HydrologicalSciences-Joumal-des Sciences Hydrologiques, 42(2) April 1997
199
Hyporheic temperature patterns within riffles
E. C. EVANS & G. E. PETTS
School of Geography, University of Birmingham, Edgbaston, Birmingham Bl 5 2TT, UK
Abstract This paper outlines the results of a pilot study using data at 12 min
intervals from 20 miniature temperature dataloggers to establish vertical and
longitudinal temperature patterns within a river bed. Data are presented from two
adjacent riffles immediately below the Blithfield Reservoir on the River Blithe,
Staffordshire, UK, for a five day period in July 1994. Hyporheic temperatures were
warmer than those of groundwater, colder than those of surface water and decreased
with depth into the river bed. At the heads of the riffles shallow sites (20 cm depth)
mirrored surface temperature patterns but were on average 1.53°C cooler while
deeper sites (40 cm depth) were 2.60°C cooler on average. Hyporheic temperatures
lagged behind the surface water pattern and lag times increased with depth. Sites at
the riffle tails generally displayed temperature patterns similar to those of the
groundwater system: on average, temperatures were 4.32°C cooler than surface
water temperatures and showed no significant variation. Hyporheic temperature
patterns at the heads of the riffles suggested downwelling surface water, while the
tails appeared to be influenced by upwelling groundwater. Both riffles displayed
similar hyporheic temperature patterns but riffle 1 was on average 1.22°C warmer
per site than riffle 2. Temperature differences between the riffles are attributed to
bed form and substratum composition. These factors may have significant ecological
implications including rates of organic matter decomposition, invertebrate life cycles
and salmonid egg hatching times.
Comportement des températures hyporhéiques dans des rapides
Résumé Cet article expose dans ses grandes lignes les résultats d'une étude pilote
utilisant les données reçues au pas de temps de 12 mn de 20 enregistreurs miniatures
de température, et destinée à reconnaître l'allure des profils de température verticaux
et longitudinaux dans le lit d'un cours d'eau. Les données proviennent de deux
rapides situés immédiatement à l'aval du réservoir de Blithfield situé sur la rivière
Blithe (Staffordshire, Grande Bretagne). Les données ont été recueillies pendant une
période de cinq jours en Juillet 1994. Les températures hyporhéiques étaient plus
chaudes que celles des eaux souterraines, plus froides que celles des eaux superficielles et elles décroissaient avec la profondeur du lit de la rivière. A l'entrée des
rapides les températures des sites peu profonds (20 cm) reflétaient les températures
superficielles mais étaient en moyenne 1.53°C plus fraîches, celles des sites plus
profonds (40 cm) étant en moyenne 2.60°C plus fraîches. Au cours du temps, les
températures hyporhéiques suivaient celles des eaux superficielles avec un retard
augmentant avec la profondeur. Les sites situés à l'exutoire des rapides montraient
un comportement des températures similaire à celui des eaux souterraines, les
températures étant en moyenne 4.32°C plus fraîches que celles des eaux superficielles et ne présentant pas de variations significatives. Le comportement des
températures hyporhéiques à l'entrée des rapides a suggéré un enfoncement des eaux
superficielles, alors que l'exutoire des rapides semble influencé par des résurgences
d'eaux souterraines. Les comportements des températures hyporhéiques des deux
rapides étaient similaires mais, pour deux sites analogues, la température au seuil
numéro 1 était en moyenne 1.22°C plus chaude que la température au seuil
numéro 2. Les différences de température entre les deux rapides ont été attribuées à
la forme du lit et à la nature du substratum. Ces éléments peuvent avoir une
importance du point de vue écologique, en particulier en ce qui concerne la
décomposition de la matière organique, le cycle de vie des invertébrés et la durée
d'incubation des oeufs de salmonidés.
