Groundwater, biogeomorphic succession and controls on river channel pattern

Abstract

Strong feedbacks between river braiding and vegetation processes are now well-recognised. Recently, this has been illustrated in the notion of biogeomorphic succession, the transition from pioneer vegetation establishment to a fully-developed floodplain forest ecosystem. This succession also results in important vegetation-mediated feedbacks, through bank stabilisation and the capture of organic matter and fine sediments, stimulating soil formation and further enhancing the succession process itself. However, there are few studies that have addressed what this succession might mean for the evolution of channel planform, and almost no studies that have considered how this succession rates might be mediated by groundwater. The latter is a key concern for gravel-bed rivers with low water retention capacity. Here, we present results from a 2 km length of braiding-wandering river system in Switzerland (Allondon River). We show that the spatio-temporal dynamics of the groundwater table drives the biogeomorphic succession process at different rates, leading to very different river channel pattern responses. In the upper braiding-anastomosing part of the reach, the groundwater table is deeper. Here, dendrochronological data show that rates of pioneer vegetation growth are strongly dependent upon groundwater table fluctuations. Bank resistance modelling shows that vegetation-reinforcement of bank resistance is below its maximum. In the meandering lower part of the reach, with a mature floodplain forest, tree growth rates are independent of groundwater fluctuations, because trees can almost always access the higher groundwater table. Bank resistance is at its maximum. Through time, in response to disturbance frequency, the meandering tendency has migrated upstream. Thus, our results suggest that groundwater access modulates biogeomorphic succession processes in ways that determine the resultant river channel pattern.

Full text

1957
? ? ?
B
C
D
a b
-2
-1.6
-1.2
-0.8
-0.4
0
0.4
0.8
1.2
1.6
2
0
10
20
30
40
50
60
70
80
90
100
500 1000 1500 2000
Significant rates of change into shurbs/forest
vegeta on cover (%/yr.)
Shrubs/forest vegeta on cover (%)
Downstream distance (m)
1957
1972
1980
1991
1996
2012
Rate
1957-2012
ABCD
1980 1985 1990 1995 2000 2005 2010 2015
0
100
200
300
Years
Ring width index
Fraxinus Quercus
1980 1985 1990 1995 2000 2005 2010 2015
0
100
200
300
Years
Ring width index
Salix A Salix B Salix C Salix D Salix tot Pointer years
a b
1986 2012
1836 1957 1972 1980 1986 1991 1996 2001 2006 2012
Min groundwater hydrological year
CorInd = 73.8 CorInd
w
= 57.2
Min groundwater hydrological year
CorInd = 53.3 CorInd
w
= 40.0
Min groundwater hydrological year
CorInd = 43.9 CorInd
w
= 32.1
Min groundwater hydrological year
CorInd = 13.7 CorInd
w
= 08.1
Max discharge vegeta on period
CorInd = 92.8 CorInd
w
= 79.4
Max discharge vegeta on period
CorInd = 62.2 CorInd
w
= 48.1
Max discharge vegeta on period
CorInd = 41.7 CorInd
w
= 29.2
Max discharge vegeta on period
CorInd = 00.0 CorInd
