Effects of a regulated flow regime on the geomorphology and vegetation of
the Rusape River, Save Basin, Zimbabwe
Faith Love a*, Christopher Chapano b, George Savanhu c†, Elisha Madamombe d and Richard
Owen a
a
Mineral Resources Centre, University of Zimbabwe, PO Box MP167, Mount Pleasant,
Harare, Zimbabwe
b
National Herbarium and Botanic Garden, Box A889, Avondale, Harare, Zimbabwe
c
Department of Geography and Environmental Science, University of Zimbabwe, PO MP
167, Mount Pleasant, Harare, Zimbabwe
d
Data and Research Branch, Zimbabwe National Water Authority, Harare, Zimbabwe
Abstract
The world’s rivers are increasingly being modified through impoundments such as dams and weirs,
which affect the downstream channel and vegetation. This in turn affects the aquatic and riparian
ecosystems as well as the people who depend on the riverine ecosystems for subsistence use. As part
of an environmental flow assessment of the Rusape River, the downstream effects of Rusape Dam on
geomorphology and vegetation were investigated. The downstream channel morphology shows a
multi-threaded channel, islands, a sandy bed and greater erosion, in contrast to the confined channel
with rocky riffles and pools upstream. There is a more diverse topography upstream than downstream.
The lower dynamic zone has increased in width downstream and the tree/shrub zone has decreased
downstream. These changes are likely to be linked to the observed increase in intra-annual floods and
decrease in some inter-annual floods, respectively. Vegetation downstream appears to have been little
affected by the dam, although there is a reduction in the abundance of the reed Phragmites
mauritianus, which could be related to the increased erosion downstream. However, human and
livestock influence on vegetation downstream are also likely, as the area is communal land. The
downstream channel thus has less diverse river resources for the local community, notably the scarcity
of reeds for basket-making and similar crafts.
Keywords: Channel morphology, Geomorphology, Managed releases, Save Basin, Vegetation
1. Introduction
The world’s rivers are increasingly being modified through impoundments such as dams and
weirs, leading to extensive physical and biological changes to the riparian environment
(Stromberg, 2001). It is now well established that such changes are a major factor influencing
the development and maintenance of riverine ecosystems and their diversity (Arthington et al,
1991; Malan and Day, 2003; Molnar et al., 2002). The structure of riparian ecosystems can be
readily disturbed by flow regulation and damming (Shafroth et al., 2002). For example, where
dams cause channel-narrowing, the former channel bed can be colonised by exotic or
indigenous pioneer vegetation (Friedman et al., 1998). Dams also cause changes in the fluviogeomorphic processes, including channel narrowing or a reduction in channel migration rate
(Friedman et al., 1998). Such changes can lead to either siltation or erosion of riparian zones
(World Commission on Dams, 2000).
Environmental flow is the water left in a river, or released into it, for the specific purpose of
managing the condition of that ecosystem (Brown and Joubert, 2003; King et al., 2003). As
part of an environmental flow assessment of the Rusape River, the downstream effects of
Rusape Dam on geomorphology and vegetation were investigated.
*
†
Corresponding author, tel.: +44-1923-350298, email.: fthmbi@yahoo.co.uk
George Savanhu passed away, 15 March 2008
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2. Methods
2.1. Study area
The Rusape is a tributary of the Save, which drains an area of south-eastern Zimbabwe before
flowing into Mozambique. The river rises on Lesapi Source farm 45 km north-east of Rusape
Dam, which has a catchment area of 674 km2, see Figure 1.
Figure 1. The study area, after Love et al. (2006).
Flow in the Rusape river is perennial but highly variable, reaching a peak of around 25 Mm 3
per month in February falling to about 2.5 Mm3 in October. The mean number of days with
zero flow ranged from two above the dam to 12 below it (Love et al., 2006).
Rusape Dam, the only impoundment on the Rusape river was built to supply water to
agricultural estates at Chisumbanje, as well as to the town of Rusape. It now operates in
conjunction with two other large dams in the Save Catchment, Osborne, and Ruti, to supply
water to Chisumbanje and most of this demand is met by releases from Osborne Dam. Thus
water from Rusape Dam is currently the main source of water for Rusape town, and
commercial farmers downstream of the dam.
