| Water is important for human survival and
ecosystem functioning. Water resources can be divided into six interrelated categories:
sources or headwaters; rivers; wetlands; estuaries; groundwater and human-made facilities,
all of which form part of catchments. Catchments in the CMA display a high seasonal
variability, given that the CMA receives most of its rain in the winter months (see Climate). |
|
| Contents of CONTEXTUAL INFO on Water: |
|
|
| Catchments |
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| A catchment is the area that drains water to a common point and is largely
influenced by climate, terrain and geology. Water flowing over sandstone-derived soils has
low nutrient levels, conductivities and pH, while rivers flowing over shales have higher
values. There are 10 major and four minor catchments in the CMA. Map
1 shows their location. |
| Headwaters |
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| The upper reaches of a river are referred to as the headwaters or the
mountain stream zone. Headwaters are generally steep gradient, high-energy (turbulent
flow) systems. Water quality is pure, and in the Western Cape Province, nutrient levels,
conductivities and pH are low (conductivities < 3.0 milliSiemens per metre (mS m-1);
pH < 6). They originate at fairly high altitudes and provide a habitat for rare and
endemic species (Picker and Samways, 1996). In general sources and headwaters of the rivers of the CMA are relatively
undisturbed and in a good ecological state, however, they are sensitive to anthropogenic
disturbance. Several upper catchments within the CMA are planted with pine trees, usually Pinus
pinaster, that reduce runoff. Little documented information exists on the
current state of headwaters of the CMA’s rivers. |
| Rivers |
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| The specific state of the rivers in the CMA varies greatly between
catchments, depending largely on the degree of urbanization within the catchment (Inland
Waters Management Team, 1994). As rivers are longitudinal, unidirectional systems, effects
of stresses accumulate in the middle and lower reaches of the river. Most of the research
undertaken has focused on these reaches. Therefore, in general, quality of the water in
the rivers and streams arising in the mountains in the CMA is good. As streams pass from
the footslopes into lower-lying areas where urban and industrial developments dominate the
landscape, water quality is influenced by the quality of run off arising from these areas.
In 1991, rivers in seven major
catchments in the CMA received effluent from sewage works (Bergman and Partners, 1991). As
a result, many of the rivers of the CMA show signs of organic pollution (measured in terms
of faecal coliform content). The European Union uses a potable standard of zero cells of E.
coli (an indicator species) per 100 ml, while for recreation the standard is 1 000
cells per 100 ml (CCC, 1994, 1996). |
High levels of faecal coliforms usually result from runoff from informal
and inadequately serviced areas, leakages in pipes, overflows from pump stations and
treated and untreated effluent flows from sewage treatment works. There are also less
obvious sources of faecal pollution, such as:
- soil contamination by animal faecal
pollution (e.g. in rural/farming areas);
- runoff from residential areas may also be
contaminated with faecal bacteria and pathogens; and,
- water bodies are occasionally polluted by
people using them specifically for ablution purposes.
