The River Evenlode
Abstract
The Evenlode is a lowland river. It rises near Moreton in the Marsh and flows
towards Oxford where it joins the Thames near Cassington. The river
changes over its course in terms of width, depth and discharge. It also
contains a good example of a gorge and a misfit river, the by-product of
events in the region during the last glacial phase. Human activity has also had
a profound impact on the river. It has been diverted, split, ponded, and water
has been removed for irrigation and for power. The Evenlode, like every river,
is unique, although it does show changes downstream in common with other
river systems.
Introduction
The River Evenlode has its headwaters in the Vale of Moreton and follows a
sinuous course across the Cotswolds to Cassington, where it joins the
Thames. It has a number of tributaries including the Cornwell Brook and the
River Glyme (Figure 1). The Cotswolds are a simple escarpment, with the
scarp face overlooking the Severn lowlands, and the dip slope trending
southeastwards into the valley of the Thames. The average annual rainfall in
the Evenlode catchment is 736 mm. This varies from over 800 mm in the
west to 650 mm in the east. The area is predominantly rural in nature –
landuse is mainly split between arable (46%) and grassland (27%) and there
are many historic market towns such as Chipping Norton, Moreton-in-Marsh
and Woodstock.
Figure 1 Map of the Evenlode basin
The streams forming the headwaters of the River Evenlode rise on driftcovered Lower Lias at an elevation of about 150m around Moreton-in-Marsh.
The river flows S.S.E. down the Vale of Moreton for about 16 km to Shiptonunder-Wychwood, where it turns a right-angle to flow E.N.E. for 7 km, then
curves round (still on Lias) to resume its original direction near Charlbury,
where it enters the Evenlode gorge through the Lower Oolites (limestones
and clays). The gorge is about 9 km long. The upper half is straight, but the
lower half consists of a series of large incised meanders, for which the
Evenlode is famous. After leaving the gorge the river flows comparatively
straight for the last 4 km over Oxford Clay. It joins the Thames near
Eynsham, shortly after crossing the 60m contour.
The total fall of the river is only about 80m in its course of about 40 km; the
effective gradient is 0.11. In the 9 km of gorge the fall is 20m, giving a
gradient of just 0.13.
Geology
The Cotswolds is a simple escarpment or cuesta, with extensive tracts over
200m and two small areas of over 300m. In parts, the upland has been
deeply eroded because of
● many high-level clay beds, and
● heavy precipitation, especially in the past.
The main rock types in the Evenlode Basin are successive banded outcrops
of the clays and limestones and scattered patches of gravels either side of the
main valley. All of these are comparatively soft and unresistant to running
water.
In the west of the basin oolitic limestone forms the Cotswolds escarpment.
The main western scarp is deeply disected, and its summit is fretted with
wind-gaps resulting from transverse faulting and stream action working to the
relatively low erosion level of the Evenlode system.
Further east, the limestone thins out and is buried by glacial drift with a lower
undulating landscape. The Lower Lias appears at the surface only in the
extreme north, with narrow strips stretching southward along the valley floors
of the Cherwell and the Evenlode. The plateau drift consists of sands and
clays containing abundant erratic pebbles and boulders.
In the dissected country east of the Evenlode, faults have encouraged the
formation of several minor rift valleys and horsts. An anticline runs along the
ridge separating the Evenlode and Glyme valleys. The faults also often form
spring lines and the seepage edges of the faulted hillocks are scalloped with
blunt, steep-sided combes, primarily the products of spring-sapping and
solifluction.
Downstream changes and Drainage network
Downstream changes
In most rivers there are noticeable changes in channel variables downstream.
Bradshaw’s model (Figure 2) suggests that there will be an increase in water
depth, occupied channel width, velocity, discharge and the quantity of the
load carried. In contrast, the particle size of load carried decreases, channel
bed roughness decreases and so too does the slope angle.
Figure 2 Bradshaw’s model
The changes shown in Figures 3 and 4 support some of Bradshaw’s laws. For
example, discharge increases from less than 0.1 cumecs at the source of the
Evenlode to 1.92 cumecs near its confluence with the Thames. Velocity
appears to increase steadily from below 0.1m/sec to 0.55 m/sec at Combe,
but thereafter decreases. This reflects the nature of some of the sites chosen.
Figure 4 shows clearly that the river varies enormously at Stock Bridge,
Combe and Cassington in the space of just a few metres. Depending on
which site is chosen i.e. closer or further away from the bridge, the readings
can alter. In this case, the readings used have been those taken further away
from the bridges.