w
= 00.0
Total precipita ons summer
CorInd = 57.1 CorInd
w
= 45.1
Total precipita ons summer
CorInd = 41.1 CorInd
w
= 29.3
Total precipita ons summer
CorInd = 08.8 CorInd
w
= 05.3
Total precipita ons summer
CorInd = 02.2 CorInd
w
= 01.6
5
7
9
11
13
15
17
5
7
9
11
13
15
17
7
11
15
19
23
27
31
7
11
15
19
23
27
7
11
15
19
23
27
31
5
7
9
11
13
15
17
5
7
9
11
13
15
17
5
7
9
11
13
15
17
19
23
21
5
7
9
11
13
15
17
19
23
21
5
7
9
11
13
15
17
19
23
21
5
7
9
11
13
15
17
5
7
9
11
13
15
17
A B C D
Chronology
Variable
Chronology
Variable
Chronology
Variable
Chronology
Variable
Chronology
Variable
Chronology
Variable
Chronology
Variable
Chronology
Variable
Chronology
Variable
Chronology
Variable
Chronology
Variable
Chronology
Variable
r
Groundwater - maximum vegeta on period (March-Oct.) CorInd 54.8 44.4 11.0 15.4 -0.846
CorInd
w40.2 30.0 6.2 8.1 -0.869
Groundwater - maximum summer (May-Sept.) CorInd 54.8 37.8 11.0 2.7 -0.947
CorInd
w40.2 25.3 6.2 1.7 -0.934
Groundwater - minimum hydrological year (Oct.-Sept.) CorInd 73.8 53.3 43.9 13.7 -0.997
CorIndw57.2 40.0 32.1 8.1 -0.997
Groundwater - minimum vegeta on period (March-Oct.) CorInd 69.0 40.0 52.7 4.9 -0.907
CorInd
w53.2 28.7 38.0 2.6 -0.915
Groundwater - minimum summer (May-Sept.) CorInd 66.7 41.1 46.1 3.3 -0.949
CorIndw51.0 29.0 31.7 1.6 -0.958
Groundwater - mean hydrological year (Oct.-Sept.) CorInd 64.3 45.0 39.6 30.8 -0.942
CorIndw49.4 32.2 28.0 17.2 -0.962
Groundwater - mean vegeta on period (March-Oct.) CorInd 80.9 60.0 26.4 23.1 -0.914
CorInd
w66.8 44.4 18.1 11.9 -0.932
Groundwater - mean summer (May-Sept.) CorInd 57.1 40.0 15.4 3.8 -0.964
CorInd
w44.3 29.1 10.2 2.1 -0.958
Discharge - maximum hydrological year (Oct.-Sept.) CorInd 16.7 5.6 11.0 2.9 -0.789
CorIndw12.3 4.1 7.0 2.5 -0.803
Discharge - maximum vegeta on period (March-Oct.) CorInd 92.8 62.2 41.7 0.0 -0.999
CorInd
w79.4 48.1 29.2 0.0 -0.991
Discharge - maximum summer (May-Sept.) CorInd 4.8 20.0 17.6 43.5 0.954
CorIndw3.8 11.5 13.6 25.9 0.990
Discharge - minimum hydrological year (Oct.-Sept.) CorInd 45.2 28.9 6.6 0.0 -0.944
CorInd
w33.5 19.1 3.9 0.0 -0.933
Discharge - minimum vegeta on period (March-Oct.) CorInd 42.9 28.9 0.0 0.0 -0.892
CorInd
w32.1 19.6 0.0 0.0 -0.893
Discharge - minimum summer (May-Sept.) CorInd 40.5 17.8 0.0 0.0 -0.878
CorIndw28.1 11.4 0.0 0.0 -0.872
Precipita ons - total vegeta on period (March-Oct.) CorInd 54.8 33.3 26.4 1.1 -0.992
CorInd
w42.3 23.8 15.8 0.8 -0.983
Precipita ons -total summer (May-Sept.) CorInd 57.1 41.1 8.8 2.2 -0.933
CorInd
w45.1 29.3 5.3 1.6 -0.924
Temperature (mul plied by-1) - average vegeta on period (March-Oct.) CorInd 0.0 27.8 13.2 30.8 0.731
CorIndw0.0 17.5 6.8 17.6 0.660
Temperature (mul plied by-1) - average summer (May-Sept.) CorInd 7.1 18.9 13.2 22.5 0.807
CorInd
w6.2 10.7 10.3 11.7 0.835
A B C D
Institute of Earth
Surface Dynamics
N. Bätz(1), P. Colombini(1), P. Cherubini(2), S.N. Lane(1) Contact: Nico.Baetz@unil.ch
(1) Institute of Earth Surface Dynamics; University of Lausanne; Switzerland; (2) Dendroecology Research Group; WSL Swiss Federal Institute for Forest, Snow and Landscape Research; Switzerland.