The recreational potential of rivers in the
CMA is limited by these pollution levels. Table 1 shows a dramatic
increase in the faecal coliform counts of a number of rivers in the CMA in recent years
(also see Table 4). |
|
| River |
Faecal coliforms/100 ml percentiles* |
1993/4 |
1995/6 |
| Liesbeek
River @ Bishopscourt |
150 |
100 |
| Liesbeek
River @ Hartleyvale |
630 |
2400 |
| Black
River @ Raapenberg Pump Station |
600 000 |
63000 |
| Black
River @ Maitland |
76 000 |
22 000 |
| Black
River mouth |
10 000 |
9000 |
| Elsieskraal
River @ Pinelands |
8000 |
22 000 |
| Vygekraal
River above Athlone Sewage Works |
23 000 |
600 000 |
| Vygekraal
River below Athlone Sewage Works |
88 000 |
110 000 |
| Langa
Canal @ Settlers Way |
26 000 |
2 400 000 |
| Little
Lotus River |
2000 |
6000 |
| Big
Lotus River |
5000 |
39 000 |
|
| * values below which 50% of measurements lie, i.e. this provides a median
value. |
(Source: adapted from CCC, 1994, 1996 by Snaddon, 1998)
|
| Many of the rivers in the CMA suffer from habitat loss due to canalisation
(Table 2). Other forms of riverine habitat destruction affecting
rivers in the CMA include encroachment by farmlands and urban development. Of particular
note are the Eerste, Lourens and Diep rivers that have been affected by farming, and the
Eerste, Liesbeek, and Lourens rivers by urban development. |
Table 2 |
Extent
(in kilometres) of canalisation of rivers in the Cape Metropolitan Area |
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|
River |
Non-canalised river channel (km) |
Canalised section (km) |
Diep |
75 |
1-1.5 |
Mosselbank |
33 |
- |
Klapmuts |
31.5 |
- |
Sout |
14 |
- |
Sir Lowry’s Pass |
13.6 |
- |
Lourens |
20 |
- |
Eerste |
34 |
4 |
Kuils |
28 |
2.8 |
Bottelary |
15 |
- |
Salt |
|
|
Black |
5.3 |
6.8 |
Elsieskraal |
10.7 |
12.8 |
Vygekraal |
5.2 |
3.2 |
Liesbeek |
8.2 |
2.8 |
Keyser |
12.1 |
8 |
Diep/Sand |
22 |
8 |
Silvermine |
12.3 |
- |
Disa |
10.5 |
- |
|
| (Source: Adapted from Bergman and Partners, 1991) |
A report produced by Southern Waters at UCT presents the findings of an
ecological assessment of 37 rivers in the CMA (Day, Harding and Brown, 1999). The 37
rivers assessed fell into four sub-regional categories, namely:
- Mountain stream (7 sub-regions assessed)
- Foothill (16 sub-regions assessed)
- Wetland transitional (37 sub-regions
assessed)
- Lower river (8 sub-regions assessed)
This study was largely a qualitative
assessment of biotic and abiotic factors, which determine the ecological status of rivers
in the CMA. Table 3 below presents the six ecological status
classes to which the sub-regions of rivers were assigned. The classes reflect the degree
of "naturalness" of the rivers.
|
|
| STATUS CLASS |
DESCRIPTION |
| Class 1 |
100% of potential
value; unmodified, natural. |
| Class 2 |
80-99% of
potential value; largely natural with few modifications. A small change in natural
habitats and biota may have taken place, but the assumption is that ecosystem functioning
is essentially unchanged. |
| Class 3 |
60-79% of
potential value; moderately modified. A loss and change of natural habitat and biota has
occurred, but basic ecosystem functioning appears to be predominantly unchanged. |
| Class 4 |
40-59% of
potential value; largely modified. A loss of natural habitat and taxa and a reduction in
basic ecosystem functioning has occurred. |
| Class 5 |
20-39% of
potential value; seriously modified. The loss of natural habitat, taxa and ecosystem
functioning is extensive. |
| Class 6 |
0-19% of
potential value; modifications have reached a critical level and there has been an almost
complete loss of natural habitat and biota. In the worst cases, basic ecosystem
functioning no longer exists. |
|
| * values below which 50% of measurements lie, i.e. this provides a median
value. |
| (Source: Day, Harding and Brown, 1999) |
|
| Ecological status of assessed sites differentiated in terms of their
classifications as:
|
(Source: Day, Harding and Brown, 1999) |
| Figure 1 |
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Mountain Stream |
 |
| A:
Mountain stream Lour1=Lourens River;