Figure 3 Changes in the Evenlode downstream
Figure 4 Changes in the Evenlode downstream
Site
Distance
Crossfrom
sectional
source
area (m-2)
1 Fire Training
1
0.456
College
2 Common
3
0.99
Bridge
3 Stock Bridge
5
0.392
4 Foxholes
14
0.35
5 Ascott under
20
1.82
Wychwood
6 Stonesfield
30
1.68
7 Combe
33
3.39
8 Cassington
38
10.67
Velocity
m/sec
Discharge
(cumecs)
< 0.01
< 0.01
< 0.01
< 0.01
0.13
0.26
0.26
0.05
0.09
0.47
0.29
0.55
0.18
0.49
1.86
1.92
Figure 5 Sites where measurements were taken
Starting near Moreton in the Marsh, the Evenlode is little more than a
drainage ditch, less than 2m wide and nowhere deeper than 24cm. Its
bedload is largely clay and silt, reflecting the Lias clays on which it is located.
As the river flows south to the village of Evenlode it widens and deepens. At
Common Bridge, just 3 km from the source, it is 2.67m wide and has reached
a depth of over 50 cm. By Stock Bridge, 5 km from the source, it is over 6m
wide and over 50 cm in places. Its discharge is over five times greater than at
its source. This is partly due to a number of tributary streams and partly to
draining a larger area.
Further south near Foxholes, the Evenlode receives water from the Westcote
and Sars Brooks and its discharge has increased to 0.47 cumecs (see Figure
4). Its velocity has doubled compared with the previous site. The river then
meanders in an overall easterly direction towards Ascott under Wychwood.
Increasingly, more tributaries and springs feed the Evenlode, but equally,
more water is diverted and taken from it, such as at Ascott Mill, Charlbury and
Fawler. In places the Evenlode has been straightened, diverted and
deepened. A good example is at Ashford Mill and Bridge in the gorge section.
Hence it is not surprising that not all of the changes indicated by Bradshaw
are to be seen.
By the time the river enters the Gorge there is meandering on two scale –
there are valley meanders and there are channel meanders (Figure 6). Near
Combe there is an excellent example of an ox-bow lake. The river continues
to widen and deepen. At Stonefield its discharge is 0.49 cumecs but by
Combe this has risen to1.86 cumecs, and by Cassington it has reached 1.92
cumecs. However, human interference increases downstream so that in many
places the river is rarely in one channel. This is especially true south of
Hanborough. Nevertheless, as the data suggests, the Evenlode illustrates
many of the expected changes, but not in a text book style.It is perhaps
unrealistic to expect any lowland river, especially one in a densely populated
part of the country to be in a ‘natural state’.
Figure 6 Valley and channel meanders
Drainage network
In addition to the Evenlode itself, data for the drainage network is presented
in Figures 7 and 8. The data conform to some of the Laws of Drainage Basin
Morphometry (stream order) i.e.
● the number of streams is inversely related to stream order
● the mean length of stream decreases with increasing order
● the mean area of drainage basin increases with increasing stream order
● the mean gradient of streams decreases with increasing order (streams
everywhere in the basin are fairly gentle in gradient but the few steep ones
are generally first order streams).
There are plenty of exceptions to these rules, but when aggregated, the data
tends to support them.
Figure 7 The River Evelode drainage network
Stream
Number Average
Average area Average
order
length (km) (km2)
gradient (º)
1
2
3
4
5
105
26
6
2
1
2.6
4.81
6.85
4.9
7.2
2
6.6
33
204
425
Figure 8 Data for the Evenlode drainage network
Location
Width (cm)
Average
depth (cm)
Moreton
114
10.67
Four Shires
195
12.2
Stock Bridge
495
18.7
Bruern
610
76.6
Aston under
655
30.2
Wychwood
Stoinesfield
1140
61.9
Combe
770
71.6
Cassington
840
67.2
1.55
1.06
0.28
0.12
0.08
Velocity
(m/sec)
0.10
0.06
0.29
0.33
0.39
0.43
0.43
0.71
The alternation of clay and limestone in the west compared with the
predominance of clays in the east effects the river pattern. In the west the
drainage pattern is a regular almost rectangular whereas in the east the river
pattern is irregular. Figure 9 shows consequent dip-slope rivers and
subsequent strike-streams that are characteristic of the Cotswolds.
Figure 9 Terraces of the Evenlode (slide of Hanborough terrace)
The main rivers occupy wide deep-set valleys cut down to Lias clays at or
near their sources, but enter upon 'gorges' incised into the oolitic limestones.
Beyond the present head of each main stream is a wind-gap, evidence
perhaps of its former north-westward extension. Within the Cotswolds the
river valleys are noted for their incised meanders and for the extensive
development of dry valleys above their head-springs.