Groundwater, biogeomorphic succession and controls on river channel pattern
ID 71980
-
2007; Zanoni et al., 2008).
•They may also be controlled by groundwater dynamics (Harner and Stanford, 2003).
•-
through groundwater-mediated changes in biogeomorphic feedback intensity.
is slow and dynamic, while downstream it is generally weaker.
(upstream) 1991 to 1996; B 1980 to 1991; C 1972 to 1980; D (downstream) highly vegetated
throughout the period.
3- Climate and groundwater dynamics - Fig. 3
-
dal pluvio-nival regime to a unimodal pluvial regime (Fig. 3a).
(shallow, steady, upwelling) gradient in the water table (Fig. 3b).
Fig.1:
zone A, B, C, D shown in Fig. 2.
Fig. 2: -
-
mits zones where dendroecological samples were taken (Fig. 4 and Fig. 5).
Fig. 3 (above) a:
Tab. 1:
different environmental variables (Fig. 6) along the four sampled pioneer fluvial landforms (A, B, C, D –
-
Fig. 5 a: Colored lines show the Salix sp. chronologies for the units A, B, C, D (color-code as for Fig. 4).
b
which all chronologies should have responded to while the boxes corrected years (double/missing rings).
-
encroachment where the groundwater is shallower.
•Salix sp. plants were sampled on pioneer fluvial landforms in zones A through D (Fig. 2 and Fig. 4).
•Tree ring widths were measured (Lintab6/TSAP) and crossdated using standard methods (Fig. 5a).
•Double and missing ring were corrected by comparing the derived chronologies with the Quercus sp.,
Fraxinus sp. and overall Salix sp. chronologies growing on stable terraces (Fig. 4 and Fig. 5b).
groundwater level measurements at unit A were available. Data were aggregated into different pe-
riods and indices (see list of variables Tab. 1)
•Derived environmental variables were compared with the four Salix sp. chronologies (A, B, C, D) using
total length of the chronology. Only significant values were considered (Fig 6).
over all the scales (total number of windows), while CorIndw weights the CorInd based on the mean
Dendroecological analysis (Tab. 1) shows that the Salix sp. dependence on groundwater decreases
6- Synthesis
•During the last six decades, a decrease in disturbance (flow regime change from bimodal pluvio-nival
-
water guaranteed.
•Thus,
-
gement of sampled trees
for dendroecological analy-
sis. The colors correspond
to the sampled Salix sp.
plants growing on pioneer
landforms in unit A (o-
range), B (yellow), C (light
green) and D (dark green).
In grey Quercus sp., Fraxi-
nus sp. and Salix sp.
growing on terraces to
which the colored Salix sp.
have been compared
lines correspond to zone A,
B, C, D shown in Fig. 2.
Fig. 6:
-
A
Rerefercens
Bätz N, Iorgulescu I, Lane SN. in prep.. The Allondon River: an evolving piedmont river system. In Landscapes and Lanforms of Switzerland , Reynard E (ed). Springer;
Bätz N, Verrecchia EP, Lane SN. 2015. The role of soil in vegetated gravelly river braid plains: more than just a passive response? Earth Surface Processes and Landforms 40 : 143–156.
Bertoldi W, Gurnell AM, Surian N, Tockner K, Zanoni L, Ziliani L, Zolezzi G. 2009. Understanding reference processes: linkages between river flows, sediment dynamics and vegetated landforms
Data sources
Climate data have been provided by MeteoSwiss; Groundwater data by the Cantonal Geology department of Geneva; Discharge data by the Swiss Federal Environmental Agency; Historical aerial
(Bätz et al., in prep.), im-
-
turbance frequency.
(above) b: Groundwater
table depths along the stu-
died river reach (based on
about 3.2m (2std) around
the mean stage. Moreover,
-
pioneer fluvial landforms
for dendroecological ana-
lysis (Fig. 2 and Fig. 4).