooms, depleting oxygen and depriving bottom-living organisms of light.  Accidental or deliberate introduction of invasive non-native plant species can have the same effects;. some discarded garden pond aquatics proliferate to the exclusion of native species when introduced into the wild.

A special responsibility

     Special habitats demand special attention and to this end, members of the Chiltern Society’s Rivers and Wetlands Conservation Group maintain a watching brief over the behaviour of our somewhat fragile streams, noting and reporting any changes that may appear to affect the riverine environment.  The Group is a partner within the Chilterns Chalk Streams Project, a working partnership with the Chilterns Conservation Board, local authorities, water companies and statutary agencies, dedicated to the conservation and enhancement of our chalk streams.

John Norris

Chiltern Society Rivers and Wetlands Conservation Group

14 July 2006

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