The winding gorge of the Evenlode in the oolitic limestones typifies the misfit
character and incised and abandoned meanders of the Cotswolds master
streams. These represent the work of large rivers, but the meanders of the
gorge sections also reflect the influence of a former mature development on a
covering of clay or interbedded-clay strata since removed by erosion. The
‘misfit’ streams and the aggradation of the valley floors denote decreased
river discharge and increased load.
The dry valleys differ widely in nature and age. Those in the uplands seem
primarily the work of surface water, while many of the deeply incised valleys
near the eastern edge of the Cotswolds, are essentially the work of springsapping, aided occasionally by surface run-off.
Misfit rivers
One of the most characteristic features of the dip slope is the great size of the
valleys in comparison with the diminutive size of the present streams flowing
in them. The streams flow in large, wide, flat-bottomed valleys meandering
across the dip slope in great curves, known as valley meanders.
Figure 10 Misfit river
Such misfit rivers may result from the climatic changes of the Ice Age. Such
changes have often been put forward to explain the dry valleys of the chalk
lands, and the valley meanders differ only in that they have greatly shrunken
streams rather than being totally dry. The channels associated with the valley
meanders are approximately ten times as wide as the existing stream beds,
and the wavelengths of the valley meanders are approximately nine times as
long as the wavelengths of the corresponding stream meanders.
One possibility is higher rainfall levels. Alternatively, when temperatures were
lower during the glacial phases of the Ice Age, much precipitation would have
fallen as snow rather than as rain. Rapid melting of the snow in the spring
thaw would supply large discharges. In addition the amount of water running
off the surface might have been increased by the presence of frozen subsoil
(permafrost) which would have hampered the percolation of water into the
permeable limestones.
River terraces
After the last glacial period, the Evenlode system subsequently lowered its
bed in a rejuvenation that cut down about 15m, and created the Evenlode
Gorge. They also spread out gravels on the clay lands such as at Wolvercote.
The dominant downward movement subsequently continued until about 4m
above present river levels and gravels accumulated.
Figure 11 Sequence of terraces in the Evenlode Basin
Anglian
Coombe
and
Freeland up to 50m above flood-plain
(oldest)
Terraces
(Platue or Northern Drift)
Hoxnian
Hanborough Terrace
up to 32m above flood-plain
Wolstonian Wolvercote Terrace
up to 16m above flood-plain
Ipswichian Summertown-Radley
up to 8m above flood-plain
Terrace
Devensian Flood-plain
(Northmoor up to 3m above flood-plain
(youngest) Terrace)
The Combe terrace is now left on the top of a watershed ridge between the
Evenlode and the Glyme. Its terrace form and parallelism with the Evenlode
valley are unmistakeable. It is continued by a further outlier on Bladon Heath.
This terrace is unusual for the abundance of large erratics, 15-20 cm long,
mainly quartzite, vein quartz and Millstone Grit.
The Freeland terrace, on the opposite side of the valley and about 10m
lower, has a still more unmistakable terrace form, and is preserved for a width
of a over 1.5 km at its widest and a continuous length of nearly 3 km. The
amphitheatre-like ridge of high Oxford Clay at North Leigh and Perrotshill
represents the last meander cliff of the old river.
The gravel of the Combe and Freeland terraces is much thinner than that of
the oolitic Hanborough and Summertown terraces. It may be doubted
whether the gravel on the Combe and Freeland terraces anywhere exceeds
1.5m in thickness.
The Hanborough terrace represents a great delta of the Evenlode. The
preserved parts are over 4km long by up to 1.5 km wide, and the gravel is
commonly 5m thick. It is the last big gravel deposit to precede the cutting of
the gorge. The Hanborough terrace gravels consist of locally derived pebbles
from the oolitic limestone and material derived from the plateau drift. The
oldest known superficial deposits are of glacial origin; they contain erratic
pebbles and boulders, some of them striated. The Plateau Drift occurs high
on the sides of the valley and on the interfluves.
Since their formation, some of the big incised meanders have since changed
their positions, owing to their narrows having become choked with limestone
gravel during the aggradation that produced the terrace. There are three of
these abandoned incised meanders, all near the lower end of the gorge, at
Combe Sawmill, below Long Hanborough, and at the very mouth of the
gorge.
Near the confluences of the Evenlode and the Thames patches of glacial drift
have protected the underlying Oxford Clay and have led to the formation of
isolated hillocks which in parts are higher than the adjacent plateau of the
Cotswold dip-slope, for example at Purwell Farm and Acre Farm .
Management issues
Figure 12 Management issues in the Evenlode
Management
Actions
Issues
Poor Water
1. Monitoring of Fire
Quality of the
College discharge
Four Shire
Stream
The Impact of
Lake
Construction in
the Upper Glyme
Valley
Channel Habitat
and Water Level
Control on the
River Evenlode –
Impact on
Fishery
2. Pollution Prevention
measures (re oil
spills).