Open for discussion until I October 1997
200
E. C. Evans & G. E. Petts
INTRODUCTION
River beds form a significant component of lotie ecosystems (Ward, 1989) and
represent an important ecotone controlling the flux of energy and materials between
surface and groundwater systems (Gibert et al., 1990). The saturated interstitial
space below the river bed and within the banks that holds a proportion of surface
water is referred to as the hyporheic zone (White, 1993). It has great ecological
significance as a hatchery for salmonid and invertebrate eggs (Hynes, 1983;
Shepherd, 1984), as a refuge for invertebrates and larval fish during spates (Hynes et
al, 1976; Poole & Stewart, 1976), low flows and pollution events (Williams, 1977;
Mestrov & Lattinger-Penko, 1981); and constitutes a prominent area of stream
metabolism (Grimm & Fisher, 1984; Valett et al., 1990). Temperature is a
fundamental biological variable (Hynes, 1970). The thermal regime of the hyporheic
zone regulates ecological processes such as organic matter decomposition, fish egg
incubation and invertebrate diapause. Heat transfer within the hyporheic zone is
complex and controlled not only by the fluxes of water that pass through it but also
by conduction of shortwave solar radiation (Silliman et al, 1995; Hondzo & Stefan,
1994).
Spatially and temporally the thermal regime of the hyporheic zone may be highly
dynamic due to variations in flows, groundwater levels, bed form and sediment
composition (White et al., 1987; Williams, 1989), and to changes of flow patterns
caused by large rocks, logs and seasonal vegetation growth (Cooper, 1965;
Hendricks & White, 1988). Channel features such as riffles may establish
longitudinal hyporheic flow and temperature patterns as demonstrated by flume
experiments (Vaux, 1968; Savant et al, 1987; Thibodeaux & Boyle, 1987). The
sediments control the permeability and porosity of the substratum and may lead to
heterogeneous hyporheic temperatures (Wickett, 1954; Ringler & Hall, 1975; Evans
et al, 1995). Hyporheic temperatures in Britain have been demonstrated to follow a
strong annual cycle with conditions warmer within the river bed compared to the
surface during the winter, cooler in summer, and brief isothermal periods in autumn
and spring (Crisp, 1990; Evans et al, 1995).
This study investigated the influence of riffle-scale bed form and sediment
characteristics on thermal patterns within the hyporheic zone using a regulated river
with constant flow to eliminate variations caused by hydrological changes. To avoid
seasonal influences, this pilot investigation focused on a 5 day period in July 1994
characterizing warm summer conditions with high levels of incoming shortwave
radiation. This period was chosen because the temperature gradients, both vertically
from the water column into the bed and longitudinally within riffles, were likely to
be at a maximum at that time.
SITES
Two adjacent riffles were selected on the River Blithe immediately below the
Blithfield Reservoir, Staffordshire, UK (Fig. 1). A tributary of the River Trent, the
Hyporheic temperature patterns within riffles
201
Fig. 1 The River Blithe below Blithfield Reservoir showing the location of the
monitoring site.
site has a catchment area of 110 km2, the majority of which is under agricultural
production. Blithfield Reservoir has a capacity of 18 x 106 m3, a maximum depth of
14 m and an area of 320 ha, creating one of the largest lowland reservoirs in the UK.
The valley floor is underlain by coarse pebbly alluvial gravel, which is at least
5 m thick. The field period was from 21 to 30 July 1994. The initial five days were
considered the settling period and data therefore were not included in this analysis.
The discharge from the reservoir was a constant 22.7 x 103 m3 a day during the study
period (0.263 m3 s"1). All releases were made via a large stilling pool and a fish farm.
The fish tanks were flushed out two or three times a week causing a rise in discharge
to approximately 30 x 103 m3 per day (0.347 m3 s4) for one hour. The average width
of the channel during the study period was 4.9 m and the average depth of water
over the study riffles was 0.15 m. The riparian zone is rough pasture, there are no
trees or bushes shading the channel, which has low sloping banks and a north-south
orientation.
METHODOLOGY
The study used Tiny talk-Temp™ (Orion Group Ltd.) temperature dataloggers. These
are miniature, self-contained dataloggers, (dimensions 50 x 55 x 33 mm) that can
store 1800 measurements over a range of logging periods. The dataloggers take spot
measurements at a chosen interval and are launched and offloaded directly onto a PC
202
E. C. Evans & G. E. Petts
via a communication cable. The dataloggers used in this study had a working range
of 42°C (-5 to +37°C) and a time constant of approximately 90 s in water. Based on
the manufacturers information [Orion Components (Chichester) Ltd.] the error of the
dataloggers should never exceed 0.17°C in the field range encountered on the River
Blithe. The 20 dataloggers used were calibrated, to identify any variation between
individual units, in a water bath, recording at 12 min intervals for 14 days. The
temperature of the waterbath was adjusted daily within a 10°C range (5-15°C). The
means of the 20 dataloggers ranged by 0.28°C. Variations in maxima, minima and
range were always less than ±0.15°C between all dataloggers. Table 1 displays the
results of the waterbath calibration and the correction factor applied to each unit. The
dataloggers have a peak resolution of 0.16°C at 15°C with negligible variation in
resolution (0.01°C) over the annual range of water temperatures encountered in the
River Blithe.