HB1/HB2=Hout Bay River; SM1/SM2=Silvermine River; GB1=Grootboschkloof; Diep1=Diep River
(Constantia); ST1=Simonstown River. |
| Figure 2 |
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Foothill river |
 |
| B:
Foothill river SM3=Silvermine River;
SL1=Sir Lowry’s Pass River; SP1=Spaanschemat River; Wlake1=Westlake River;
HB3/HB4=Hout Bay; Lour2/Lour3=Lourens River; GB2=Grootboschkloof; Diep2=Diep River;
Brom=Brommersvlei River; Bur=Burgersbos River; PK1=Prinskasteel River;
Liesb1/Liesb2=Liesbeek River; ST2=Simonstown River; Bok1=Bokramspruit River. |
| Figure 3 |
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Wetland Traditional Rivers |
|
|
| Figure 4 |
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Lowland Rivers |
|
| D1/D2/D3/D4/D5=Diep River (Rietvlei); Mossel4/Mossel5=Mosselbank River;
Eerste/1Eerste2=Eerste River; Liesb3=Liesbeek River; Black1/Black2/Black3=Black River. |
| Estuaries |
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| An estuary is the ecosystem that defines the area where a river meets the
sea. The most important estuaries in the CMA in terms of their nursery function for marine
fish include the Milnerton Lagoon on the Diep River, Sandvlei and the Eerste River
Estuary. There are 11 estuaries around the CMA coastline, namely: |
- Sout
- Diep (Milnerton Lagoon)
- Salt
- Disa
- Wildevoëlvlei
- De Mond
- Silvermine
- Sandvlei (mouth is canalised)
- Eerste
- Lourens
- Steenbras
|
|
| These estuaries vary in terms of their ecological state, depending on
their proximity to urban development. In addition, estuaries also suffer the cumulative
effects of upstream perturbations such as pollution. Most have retained their natural
characteristics, while others, mostly the larger estuaries such as the Diep, Eerste and
Sandvlei, have physically been altered and polluted (Snaddon, 1998). |
| Wetlands |
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Wetlands are valuable habitats, in terms of the role they play in the
maintenance of water quality and flood attenuation (Silberbauer and King, 1991). Wetlands
are efficient assimilators of excess nutrients, particularly phosphates and nitrates due
to the nature of the vegetation and fine sediments. There are many wetlands within the
CMA, ranging in size and state. The larger wetlands or vleis are situated towards the
river mouths and include:
- Zeekoeivlei, Little Princessvlei and
Princessvlei, and Rondevlei (Big Lotus/Little Lotus catchment)
- Zandvlei (Diep/Sand catchment) - also
included in the list of estuaries
- Rietvlei (Diep catchment) - a highly
threatened saltmarsh habitat
- Wildevoëlvlei (Noordhoek catchment)
Seasonal wetlands in the CMA include:
- Blouvlei,
- Noordhoek Saltpan, and
- Pans behind the Cape Flats sewage works,
below Rondevlei, and within the Silvermine Reserve.
Wetlands in the CMA have been the most
affected by pollution, loss of habitat and hydrological and physical alteration. Many of
the small perennial and seasonal wetlands have been filled in or "reclaimed" for
development or badly polluted. Figure 5 provides a class
classification of the larger vleis according to their Habitat Integrity Index. It should
be noted that trophic levels are of greater ecological significance in summer when water
levels are low and temperatures are high.
|
| Figure 5 |
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Classes assigned to estuaries and wetlands in the
Cape Metropolitan Area based on Habitat Integrity Assessment Scores |
|
(Source: Day, Harding and Brown, 1999)
|
| Table 4 below presents water quality measurements
for dissolved oxygen, pH, ammonia, phosphorus and faecal coliform for a selection of
rivers and vleis in the in the CMA from July 1997 to June 1998. |
Table 4 |
Water
quality measurements for a selection of rivers and vleis in the Cape Metropolitan Area |
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|
| Selected
rivers and vleis |
Dissolved oxygen |
pH |
Ammonia |
Phosphorus |
Faecal coliform |
| Sand River (below confluence of Langvlei/Sand canals) |
79 |
7.8 |
0.10 |
0.025 |
3
000 |
| Keysers River (at Military Road, Steenberg) |