Major changes in the flow regime of the Rusape River, downstream of Rusape Dam, were
reported by Love et al. (2007). Two main issues can be considered: (i) The large number of
intra-annual floods occurring in the dry season downstream as compared to upstream of the
dam. (ii) The magnitude of the inter-annual floods has been significantly reduced
downstream.
2
2.2. Field data collection
Fieldwork at both sites was carried out at the start of the rainy season. Two transect lines were
made across the river with one transect made on both upstream and downstream sites of the
dam. The first transect, that is 72 m in length is approximately 4 km away from the dam
upstream, this was followed by the second transect 112 m long, is approximately 3 km away
from the dam wall downstream. Along each transect, a topographic survey was made, the
vegetation was recorded and the geomorphology was analysed.
3. Results
3.1. Geomorphology
The Rusape river bed is largely composed of coarse-to-fine sands. This is a result of the
geology: the Rusape catchment is mainly granitic terrain, characterised by rock outcrops that
include domes and castle kopjes, and by sandy soils that are poor in nutrient content. These
are fragile soils susceptible to erosion upon disturbances such as deforestation and
overgrazing.
The Rusape River’s channel pattern is greatly controlled by the structure of the underlying
granitic bedrock. It follows fault-lines for most of its length. Rock outcrops are common
along the river length; forming natural local base levels. Thus the river long profile shows
localised areas of steep gradient with rapids, interspaced by zones of low gradient with
developed pool-riffle sequences. Both transects were made in between rapids.
Examination of the active channel morpohology (figure 2) shows that upstream, the river is in
a confined channel with rocky riffles, rapids and pools with no sandy bed development. The
downstream channel morphology shows a multi-threaded channel, with much larger islands
and a sandy bed. There is greater erosion, in contrast to the upstream, including bank erosion
and gully erosion.
3
Figure 2. Active channel morphology, upstream (left) and downstream (right).
The multi-threaded channel is also apparent downstream in cross-section, leading to wider
active channel and wet bank zones (figure 3). The lower dynamic zone is broader
downstream, and composed entirely of silty deposits, with no rock outcrops, which are
present upstream. The tree/shrub zone is greatly reduced downstream, and is the locus of bank
erosion.
Figure 3. River cross-sectional morphology, upstream (top) and downstream (above).
3.2. Vegetation
The regional vegetation in the area is tropical savanna. The upper reaches of the Rusape River
is dominated by savanna grassland dominated by the species hyparrhenia. The area around
Rusape Town is dominated by tree savanna characterized by the acacia species. Beyond
Rusape Town the natural was savanna woodland but over time this has degraded to tree
savanna (acacia) with isolated pockets of woodland (dominated by the brachystegia
speciformis and julbernadia globiflora species) along river valleys and on residual granite
kopjes.
4
Table 1. Botanical characteristics of the surveys sites. With a few exceptions (identified in the
table), the botanical assemblage was the same upstream and downstream.
Zone
Upstream Site
Downstream Site
Aquatic
This zone was covered by flowing
The active channel is flowing with
zone
water.
some few plant species rooted in
shallow depth close to the banks.
These plant species include
Phragmites mauritianus and
Persicaria senegalensis. Part of the
active channel is now under sandy
deposits, with plant species Salix
mucronata, Cynodon dactylon, and
Schoenoplectus and Syzygium
cordatum present.
Wet bank
Plant species identified on both sides This bank comprised of Hemarthria
zone
of this zone include Eulalia sp.
altissima, Centella asiatica, Cyperus
(forming dense clusters here and
denudatus var. denudatus,
there), Phragmites mauritianus (this Schoenoplectus sp., Persicaria
was growing as a runner into the
senegalensis, Sacciolepis africana,
flowing channel), Ischaemum afrum Lipocarpha chinensis, Fimbristylis
(on small islands on the river),
dichotoma, Paspalum scrobiculatum,
Imperata cylindrica (only recorded
Kyllinga alba, Verbena bonariensis,
here in clusters), Persicaria
Juncus oxycarpus and Pycreus
senegalensis and Thelypteris
flavescens var. flavescens on the left
confluens (grows where there is
side of the channel. The wet zone on
permanent moisture in grasslands or the right side of the channel
on shores of lakes, rivers and dams). comprised of Lipocarpha chinensis,
In addition to this was Cyperus sp.
Centella asiatica, Panicum repens,
and Verbena bonariensis. More
Pycreus polystachyos, Syzygium
species were recorded in this zone
cordatum and Cynodon dactylon.
downstream than here however, at
this site few species recorded were
dense together covering much of the
ground.