In the field, the dataloggers were housed in five sampling tubes at each riffle,
located at 1 m intervals longitudinally down the centre of each, two sampling tubes at
the head, two at the tail, and one in the centre (Fig. 2). The tubes were placed just
below the interface of the river bed and covered with adjacent armour layer material
to prevent direct exposure to solar radiation. The site was established to minimize
hyporheic disturbance whilst removing and replacing dataloggers in the river bed.
Sampling tubes consisted of outer tubes which remained permanently in the river bed
and inner tubes, containing dataloggers, which could be easily removed. The inner
and outer tubes were perforated with 10 mm holes around each datalogger housing,
allowing hyporheic water to flow freely around the dataloggers. The outer tubes had
Table 1 Results in degrees Celsius (°C) of the 14 day laboratory calibration of temperature dataloggers.
Datalogger number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Mean figure in water bath
(N = 1680)
9.78
9.61
9.77
9.62
9.83
9.62
9.61
9.63
9.85
9.57
9.82
9.76
9.61
9.69
9.64
9.65
9.81
9.68
9.81
9.71
Mean of all dataloggers
(iV = 20)
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
9.70
Difference
-0.08
+0.09
-0.07
+0.08
-0.13
+0.08
+0.09
+0.07
-0.15
+0.13
-0.12
-0.06
+0.09
+0.01
+0.06
+0.05
-0.11
+0.02
-0.11
-0.01
Hyporheic temperature patterns within riffles
203
0m
1m
2m
3m
4m
Fig. 2 Hyporheic dataloggers 1-9 on riffle 1 at 20 and 40 cm depths with the central
armour layer site at 5 cm depth.
1 mm mesh placed over the holes to inhibit sediment from becoming lodged between
the two tubes, preventing separation. Trials showed that the mesh allowed easy
access to the dataloggers and did not inhibit the free flow of hyporheic water through
the tubes even after several months in the field. "Head" and "tail" dataloggers were
positioned within the sampling tubes corresponding to depths of 20 cm and 40 cm
into the river bed. The "centre" tube contained only one datalogger, at depth 5 cm,
positioned to monitor the armour layer temperature. The inner tubes containing two
dataloggers had separators between the two compartments and at the base, and only
allowed water to flow laterally through the datalogger compartments. The uppermost
compartment of the inner tubes was filled with surrounding bed material to hold the
tubes in position. The inner tubes for the centre riffle sites had separators at the top
and bottom to prevent the vertical intrusion of surface water.
Calibration tests during August 1994 involving dataloggers being placed directly
into the sediment in close proximity to the sampling tubes did not show any appreciable temperature differences. At 20 cm into the river bed, four freely positioned
dataloggers gave mean results that were on average 0.12°C higher than those positioned in the sampling tubes; no lags were evident between patterns. The discrepancy
may be due to the tubes allowing less conduction of solar radiation and avoiding
direct contact between the dataloggers and the sediments. Such minor differences
could be caused by slight inconsistencies in depth of burial or reflect the heterogeneous nature of temperature patterns in the hyporheic zone. Values were virtually
identical between the sampling tubes and the free sediment at 40 cm into the river
bed suggesting that conductive processes may explain the minor differences at the
20 cm depth. The advantages gained by inserting permanent sampling tubes and not
continually disturbing the hyporheic zone were considered of more importance than
such slight temperature variations.
A datalogger was also used to record surface water temperatures. This was
secured in a section of white tube, to reflect as much solar radiation as possible, and
204
E. C. Evans & G. E. Petts
attached to a length of metal driven into the river bed. The tube was fixed midway
down the water column in a position that allowed water to flow constantly through it.
