Mgeni Estuary
- General Information and location
- Catchment Characteristics
- Land and Estuary use
- Ecology
- Water Quality
- References
General Information and location
Synonyms – Umgeni is the most commonly used synonym. The Mgeni estuary is also called the “Blue lagoon” which is most certainly a misnomer.
Zulu meaning – “Mgeni means the river that flows through the Acacia or thorn trees”
Location
The general location of the Mgeni Mouth is 29°48’S:30°02’E, approximately 5km north of the centre of Durban. The river is 230km long, with a catchment area of approximately 5 000 km2 and a mean annual run-off of 707 mil.cub.m”.
Catchment Characteristics
Main Rivers and tributaries
The Mgeni River rises in the Natal Midlands and has been largely modified by human activities, mainly intensive agriculture, forestry and urban settlements.
Catchment characteristics: “AREA – Various references provided a range from 4 385 km2 to 5 850 km2. RIVER LENGTH – 230 km to 235 km.
Dams
| Dam | Constructed | Capacity |
|---|---|---|
| Henley Dam | 1942 | |
| Nagle Dam | 1950 | 20,8 x 106m3 |
| Midmar Dam | 1963 | 172 x 106m3 |
| Albert Falls Dam | 1975 | 261 x 106m3 |
| Inanda Dam | 241,7 x 106m3 |
River run-off
Mean annual run-off – 707 x 106m3
Geomorphology
The original source of the pink sands in the Mgeni Estuary, is supposedly the Archaean granites in the Valley of a thousand hills. The bedrock below the estuary is weathered and fractured shale, with dolomite intrusions.
Geology
In the catchment, the river traverses a whole variety of rocks of different geological characteristics. The Archaean granites (in the Valley of a Thousand Hills) are said to be the source of the pink sands in the Estuary. Orme described bedrock below the estuary as shale which was “fractured and weathered towards the surface with some dolerite intrusions. At the Athlone bridge, bedrock was found to be 52 m below mean sea level, although reported to be at 67 m below mean sea level at another locality. At the Connaught bidge bedrock lay at a depth in excess of 28 m below mean sea level.
Land and Estuary Uses
Agricultural
Sugar, fodder crops, tropical fruits, beef and cattle farming are the main agricultural activities in the catchment.
Recreation
“The Mgeni estuary is a popular recreational centre used for angling, bait collection and the occasional water sport event (such as the Mgeni raft race and the ‘Duzi canoe marathon. Sand mining may feature as a use to which the estuary is put in the future)”
Conservation
The estuary also functions as a means for effluent and storm-water disposal.
The Beachwood nature reserve serves as a scientific and educational facility and nature trails through the swamp are planned.
Ecology
Flora
The mangrove community of the Beachwood nature reserve, is the only feature of real botanical significance which remains around the Mgeni estuary. This is largely as a result of urban encroachment in the form of recreational facilities on the southern bank. A fair proportion of the reserve itself is a shooting range. The islands in the center of the Mgeni estuary are as yet undeveloped.
Halophytes
Of the 50 ha set aside as the Beachwood nature reserve, 12,14 ha are undermangroves. The two common species are the White (Avicennia) and Black (Bruguiera) mangrove. Transplantation of specimens of the Red (Rhizophora) mangrove from Durban Bay and Ceriops from Kosi Bay have been made. The introduction of Lumnitzera is envisaged. The fresh water mangrove (Barringtonia) has also been introduced into the reserve. Amongst the mangrove community, are various salt tolerant species of grasses, reeds and rushes. The plant community as a whole supports a very characteristic and interesting estuarine adapted fauna, dependent on the rich supply of organic detritus which the swamp provides. Because of the educational value of this area (by virtue of its proximity to the main centers of education in Natal) it has been set as a nature reserve, to protect the area from the pressures which formerly threatened it (such as reclamation, “Disneyland” development, uncontrolled public access, sand removal, etc.).The Natal Parks Board, presently in control of the area, are restoring the swamp. This involves among other things, the constuction of gabions, dune stabilization, fencing, dredging and debris removal. Other suggestions have included the building of a weir which could be closed when necessary to exclude the Mgeni should it be polluted, excessively silt laden or in some other way a threat to the nursery function of the reserve. The use of high pressure hoses to wash away sand which has accumulated amongst the root masses of the mangroves has also been suggested.
Aquatic Vegetation
Aquatic flora species spotted in the estuarine area of the Mgeni, are sea-lettuce (Ulva), during winter. Water hyacinth (Eichornia) and eelgrass (Vallisneria).Conditions are reported as too turbid for the development of Zostera and Ruppia, however blooms of sea-lettuce (a large marine alga named Ulva) have been reported in winter. Water hyacinth (Eichornia) has been reported amongst the mangroves in the past, and in the extreme north of the reserve, water conditions are sufficiently fresh for Vallisneria (eelgrass) and Savinia (Kariba weed) to develop.
Terrestrial Vegetation
The most significant botanical feature of the Mgeni estuary is the Mangroves in the Beachwood nature reserve. The 2 most common mangrove species are the White mangroves (Avicennia) and the Black mangroves (Bruguiera) amongst which there are various salt tolerant grass, reed and rush species.
Fauna
Zooplankton
Studies of the zooplankton of the lower reaches of Beachwood concluded that conditions in the creek did not favour the survival of estuarine and marine forms of zooplankton. Fresh water forms of zooplankton were dominant. Crab, fish and prawn larvae were the most abundant meroplankton forms.