52 |
7.4 |
0.04 |
0.019 |
1
100 |
| Westlake River (At corner Chenel and Main Road) |
60 |
7.4 |
0.08 |
0.018 |
270 |
| Zandvlei (vlei centre) |
94 |
8.4 |
0.05 |
0.008 |
100 |
| Zandvlei (outlet channel) |
86 |
8.0 |
- |
- |
- |
| Marina da Gama (at Visitors’ Bureau) |
86 |
8.4 |
- |
- |
80 |
| Big Lotus
River (at 5th Avenue,
Grassy Park) |
110 |
8.4 |
0.38 |
0.352 |
36
000 |
| Little Lotus
River (at 5th Avenue,
Grassy Park) |
94 |
8.3 |
0.06 |
0.071 |
5
000 |
| Zeekoeivlei (in Home Bay) |
120 |
9.7 |
- |
- |
240 |
| Zeeloeivlei (opposite Peninsula Aquatic Club) |
110 |
9.4 |
0.02 |
0.274 |
340 |
| Princessvlei (vlei centre) |
93 |
8.4 |
0.20 |
0.019 |
60 |
| Little
Princessvlei (south point in
vlei) |
60 |
7.7 |
0.11 |
0.025 |
1
000 |
| Liesbeek River (at Winchester Road, Bishopscourt) |
97 |
7.48 |
0.08 |
0.014 |
250 |
| Liesbeek River (opposite Hartleyvale) |
75 |
7.53 |
0.12 |
0.014 |
5
000 |
| Jakkalsvlei
River (near Settlers Way) |
90 |
8.02 |
7.71 |
5.50 |
42
000 |
| Fygekraal
River (upstream of Athlone WWTW) |
67 |
7.83 |
6.25 |
2.38 |
200
000 |
| Fygekraal
River (downstream of Athlone
WWTW) |
64 |
7.66 |
4.13 |
1.85 |
120
000 |
| Black River (near Raapenberg Pump Station) |
47 |
7.59 |
2.21 |
1.46 |
13
000 |
| Black River (Mouth) |
79 |
7.85 |
0.78 |
0.966 |
8
000 |
|
| NOTE: This data
is extracted from the Cape Metropolitan Council Directorate:Water and Waste Annual Report,
July 1997-June 1998. (Source: CMC, 1998a) |
|
| EXPLANATORY NOTE: Dissolved oxygen is essential for all aquatic
life. The amount of oxygen dissolved in water may vary with time of day, season, altitude,
temperature, salt content and other environmental factors. Organisms exposed to dissolved
oxygen levels less than 40% for extended periods will tend to suffer severe adverse
effects.
pH is measured on a scale of 1-14
and indicates how alkaline (pH>7) or acidic (pH<7) the water is. Water with a pH of
7 is neutral. Most aquatic life can tolerate pH in a range from approximately 6 to 8.
Streams flowing through areas vegetated with Fynbos tend to have low pH to which biota has
adapted.
Ammonia usually occurs in low
concentrations in natural waters. It is a common pollutant associated with sewage and
industrial effluents. Ammonia occurs in two chemical forms, namely unionized ammonia,
which can be toxic to aquatic life, and the ammonium ion. The relative amount of the toxic
form increases at high pH and high temperatures. The General Limit for ammonia in effluent
discharges is 2mg/l.
Phosphorus is measured in its
soluble reactive form, which is the portion of total phosphorus available and actively
used by aquatic plants and algae. High levels of phosphorus will tend to promote rapid
plant growth, while concentrations less than 0.005mg/l are considered sufficiently low to
reduce the likelihood of algal or other plant growth. Sources of phosphorus include
detergents, fertilizers and sewage.
Faecal coliform results are depicted
as percentiles, which indicate that 50% of the samples taken have faecal coliform counts
less than and equal to the figure listed for that sample point. The Target Water Quality
Range for "full-contact" recreation (swimming) is 0 to 130 counts per 100 ml and
1 to 1 000 counts per 100 ml for "intermediate-contact" recreation. Above these
levels, the risk of gastrointestinal effects increases. |
|
|
| Groundwater |
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| There are three significant aquifers within the CMA: the Newlands aquifer,
the Atlantis aquifer and the Cape Flats aquifer. The sandy substrate of the Cape Flats and
Atlantis areas has a low filtering efficiency and, as groundwater is recharged by slow
seepage from the surface, this water resource is particularly vulnerable to pollution from
human activities. The water quality
in the Newlands aquifer is relatively good and uncontaminated. Currently, approximately
3.6 x 106 m3 of water per annum is extracted from this aquifer, for
irrigation of sports fields and for the Ohlssons Brewery (Western Cape System Analysis,
1994). It has been identified as a potential source of water for the CMA.