Trials revealed that filamentous algae clogged the tube, retarding flows. A deflector
was positioned 1 m upstream and this worked efficiently over the study period,
preventing excessive fouling of the tube.
Groundwater temperature was recorded by a datalogger positioned in a tube with
a design similar to the hyporheic tubes and located on the vegetated section of a point
bar approximately 10 m away from either riffle. The groundwater tube was sunk
1.5 m into the point bar, approximately 70 cm below the surface water/river bed
interface, using a combination of digging and auguring. The groundwater inner tube
was packed with foam to retard advection of heat down the tube from the surface.
The surface end of the groundwater tube was slightly below ground level and was
covered with turf. The groundwater and hyporheic tubes were inserted two weeks
before sampling began to allow them to settle.
RESULTS
Meteorological data are recorded at the reservoir. Air temperatures on day 1 had a
maximum of 20°C and a range of 4°C. Days 2, 3 and 4 had air temperature maxima
of 24°C and ranges of approximately 12°C with day 5 experiencing the warmest air
temperatures of 27°C and a range of 11°C. Rainfall was 3.9 and 0.8 mm for days 1
and 2 respectively, zero on days 3 and 4, and 0.9 mm on day 5. Wind speed and
directions were described as calm or light northwesterlies. Data from an automatic
weather station positioned adjacent to the study site (Evans et al., in press) recorded
average incoming shortwave solar radiation levels in excess of 200 Wm2 during
daylight hours on all of the study days. Maximum daily values of incoming
shortwave solar radiation were in excess of 750 Wm"2 during the sampling period and
tended to occur during the early afternoon. Surface water temperatures in the river
followed a normal diurnal cycle although daily temperature ranges appeared to be
depressed by the reservoir in view of the high levels of incoming shortwave solar
radiation. The daily temperature range of the water was 0°C before being released
from the reservoir (based on water temperature data from South Staffordshire Water
Company Pic) and rose to 1.8°C at the study site 300 m downstream of the reservoir.
Daily air temperature maxima have a high correlation (r2 = 0.927) with daily surface
water temperature maxima at the site but daily air temperature minima have a very
low correlation (r2 = 0.020) with daily surface water temperature minima, reflecting
the influence of the reservoir (Evans et al., in press).
The average daily mean surface water temperature at the study site during the
five day period of this pilot study was 17.82°C with an average daily temperature
range of 1.80 °C. In comparison, the groundwater temperature pattern was virtually
constant with an average daily mean of 11.20°C and an average daily temperature
range of 0.04°C. All hyporheic temperatures fell between these two extremes and the
temperatures at 20 cm depth were constantly warmer than the corresponding
Hyporheic temperature patterns within riffles
205
Table 2 Summary temperature data (°C) for all sites.
Datalogger
number
Air
Surface
5
14
1
3
10
12
6
8
15
17
2
4
11
13
7
9
16
18
GW
Position on riffle
(R)
N/A
N/A
Armour R1-5 cm
Armour R2-5 cm
Head Rl-20 cm
Head Rl-20 cm
Head R2-20 cm
Head R2-20 cm
Tail Rl-20 cm
Tail Rl-20 cm
Tail R2-20 cm
Tail R2-20 cm
Head Rl-40 cm
Head Rl-40 cm
Head R2-40 cm
Head R2-40 cm
Tail Rl-40 cm
Tail Rl-40 cm
Tail R2-40 cm
Tail R2-40 cm
Point bar-70 cm
Mean
(N = 600)
N/A
17.82
17.60
N/A
16.80
16.80
15.23
16.32
14.00
14.55
12.71
13.01
15.84
15.63
14.70
14.73
13.84
14.42
12.58
12.91
11.20
Maximum
Minimum
27.00
19.30
18.80
N/A
17.20
17.20
15.50
16.90
14.50
14.80
12.80
13.10
16.10
15.80
14.80
14.80
14.10
14.50
12.70
13.00
11.3
10.00
16.90
16.90
N/A
16.60
16.40
15.00
15.90
13.90
14.40
12.50
12.80
15.80
15.50
14.50
14.70
13.80
14.40
12.40
12.80
11.10
Mean daily
range
10.40
1.80
1.40
N/A
0.38
0.44
0.40
0.56
0.18
0.28
0.08
0.14
0.14
0.30
0.18
0.10
0.08
0.06
0.14
0.08
0.04
Maximum
daily range
14.00
2.30
1.90
N/A
0.60
0.60
0.50
0.80
0.60
0.40
0.10
0.20
0.30
0.30
0.30
0.10
0.20
0.10
0.20
0.20
0.20
temperature at 40 cm depth for all sites. However, the hyporheic temperature
gradients varied greatly between sites (Fig. 3). The datalogger at site 14 (armour
layer, riffle 2) failed and was consequently omitted from all calculations. Table 2
displays the summary temperature data for the sites. All daily temperature means at
the study site increased over the study period, as did the air temperatures recorded at
the reservoir. Riffle 2 was appreciably colder than riffle 1 with a mean difference of
1.22°C (discounting armour layer sites). Compared to riffle 1 the head and tail of
riffle 2 were 1.02 and 1.40°C colder respectively. Overall daily hyporheic maxima,
minima and ranges experienced little variation during the study period. There were
no outliers in the data set to affect these descriptive statistics adversely.