Benthos
In 1972, 16 species of benthic animals were found to be present in the estuary, dominated by the polychaetes Ceratonereis and Desdemona (both being tolerant of low oxygen conditions). The lack of amphipods, isopods and tanaids was considered to be striking, since these forms normally comprise a considerable part of the biomass in Natal’s estuaries. This is because they are probably less tolerant of pollution. Production was limited to the top two centimeters of sediment, below which the sediment was anaerobic. At the head of the estuary, some oligochaetes and entomostraca (Pseudodiaptomus and Cyclops) occur, indicating the influence of fresh water. Attention has been given to the distribution and occurrence of fiddler (four species of Uca) and gastropod mollusks, in Beachwood swamp. In 1964 Brown found 10 species of gastropods (out of the 13 species listed as occurring in South African mangroves) present. This represented the richest assemblage known at any one locality. The greatest densities were 370/sq metre. Brown concluded that the area might support populations ranging from 200 000 Cassidula to 8 million Assiminea. Beachwood is the type locality for Neritina natalensis. Gastropod densities varied according to texture and moisture content of the substrate as well as the amount of shade. The same sort of factors govern the distribution of fiddler crabs. The mangrove whelk (Terebralia) has since been introduced from Durban Bay.
Epifauna
In the estuary, animal communities of the marginal vegetation were poorly developed. Thirteen species typical of estuarine conditions were found, these being markedly different in May (when fresh water influenced) and in August (when more typical of brack water forms). Above the estuary the fauna of the marginal vegetation is slightly better developed, with 18 species listed as present 2,5 km above the estuary in 1965. Of the larger crustacea in the Beachwood swamp, shrimps (Caridina) are said to be dominant, particularly in spring when thousands of juveniles are said to be present. Two species of prawns occur in the estuary, but they are not common.
Fish
30 Species of fish (mainly mullet and “glassies” species) have been reported in the Mgeni estuary. 5 Species of fish are reported to breed in the estuary and the marine species, tigerfish, stumpnose and perch occur there as well. Amongst the fresh water fish species are “guppies”and mud bream. “Of the 30 species of fish reported as present in the Beachwood creek a striking proportion belong to the gobioid family. Mullet, followed by “glassies” (Ambassis) are the dominant species, the ratio of juvenile mullet to all other species being in excess of 20:1. Juveniles 1,5 cm in length representing fish only a few weeks old, have been caught after coming into the estuary from the sea. The Beachwood population of Periophthalmus (the mudskipper) is one of the most southerly in terms of its distribution. Five species of fish are reported to breed in the estuary.
Birds
71 bird species have been reported from around the Mgeni estuary at the Beachwood reserve.
The Mgeni sandbar is an important roost for terns and gulls, in particular for the White-winged black terns. There have reports of 23 wader species, of which Sanderling, Common and Curlew sandpipers are the most common. A rare bird that of the area is the Crab plover. Fish eating birds, such as cormorants, herons and egrets are common.
Among the other fauna that have been reported, were vervet monkeys, grey duiker, banded mongoose, leguaans, lizards, geckoes, skinks, snakes, frogs, toads and insects.
Water Quality
Physio-chemical characteristics
Whilst the Mgeni river system is of reasonably good water quality, conditions deteriorate markedly in the estuary due to sources of pollution in the lower Mgeni. Van der Riet has recently studied certain aspects of the chemistry of the Beachwood creek, mainly to investigate the exchange of suspended and dissolved material between the estuary and the mangroves, over a full (both spring and neap) tidal cycle. The results obtained reveal the net gain or loss of such materials with each tide. On three of the four tidal cycles studied, there was a net export of particulate organic matter, nitrite and total phosphorus from the mangrove swamp into the estuary.
The water in the estuary is rarely clear. Dredging at the Umgeni Sandworks upstream is constantly stirring the bottom ooze. Only in winter, with low river flows, and on a high spring tide can the water be considered as clear.
Salinity – At spring tide, salinities of 31% can be recorded at the bottom of the estuary, 2 km from the mouth. Distinct stratification becomes evident ±1 km from the mouth, but at the mouth itself salinities are high (35%) and evenly distributed from top to bottom. During high flow periods, salinities naturally fall dramatically and the system assumes ‘river mouth’ characteristics. In May and August 1972 salinities of 4,6% and 3,3% respectively were recorded from near the mouth of the estuary. Information on salinities inn the Beachwood creek are available for three months of 1976. At low tide the creek carries water which is almost fresh.
Pollution
Industrial areas on the Palmiet river have been known to pollute the system, but the chemistry of the estuary is dominated by the overpowering degree of pollution from the Zeekoe (or Piesang) river sewage discharge. Indices such as a rise in phosphorus from 16 micrograms/litre to 270 micrograms/litre in August 1972 are indicative of the magnitude thereof. Industries immediately above the estuary also pollute the system at times. In July 1971 a spill of crude furnace oil and on two occasions the release of some respiratory poison, all had disastrous results in the estuary. Fibre desposal by the S.A. Board Mills is a problem. Pollutants are said to seldom enter the Beachwood creek, with the result that this area acts as a reservoir of life to re-populate the estuary.
References
Begg, G (1978). The Estuaries of Natal; Natal Town and Regional Planning Report Vol 41;
Department of Water Affairs and Forestry, South Africa. 2004. Internal Strategic Perspective: Mvoti to Mzimkulu Water Management Area : Prepared by Tlou & Matji (Pty) Ltd, WRP (Pty) Ltd, and DMM cc on behalf of the Directorate: National Water Resource Planning (East). DWAF Report No. P WMA 11/000/00/0304