The Durban Metropolitan Area has 14 rivers and is fortunate
to receive high quality potable water which is mainly taken
from the upper Umgeni river. The coastal reaches of several
DMA rivers suffer from eutrophication (nutrient enrichment).
Most, if not all, of the DMA's rivers are contaminated with
bacteria originating from untreated sewage arising
from poorly serviced settlements or broken and
blocked sewerage pipes. As a result, many of the DMA's
estuaries are, at times, unfit for contact recreation. There
are some large valley marshlands in the coastal reaches
of the DMA's rivers and these aquatic systems perform
valuable self-purification and ecological functions within the
river system (eg the Ohlanga flood plain). Durban Bay, a
marine dominated system associated with the Port, is the
largest and most important estuary which still supports a wide
range of marine species and habitats.
Rivers
|
Durban's River
Systems (S to
N) |
|
Msimbazi
Lovu
Little
Amanzimtoti
Amanzimtoti
Isipingo
Mbokodweni
Umlaas |
Umhlatuzana
Amanzimnyama
Umbilo
Umgeni
Ohlanga
Umdloti
Tongaat |
The
Durban Metropolitan Area has 14
rivers of which the most significant resource is the
Umgeni River. Most of these rivers are sources of potable
water. Recreation is the main use where rivers extend into
estuaries. In a few poorly serviced areas, some residents
still rely directly on streams for their daily water
supply.
The functioning of many of these
rivers has been heavily modified. Several of the
Metro rivers (eg Umdloti, Ohlanga, Umgeni, Umbilo,
Umhlatuzana, Umlaas, Isipingo and Little Amanzimtoti) receive
sewage effluent after treatment. This means that these river
waters are exposed to contamination, especially nutrient
enrichment (eutrophication) even if uniform effluent standards
have been met. The extent of the impact is a measure of the
environmental capacity of the river to assimilate the
pollutant load from these point sources and from runoff from
non-point sources. The excessive growth of water hyacinth
(Eicchornia crassipes) or water lettuce (Pistia
stratiotes) and sometimes algal blooms in the coastal
reaches is a visible manifestation of the imbalance in some of
these systems. Typically, ambient oxygen concentrations are
also reduced and there is a measurable shift in the Nitrogen/
Phosphorus ratio from 14 to less than 8 which is an
indication of environmental
stress.
Recent water quality data sets show
that this kind of response has occurred in the Umdloti,
Ohlanga, Umgeni, Umbilo, Umlaas, Isipingo and Little
Amanzimtoti to a greater or lesser extent at certain times of
the year. Evidence of an enriched condition will be more
pronounced where the base flow of the river is dominated by
the continuous flow from the waste water works - known as the
return flow volume ( ie water returned to a river from waste
water treatment plants). Such a situation pertains during
winter low flows in the Little Amanzimtoti, Umbilo, and
Ohlanga and potentially in the Umgeni, Umlaas and Umdloti
through flow regulation in the main channel downstream of the
Inanda, Shongweni and Hazlemere dams respectively. The
distance between the waste water works discharge and the
estuary is often insufficient to allow the river to immobilise
the pollutants and perform the so called "ecological service
function" adequately - hence the estuarine reaches become the
primary repository for land derived pollutants.
Click here
to view a table of the main characteristics and uses
of the rivers in the DMA.
Potable water supply
The Durban Metropolitan Area receives
high quality potable water from the Umgeni River. Much of the
metropolitan area is served by adequate modern water supply
infrastructure which is operated efficiently and safely.
Responsibility for bulk water supply for the Durban
Metropolitan Area (DMA) lies with Umgeni Water, a statutory
body. Most of the water stored to supply Durban (98%) is
located outside the metropolitan boundaries. This means that
the DMA does not have direct control of its most critical
water resources. Future water needs for the DMA may be in
direct competition with the neighbouring capital city of
Pietermaritzburg and other agricultural users.
Wetlands
There are some unique wetland areas
in the DMA and these are located in the Umgeni River mouth
(Beachwood Mangroves), the Durban Bay (Bayhead Mangroves) and
the mangrove stands at Isipingo. There are, however, some
other marshlands in the river valleys, such as the Ohlanga
flood plain. These collectively perform valuable ecological
functions in reducing flooding and improving water quality
downstream. They also provide a variety of habitats and
generate resources such as reeds and other commercially useful
plants for home industry applications.
Estuaries
Most DMA rivers enter the Indian Ocean
via estuarine or coastal lagoons. These provide attractive
opportunities for passive and active recreational activities.
Several estuaries are exposed to conditions in which high
bacterial contamination occurs and can, at times, be
unsuitable for recreational use.
The largest and most valuable of all these systems is
Durban Bay (covering an area at full tide of approximately 2000ha). This
is regarded as the most important estuarine system on
the eastern seaboard. Like many of the smaller estuaries it
provides an important primary conservation function as
a refuge and nursery area for marine species, while
also sustaining a most critical business enterprise - the
functioning of the busiest port in Africa.
The SA coast is extremely open, with
high wave action and little shelter and therefore
many marine species are restricted to estuarine areas for
protection from these harsh conditions. In the
KwaZulu-Natal (Eastern seaboard) context, Kosi Bay, St Lucia, Richards Bay and Durban
Bay are the major systems but are separated by great distances and
are very different. Durban Bay is one of the few, if not the
only large estuary to have permanent access to the sea. It is
the last of the large subtropical systems (going south) and
possesses the only sheltered and permanently
intertidal sandbanks of KwaZulu-Natal. Furthermore the species diversity
in Durban Bay was greater than any of the other larger
systems. Surveys by Day & Morgans in 1956 and more
recently (Breen et al 1993) show that Durban bay is still a
vital system. Forbes et al (1998) state that Durban Bay still
has "flagship status as an estuarine environment in terms of
biological richness in central KwaZulu-Natal".
The 70 other estuaries between
Mozambique and Mtamvuna are small and frequently closed and
degraded (Begg, 1978, 1984). The Tugela and Umkomaas are more
riverine in their configuration and behaviour as strong fresh
water flows mean that there is little tidal influence within
the riverine system.
Rivers and stormwater canals discharge
bacterially contaminated water into the Bay. In addition, oil
spills from harbour activities pollute the water. Despite
a long history of contamination from urban and industrial
activities, the quality of the water in Durban Bay is
sufficiently good to sustain aquatic life and is safe for
recreational activities. This is largely because of the marine
tidal exchanges through the harbour mouth. These exchanges
enable the water to mix and purge excessive pollution through
self-purification processes. One of the most important
habitats recognised in the Durban Bay is the stand of Bayhead
mangroves, which is now a Natural Heritage Site by
agreement with the Port authorities. The sediments of the Bay
are, however, contaminated with heavy metals to a degree. The
risk of uptake and negative impacts on aquatic life of the
concentration of such metals is not well
understood.