The two riffles generally displayed the same longitudinal temperature pattern
which was constant during the study period with little daily variation at either riffle.
Figure 3 clearly demonstrates that the surface temperature pattern is transmitted in a
lagged and buffered form into the hyporheic zone to a depth of at least 20 cm at the
heads of the riffles. In contrast, the tails of the riffles display thermal patterns with
little or no relationship to that of the surface. The temperatures experienced at the
heads of the riffles, 15.76°C on average, are 2.26°C warmer than the mean figure of
13.51°C experienced in the tails of the riffles. Temperature differences between 20
cm depth and 40 cm depth were distinct at the heads of the riffles and virtually undetectable at the tails of the riffles. Temperature ranges tended to decrease with depth
and were generally lower at the tails of the riffles compared to the heads.
E. C. Evans & G. E. Petts
Head of Riffle 1
Sample Tube 1
Saniple Tube 2
20
Surface,
u
_
Site 3
Site 4
Groundwater
26th
27th
28th
29th
Date in July 1994
30th
27th
28th
29th
Date in July 1994
26th
30th
Centre of Riffle 1 Sample Tube 3
19.5
26th
27th
28th
29th
Date in July 1994
30th
Tail of Riffle 1
Sample Tube 4
Sample Tube 5
20
Surfacei
Site 6
L
Site 7
Groundwater
26th
27th
28th
29th
Date in July 1994
30th
Head of Riffle 2
Sample Tube 6
26th
27th
28th
29th
Date in July 1994
Sample Tiibe9
Surface,
<\
26th
26th
Tail of Riffle 2
">0
18
30th
Sample Tube 7
20
30th
27th
28th
29th
Date in July 1994
27th
28th
29th
Date in July 1994
30th
Sample Tube 10
Surface,
t\
n*
M
16
14
Site 15
Groundwater
26th
Site 16
12
Site 17
Groundwater
Site 18
27th
28th
29th
30th
26th
27th
28th
29th
30th
Date in July 1994
Date in July 1994
Fig. 3 Sequence plots of the vertical thermal profiles for all sites during the five day
study period. Note that the central site (tube 3) on riffle 1 is plotted against surface
temperature only and is on a different scale.
Hyporheic temperature patterns within riffles
207
The extension of surface patterns into the hyporheic zone at the riffle heads
allowed lag times to be calculated using cross-correlation functions of the time series
data. Following the calculation of the lags it was possible to align the surface, 20 cm
depth and 40 cm depth temperatures. Table 3 displays the lag and correlation data for
the riffle heads. The time series data drifted and needed to be first differenced to
induce stationarity. This involved the original series being replaced by the
differences between adjacent values. Figure 4 displays an example of the lags
between the surface, site 1 and site 2, the associated cross-correlation functions and
the alignment of the patterns.
Table 3 Correlation and regression data for surface and hyporheic temperature patterns at the riffle
heads after the removal of the lags.