Lower
This zone comprised of Combretum
This zone was steep on the left side
dynamic
erythrophyllum that is a riparian
as compared to the right hand side of
zone
species, occurring alongside rivers or the channel. Plant species like Salix
away from rivers where sufficient
mucronata, Sida alba, Cynodon
groundwater is available. Other
dactylon, Vernonia glabra,
species recorded here were
Richardia scabra and Sporobolus
Ledebouria sp., Eriosema
pyramidalis. The right hand side of
psoraleoides, Panicum repens,
the channel had equally the same
Flueggea virosa, Cyperus denudatus species with Syzygium cordatum,
var. denudatus, Salix mucronata
Nuxia oppositifolia, Crinum
(riparian tree), Aeschynomene
macowanii, Senna petersiana,
abyssinica, Carissa bispinosa,
Psidium guajava and Verbena
Trichopteryx sp., Cynodon dactylon, bonariensis as additional.
Mariscus pubens, Vigna unguiculata,
Justicia sp., Rumex sagittatus,
Acacia sieberiana, Asparagus
5
Zone
Tree/shrub
zone
Back
dynamic
zone
Upstream Site
africanus, Verbena bonariensis
Tagetes minuta, Persicaria
senegalensis and Phragmites almost
common in this zone especially on
the right bank.
This zone was made up of Ziziphus
mucronata, Pterocarpus
rotundifolia, Acacia karroo, Acacia
rehmanniana, Tagetes minuta,
Commelina erecta, Acalypha
petiolaris, Lippia javanica, Lannea
edulis, Aloe zebrina, Eragrostis
lappula, Verbena bonariensis,
Eriosema psoraleoides, Hemizygia
bracteosa, and Asparagus virgatus.
Also recorded were Combretum
molle, Hyparrhenia filipendula, and
Euclea crisp.
Downstream Site
This zone on the left side of the
channel has Lantana camara
(recorded downstream only),
Dodonaea viscosa, Syzygium
cordatum, Psidium guajava
(recorded downstream only),
Combretum erythrophyllum, Salix
mucronata and grasses like Perotis
patens, Pogonarthria squarrosa,
Hyparrhenia filipendula, and
Richardia scabra. Other species
recorded here include Senna
petersiana, Mundulea sericea, Rhus
chirindensis, Clerodendron
eriophyllum and Dicerocaryum
senecioides. Other plant species
recorded on the right hand side of the
channel include Ziziphus mucronata,
Senna petersiana, Clerodendron
eriophyllum, Lippia javanica, Acacia
karroo, Flueggea virosa, Rhus
chirindensis and Combretum
erythrophyllum.
Recorded plant species include
Plants recorded on the left side of the
Asparagus africanus, Acacia karroo, channel were Lantana camara,
Diospyros mespiliformis, Diospyros
Dodonaea viscosa, Vitex payos,
lycioides, Senecio oleracea, Carissa Terminalia sericea, Acacia
bispinosa, Portulaca foliosa, Fuirena sieberiana and Rhus longipes var.
sp, Fuirena pachyrrhiza, Cyperus
longipes. The herbs and grasses
sp., Morella sp., Themeda triandra,
include Dicerocaryum senecioides,
Verbena bonariensis, Kyllinga alba, Sida alba, Richardia scabra, and
Eragrostis plana?, Acacia sieberiana Sporobolus pyramidalis. The right
and Pterocarpus rotundifolia. Also
hand side of the channel composed
recorded are Elephantorrhiza
of Dichrostachys cinerea, Senna
goetzei, Dolichos kilimandscharicus, petersiana, Dodonaea viscosa,
Commelina-yellow flowers, Ochna
Acacia sieberiana, Combretum molle
pulchra, Vitex payos, Asparagus sp1, and Mundulea sericea.
Asparagus sp2., Terminalia sericea,
Ziziphus mucronata and Burkea
africana.
Although the diversity of plant species does not appear to be very different between the two
survey sites, the abundance of some species varied. Notably, the mature population of the
reed Phragmites mauritianus was much greater upstream.
6
Discussion
The downstream channel morphology shows a multi-threaded channel, islands, a sandy bed
and greater erosion, in contrast to the confined channel with rocky riffles and pools upstream.
There is a much more diverse topography upstream than downstream.