The Cape Flats aquifer is of variable, and
to some extent, unknown, water quality. Variable amounts of water are abstracted on an ad
hoc basis. Illegal dumping of waste at industrial and building sites throughout the Cape
Flats poses a threat to this aquifer.
The Atlantis aquifer is used most
intensively, and the water supply for the town of Atlantis is abstracted from this source.
Thus, most monitoring and exploratory work has been focused here. The Atlantis aquifer
currently yields approximately 5.5 x 106 m3 per annum (Cavé et
al, 1996). The Atlantis aquifer is considered to be polluted, and is threatened by
industrial contamination, in particular by iron precipitation even though the main aquifer
is 2-3 km away from the major industrial area (Snaddon, 1998). |
| Human-made resources |
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| Much of the CMA’s water supply has been diverted from rivers that are
located beyond its boundaries. The water is brought into the CMA via inter-basin
water transfer schemes (IBTs). The Riviersonderend/Berg River Government Water Scheme
regulates the flows from the Riviersonderend, the Berg River and the Eerste River for
urban, industrial and agricultural use (see Table 5). The Palmiet
River Scheme (Phase I) was completed in mid-1998 and has commenced with the transfer of
water from the Palmiet River via the Palmiet Pumped Storage Scheme to the Steenbras Upper
Dam and ultimately to the CMA. During
summer, algal blooms threaten some of the reservoirs in these catchments. The water from
these sources requires extensive treatment before consumption and this is a costly and
time-consuming process. Sewage ponds and reservoirs, although unnatural, provide habitat
for aquatic fauna and flora.
Currently, water supply and utilisation are
measured in terms of quantities. Table 6 shows the water supplied
by the CMC to the MLCs for 1997/1998. The metropolitan meterage system is currently being
extended to measure water consumption and calculate the amount of water that is
unaccounted for. In 1993/1994, figures for the City of Cape Town, which then included the
South Peninsula area, indicated a total consumption of 109 893 155 kl with 22.8% of this
being unaccounted for (Cape Town City Council, City Engineer, 1994).
|
Table 5 |
Supply of
water to the Cape Metropolitan Area, showing supply catchments and reservoirs and the
percentage of supply from July 1993 to June 1996 |
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|
RESERVOIR |
RIVER |
PERCENTAGE OF SUPPLY TO THE CMA |
| 1993/4 |
1994/5 |
1995/6 |
| CMA catchments: |
| - |
Liesbeek
(Albion Springs) |
- |
0.1 |
0.3 |
| *Hely
Hutchinson, Woodhead, De Villiers, Victoria, Alexandra |
Table
Mountain rivers |
1.3 |
1.2 |
1.3 |
| Catchments beyond the CMA: |
| Steenbras |
Steenbras |
13.7 |
10.4 |
11.5 |
| Wemmershoek |
Berg |
21.5 |
21 |
20.2 |
| Voëlvlei |
Berg |
25.6 |
25.6 |
22.2 |
| Theewaterskloof |
Riviersonderend |
37.9 |
41.7 |
44.5 |
|
| * The total volume from
these sources comprises 3.7 x 106 m3 yr-1, compared to
185.9 x 106 m3 yr-1 from catchments beyond the boundary
of the CMA. |
|
(Source: CCC, 1994 and 1996) |
Table 6 |
Water
supplied to local authorities (1 July 1997 to 30 June 1998) by CMC |
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|
Local authority |
Amount
(kl) |
Blaauwberg Municipality |
27
985 557 |
City of Cape Town |
95
160 582 |
Helderberg Municipality |
11
371 496 |
Oostenberg Municipality |
20
310 808 |
South Peninsula Municipality |
42
328 905 |
City of Tygerberg |
82
828 004 |
Total |
279 985 352 |
|
| NOTE: The figures for
new Metropolitan Local Councils include estimates as the separation of the network is not
yet completed. Riparian owners and other private consumers are not included in the above
figures. |
|
(Source: CMC Water Department, 1998) |
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