Depth (cm)
-20
-20
-40
-40
-5
-20
-20
-40
-40
-5
Datalogger
number
1
3
2
4
5
10
11
12
13
14
Lag (h)
5.0
5.2
8.4
8.4
0.4
5.8
6.0
8.8
9.2
N/A
Linear regression
with surface (r2)
.765
.820
.545
.557
.943
.757
.699
.216
.549
N/A
S mode Principal Components Analysis (PCA) was used to identify inter-site
correlations for the single variable of temperature over time. Three principal
components (PCs) were extracted from the nineteen variables (site 14 was excluded)
by using Kaiser's criterion selecting the number of Eigenvalues over 1 (Table 4).
This was in agreement with a visual scree test. An arbitrary figure of +0.4 was set
for the significance of factor loadings. The first PC (PCI) displayed the dominant
source of variation with high factor loadings for all deep hyporheic sites, i.e. all sites
bar 5 and the surface. This major source of variance was masking smaller trends in
the data set. The second PC (PC2) displayed high factor loadings for the surface and
armour layer (site 5). The third PC (PC3) exhibited high factor loadings for sites
with a high degree of connection to the surface system i.e. the surface, armour layer
(site 5) and the heads of the riffles at the 20 cm depth level.
Table 4 Statistics on the three Principal Components (PCs) extracted from the 19
variables.
PC
1
2
3
Eigenvalue
8.14307
3.31532
2.72358
% variance
42.9
17.4
14.3
Cumulative %
42.9
60.3
74.6
208
E. C. Evans & G. E. Petts
Cross correlation function of surface with site 1
( 1 st differenced) with peak at lag 25
Sequence plot of surface, site 1 and site 2 with lags
Site 2 lags 8.4 hours and site 1 lags 5 hours behind the surface
i
0.0
| i
1 -
1
niiip *"i w " ••* r" r ""
,i,j..i!uii!iiyy>ii>
-.5
-45 -35 -25 -15
-5
5
15
25
35
45
Lag Number
Confidence Limits WE Coefficient
26th
Cross correlation function of surface with site 2
( 1 st differenced) with peakatlag42
0.0
27th
30Û1
28th
29th
Date in M y 1994
Sequence plot of surface, site 1 and site 2
Surface moved forward 42 lags and site 1 25 lags into alignment
20
i u ,M.II»X
-T 1 r^fyr^|i'||i|f|'|| l y"^ , ''yy"i[I.Ai,"'
^ i i r"
-.5
-45 -35 -25 -15
-5
5
15
25
35
45
Lag Number
28th
29th
30th
Date in M y 1994
Fig. 4 Examples of the time series cross-correlation functions showing the number
of lags between pairs of sites and sequence plots of the time series before and after
the profiles were moved into alignment.
" Confidence Limits
Coefficient
26th
27th
The first PC did not clearly separate the variables because they were closely
interrelated causing complex variable groupings (Table 5). Consequently the axes
were rotated in an attempt to clarify the groupings. Kaiser's Varimax orthogonal
rotation was used to maximize the variance and improve the simple structure
(Fig. 5). The groupings were clarified (Table 5) with PCI representing the sites at
the heads of the riffles and site 8 with lower factor loadings for the 40 cm depth
sites. PC2 grouped groundwater and groundwater-related sites at the tails of the
riffles. PC3 accounted for the 40 cm depth sites at the heads of the riffles and there
was a link to site 9 at 40 cm depth at the tail of riffle 1. Quartimax and Equamax
orthogonal rotations were also executed. The order of principal components and
factor loadings differed slightly but the same groups persisted.
DISCUSSION
Typically, in midsummer the hyporheic zone displayed lower water temperatures
than those of the surface due to the buffering capacity of the substratum which
Hyporheic temperature patterns within riffles
209
Table 5 Principal Component (PC) loadings for the unrotated and Varimax rotation (significant PC
loadings > .4 are shown in bold).