1. Investigate ecological
impact of on line lakes
and recent
unconsented off-line
lakes and identify
possible mitigation
actions.
2. Investigate the high
turbidity levels
experienced in the
Glyme valley
1. Habitat
enhancements, e.g.
creation of backwater
channels and in
channel works.
2. Fisheries and
biological monitoring
of past and future
enhancements.
Progress
Monitoring has been
completed. A new discharge
consent is to be issued
shortly to Moreton-in-theMarsh Fire College.
Completed 1999
Discussions held with the
Estates concerned. One of
them (Glympton Park Estate)
has ceased their duckrearing activities, which were
contributing to the ecological
impacts on the Glyme, and
has begun a programme of
lake and river restoration.
Internal meeting held to
discuss the causes of high
turbidity and possible
mitigation measures.
Past habitat enhancements
undertaken with fisheries
benefits in mind include the
restoration of a backwater at
Combe, and the installation
of flow deflectors at Long
Hanborough. No recent
schemes implemented.
River Corridor Surveys
(RSC) were undertaken on
the mid-Evenlode, at
Hanborough and Cassington,
to further ascertain the status
of the fish population. The
results showed a
predominance of larger
species with few signs of
recruitment. This could be
due to a lack of
cover/juvenile habitat and the
survey has helped identify
several opportunities for
substantial fishery
enhancement – these will be
pursued
3. Identify key
structures,
working in
partnership with
owners to develop
an operating
agreement that
benefits the
aquatic
environment.
This work has not been
progressed as yet
4. Actively
investigate
opportunities to
remove structures
where
appropriate.
Removal of impoundments is
desirable where benefits to
river habitats and processes
will accrue. No recent
opportunities have arisen.
5. Using existing
RCS and
Fisheries Surveys
of River Evenlode
to identify most
degraded habitat
and reaches with
poor fish
population
structure.
The need for a strategic
habitat enhancement
strategy for the River
Evenlode has been identified.
This will enable a costed and
prioritised list of projects to
be drawn up and
systematically implemented.
funding for the strategy
(approx. £20K) was sought in
2002
There are a number of management issues in the Evenlode Basin (Figure
12).
Increased flooding at Moreton-in-the Marsh is related to a relatively new
housing development, within the floodplain.
The poor water quality of the Four Shires Stream downstream of Moreton-inMarsh has given cause for concern due to a marginal failure to comply with
the zinc standard in the EU Dangerous Substances Directive as well as minor
but persistent oil pollution. Nevertheless, the upper reaches of the River
Evenlode are fed with high quality water from springs in the Cotswolds that
means that chemical river quality in this catchment is predominantly classed
as ‘good’.
There are a number of off-line and on-line lakes in the Glyme Valley that have
the potential to deplete flows along the river. They lead to a substantial
increase in evaporation rates, which has resulted in destruction of river
habitats and loss of floodplain, and has increased the level of turbidity in the
river.
Due to the rural nature of the LEAP (Local Environment Agency Plan) area,
one of the key concerns is the effective disposal of farm waste and
application of artificial fertilisers, which if unregulated, can pollute
groundwater. There is currently a Nitrate Sensitive Area at Old Chalford
(650ha) around the source of the River Glyme. It is designated as such in
order to protect the use of groundwater from boreholes for potable supply.
NSAs require farmers in areas around water sources that are high in nitrate,
or expect to be high in nitrate in the future, to limit the timing and volume of
organic manure and inorganic fertiliser put on the land.
Land between Cassington and Yarnton provides for a sand and gravel
extraction area. The gravel pits can provide a facility for coarse and game
fishing in still waters – Blenheim lake is utilised by a large number of anglers.
In terms of after uses, the lakes so far created in the valley already provide a
recreational and ecological resource. Existing after-uses include sailing,
water skiing, fishing and a nature reserve. However, the distribution of these
uses has evolved in a fairly haphazard manner.
The River Evenlode is predominantly a clay catchment and so is ‘flashy’ i.e. it
responds rapidly to rainfall events. This creates problems for small fish that
can get swept downstream in the fast flows. The Environment Agency plans
to improve local habitats to safeguard fry and generally improve the river
habitat for all species, as historically dredging and impoundment for milling
have produced an impoverished habitat for fish.
Conclusion
The Evenlode, like all rivers, is unique. It has many features that are
characteristic of most rivers but it also has features, such as the Gorge, the
terraces and the misfit rivers, that reflect long-term climate change. The river
has been used for centuries by people, and continues to be used. There are a
range of pressures on the river, and a number of plans to manage these
pressures.