The lower dynamic zone, which is maintained by intra-annual floods (King et al., 2000), is
greatly extended downstream. This is consistent with the observed increase in number and
magnitude of intra-annual floods downstream compared to upstream (Love et al., 2007).
The tree/shrub zone, which is maintained by 1: 2 to 1 : 20 year floods (King et al., 2000),, is
greatly decreased downstream. This is consistent with the observed downstream decline in
magnitude of 1: 5 to 1 : 20 year floods and decline in number of 1 : 10 to 1 : 20 year floods
(Love et al., 2007). However, the decline in the tree/shrub zone could also have been caused
by the bank erosion taking place in this zone.
There is not much difference in species composition between the upstream site and the
downstream site. However, invasive plant species like Lantana camara and Psidium guajava
were recorded only on the downstream site with Imperata cylindrica and Ischaemum afrum
recorded upstream only. This could be as a result of the fact that this site lies within the
communal area unlike the upstream site that is close to Rusape town. Grazing animals and
human beings could have played an important role in the spread of these invasive plants.
The reduction in the abundance of mature Phragmites mauritianus could be related to the
increased erosion downstream. However, human and livestock influence on vegetation
downstream are also likely. The downstream channel thus has less diverse river resources for
the local community, notably the scarcity of reeds for basket-making and similar crafts.
Acknowledgements
This paper is a contribution to the “Environmental flow requirements for the Rusape River,
downstream of Rusape Dam” project, funded by the Water Research Fund for Southern Africa
(WARFSA, Project 238). The support and cooperation of the Zimbabwe National Water Authority
(especially the Data and Research Branch and the office of the Catchment Manager: Save), made this
study possible. The opinions and results presented in this paper are those of the authors and do not
necessarily represent the donors or participating institutions.
References
Arthington, A. H., King, J. M., O’Keeffe, J.H., Bunn, S. E., Day, J. A., Pusey, B. J.,
Bluhdorn, D. R. Tharme, R. 1991. Development of a Holistic Approach for Assessing
Environmental Flow requirements of Riverine Ecosystems. In: Pigram, J.J., Hooper,
B.P. (eds.) Water Allocation for the Environment: Proceedings of an International
Seminar and Workshop. Centre for Water Policy Research, University of New
England, Armidale.
Brown, C. and Joubert, A. 2003. Using multicriteria analysis to develop environmental flow
scenarios for rivers targeted for water resource management. Water SA, 29, 365-374.
Friedman, J.M., Osterkamp, W.R., Scott, M.L. and Auble, G.T. 1998. Downstream effects of
dams on channel geometry and bottomland vegetation: Regional patterns in the great
plains. Wetlands, 18, 619-633.
King, J. M., Tharme, R, E., De Villiers, M, S. and 2000. Environmental Flow Assessments for
River: Manual for the Building Block Methodology. Water Research Commission
Report, TT 131/00, Pretoria.
7
King, J., Brown, C. and Sabet, H., 2003. A scenario-based approach to environmental flow
assessments for rivers. River Research and Applications, 19, 619-639.
Love, F., Madamombe, E., Marshall, B. and Kaseke, E. 2006. Estimating environmental flow
requirements of the Rusape River, Zimbabwe. Physics and Chemistry of the Earth, 31,
864-869.
Love, F., Madamombe, E., Owen, R. and Chikomo, T. 2007. Flood analyses of the Rusape
River, Save Basin, Zimbabwe. 8th WaterNet-WARFSA-GWP Symposium, Zambia,
November 2007.
Malan, H. and Day, J. 2003. Linking flow, water quality and potential effects on aquatic biota
within the reserve determination process. Water SA, 29, 297-304.
Molnar, P., Burlando, P. and Ruf, W. 2002. Integrated catchment assessment of riverine
landscape dynamics. Aquatic Sciences, 64, 129-140.
Shafroth, P.B., Stromberg, J.C. and Patten, D.T. 2002. Riparian vegetation response to altered
disturbance and stress regimes. Ecological Applications, 12, 107-123.
Stromberg, J.C. 2001. Restoration of riparian vegetation in the south-western United States:
importance of flow regimes and fluvial dynamism. Journal of Arid Environments, 49,
17-34.
World Commission on Dams, 2000. Dams and development: a new framework for decisionmaking: the Report on the World Commission on Dams. Earthscan, London.
8