Site
1
2
3
4
5
6
7
8
9
10
11
12
13
15
16
17
18
Surf
GW
Unrotated solution
PC 2
1PC 1
79353
59149
73498
51551
18281
80245
79418
77026
71503
80128
62539
84067
73575
41124
51144
56271
66308
01373
69032
-.10010
-.61000
-.11105
-.65011
.78245
.34681
.11091
-.01362
-.24009
-.13933
-.20648
.15222
-.46108
.37925
.39065
-.00429
.39191
.80599
.39392
PC 3
Varimax rotation
PC 1
PC 2
PC 3
.53581
.00068
.60561
.04129
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-.00609
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-.03840
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.16492
-.04888
.61703
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.62173
-.90233
-.04481
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.01003
.48366
.03853
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.51170
-.30719
-.03528
.26653
-.05256
-.92389
-.08527
.14035
.11373
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.89221
shielded the hyporheic water from direct atmospheric contact. Generally, the data
from the River Blithe followed the same vertical hyporheic temperature patterns as
those outlined by Crisp (1990) and Evans et al. (1995) for unregulated sites in
Britain. The main exception is that Crisp (1990) reported that hyporheic minima
PC 2
0.0
Fig. 5 Principal Components Analysis (PCA) factor plot of the Varimax rotation
showing the site groupings for different sections of the riffles.
210
E. C. Evans & G. E. Petts
were higher than surface minima in Carl Beck. In the River Blithe hyporheic minima
fell below surface minima due to the reservoir maintaining a constant high surface
water temperature.
The hyporheic temperature patterns on the River Blithe displayed distinct differences between riffle heads and tails. The resulting longitudinal temperature pattern
manifested by both riffles was in close agreement with existing conceptual models for
water flow through a riffle (Vaux, 1968; Thibodeaux & Boyle, 1987; Savant et al,
1987). These models have been based upon laboratory flume experiments which
indicate that longitudinally advected water is expected to downwell into the concave
head of the riffle, travel for some distance through the riffle and up well at the convex
foot of the riffle. It is well documented that hyporheic temperatures can be used to
determine the origin of the hyporheic water, as signatures will be left in the
substratum by the daily temperature cycles of surface water or by the fairly stable
thermal regime of groundwater (White et al, 1987; Shepherd et al, 1986; Silliman
& Booth, 1993). Groundwater would be expected to occur nearest the stream bed
surface at the downstream ends of riffles. Lateral variations in hyporheic water
temperature may also occur (White et al, 1987) but were not considered in this
study.
The hyporheic temperature patterns at the heads of the riffles were probably
caused by a combination of factors: advection and convection of thermal energy by
downwelling surface water; conduction of thermal energy caused by shortwave solar
radiation penetrating the water column; and vertical temperature gradients within the
substratum. Previous authors have suggested that shallow, unshaded, gravel bed
rivers which receive large inputs of shortwave solar energy, like the River Blithe,
can experience considerable amounts of bed conduction of thermal energy which may
form a significant component of the energy budget (Comer & Grenney, 1977;
Hondzo & Stefan, 1994). The subdued diurnal hyporheic temperature patterns at the
heads of the riffles lagged temporally behind the diurnal surface temperature pattern.
Lags occurred due to slower water velocities in the hyporheic zone and the time
taken for thermal energy to be conducted through the substratum. Lag times
increased with depth as the mass of sediment to penetrate became larger.
The hyporheic temperatures at the riffle tails are likely to indicate local groundwater upwelling into the hyporheic zone or conduction of thermal energy from the
groundwater system, displaying characteristic thermal stability with little diurnal
response to surface heating. The cooler temperature conditions at the riffle tails also
suggest a groundwater influence, although the lower temperatures may have been
caused by decreased connectivity with the surface system as water flowed through
the riffles. Thermal conductivity values calculated from sedimentological data
displayed only minor differences between the head and tail of riffle 1 demonstrating
that the temperature differences were not attributable to thermal conduction alone.
The sediments on the head and tail of riffle 2 displayed greater differences in thermal
conductivity compared to riffle 1. However, the head of riffle 2 possessed sediments
with lower thermal conductivity values compared to the tail yet experienced higher
hyporheic temperatures. Heterogeneous sedimentology results in an uneven upward
groundwater flow (Godbout & Hynes, 1982) in the same manner that variation in the
Hyporheic temperature patterns within riffles
211
shallower sediments causes irregular surface inputs (Marmonier & Creuze des
Chatelliers, 1991).
Preliminary sedimentological analyses demonstrated a high degree of variability
in substratum composition on a micro-scale with lenses of eroded bank material
present within riffle 2. Sedimentological differences such as these may explain
seemingly anomalous hyporheic temperature patterns such as the closer connection
between sites 9 and 10 in tube 6 and the slight diurnal trend at site 8. Sedimentological factors may have contributed to the temperature differences between the riffle
heads and tails by depressing advective surface flows from travelling longitudinally
through the riffles.
