Celebrating 25 Years of the Chilterns Chalk Streams Project

The Misbourne is an intriguing
chalk stream. The river flows for
27 km from its source at
Mobwell Pond to the River Colne
at Denham Country Park, but it
frequently dries up in its middle
reaches between Amersham and
Chalfont Park.. Here the chalk
aquifer is highly fractured, and
the river is ‘perched’ above
the water table.
At Chalfont St Giles, the groundwater
can be as much as 10 metres
below the riverbed. In the 1940s,
groundwater abstraction levels in
the catchment were approximately
4 ML/d but rose to a licensed
abstraction of c. 33 ML/d by the
1960s. In 1998, an Alleviation of
Low Flows scheme was carried
out by Thames Water and Three
Valleys Water (now Affinity Water),
which reduced licensed abstraction
to 14 ML/d. In 2018, Affinity Water
reduced abstraction by 3 ML/d
and are committed to a further
reduction of 2 ML/d by 2025.
In recent years the Misbourne
has benefitted from multiple
restoration schemes.
In 2012, Chilterns Chalk Streams
Project, in partnership with the
Environment Agency and the
landowner, led a wetland habitat
creation scheme at Doctor’s
Meadow. Banks were regraded,
wetland pools and scrapes were
dug and the river channel was
narrowed and made more sinuous
in the lower section of the reach.
To date, the Revitalising Chalk
Rivers programme, a partnership
between the Environment Agency
and Affinity Water, has led to
improvements in wildlife corridors
at four sites along the Misbourne.
The most recent project, in 2020,
created a new, meandering 1 km
channel more closely aligned
with the river’s original path
between the Amersham Bypass
and Quarrendon Mill, to improve
river habitat.
The Environment Agency identifies
both water quantity and water
quality issues on the Misbourne.
The chart to the right shows a time
series of ortho-phosphate levels
in the Misbourne upstream and
downstream of Gerrards Cross.
Environment Agency analysis
suggests that 97% of the phosphate
in the River Misbourne is sourced
from wastewater treatment works,
and concentrations contribute to
the ‘Poor Ecological Status’ of the
river. High levels of phosphate
encourage filamentous algal
growth, which can reduce
dissolved oxygen levels at night
and harm the balance of the river
ecosystem.
Ortho-phosphate (mg/L)
1.00
Ortho−Phosphate (mg/L)
1.00
0.75
0.75
0.50
0.50
0.25
0.25
0.0
0.00
Ortho-phosphate concentrations over time
MISBOURNE ABOVE GERRARDS CROSS STW
MISBOURNE BELOW GERRARDS CROSS STW
2012 2014 2016 2018 2020 2022
2012 2014 2016 Year2018 2020 2022
Misbourne above
Gerrards Cross STW
Perched River Misbourne
at Chalfont St Giles
Poor status
Moderate status
Good status
Misbourne below
Gerrards Cross STW
Source: Environment Agency data
River Misbourne
at Little Missenden
CURRENT STATE
RIVER MISBOURNE | 45