Despite the similarities in longitudinal hyporheic temperature patterns and lag
times exhibited by the two riffles, the actual temperatures experienced on the riffles
differed significantly, 1.22°C per site on average. The surface flow, water depth,
groundwater level and receipt of solar radiation would have been virtually identical
for both riffles. The influence of bank shading was considered to be minimal in view
of the wide channel and low sloping banks. Shallow water depths and turbulent flow
over the riffles discounted the possibility of any temperature gradients forming
within the water column. In the presence of a stable hydrological and meteorological
regime it may be postulated that the temperature differences between the riffles was
caused by heterogeneous bed form and substratum sediment composition. Riffle 1
had higher thermal conductivity and diffusivity values, larger grain sizes and a
higher degree of sorting than riffle 2. Previous studies have suggested that
substratum characteristics can cause significantly different thermal patterns in the
hyporheic zone (Ringler & Hall, 1975; Evans et al, 1995). The sedimentological
composition of the substratum acts as a "mechanical filter" {sensu Vervier et al.,
1992) influencing hyporheic temperature patterns by controlling surface inputs of
thermal energy via conductive, convective and advective processes. In turn the
hydraulic pressures exerted by surface inputs will strongly influence the upward flow
of thermally disparate groundwater. The bed form may exert an important influence
on surface water exchanges with the hyporheic zone through riffle shape and
roughness. The two riffles had a similar shape although minor differences in bed
roughness could have induced convective flows into the bed and may account for the
slight diurnal trend at site 8. Porosity, permeability, grain size, packing, sorting,
composition of framework and matrix sediments and colour will all exert a control
on hyporheic temperature. These factors are the subject of a wider investigation.
Ecological Implications
The longitudinal hyporheic temperature pattern may have a significant influence on
the distribution of aquatic invertebrates as different species have different thermal
optima (Vannote & Sweeney, 1980). The heads of the riffles were approximately 11
degree-days warmer on average than the tails during the study period. On average
hyporheic temperature in the two riffles on the River Blithe were approximately 13
and 19 degree-days cooler than the surface water during the study period.
212
E. C. Evans & G. E. Petts
Invertebrate distributions may be linked to downwelling and upwelling zones which
can differ in chemical and nutrient composition, as well as temperature. Such
differences within an annual cycle may have significance for hatching times of
invertebrate and salmonid eggs. The thermal component may also exert controls on
the water chemistry (Cairns et al., 1975) and detrital decay rates (Cummins, 1974).
River regulation can profoundly effect the thermal regime of the receiving river
and lead to alterations in maxima, minima, range and the timing of seasonal temperature cycles (Lavis & Smith, 1972; Crisp, 1977; Webb & Walling, 1988, 1993).
Previous studies have highlighted the ecological significance of such changes and
linked them to alterations in benthic macroinvertebrate communities (Spence &
Hynes, 1971; Lehmkuhl, 1972; Ward, 1974; Saltveit et al., 1994).
Invertebrates show little ability to acclimate or compensate to temperature
changes and actual temperature ranges may be as important as the temperature values
themselves. Vannote & Sweeney (1980) considered diel temperature fluctuations to
be important in selecting and maintaining a range of insect species. The presence of
Blithfield Reservoir has probably reduced the temperature ranges experienced in the
hyporheic zones at the riffle heads and reduced temperature differences between
surface and hyporheic waters during the day but increased them at night, especially
in areas of groundwater influence. Such ecological implications are the subject of
further investigations.
Acknowledgements C. Evans wishes to thank the Grundy Educational Trust for
financial support during this study. Access to the site and data concerning Blithfield
Reservoir were kindly provided by South Staffordshire Water Company Pic. Thanks
are due to A. Courtnage at Orion Components (Chichester) Ltd. for technical
assistance with the dataloggers and to R. Johnson for field assistance.
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Received 11
accepted
4 September
1996 definition of the hyporheic zone in two Canadian rivers.
Williams,
D.December
D. (1989)1995;
Towards
a biological
and chemical
Freshwat. Biol. 22, 189-208.