Mgeni River
- Overview
- Tributaries
- Geophysical
- Hydrology; Geohydrology
- Ecology
- Climate
- Land-uses
- Populated Places
- Water Infrastructure
Overview
The Mgeni River rises in the Natal Midlands and has been largely modified by human activities, mainly intensive agriculture, forestry and urban settlements.
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 (4439 km² according to Ref.2)and a mean annual run-off of 707 mil.cub.m. The uMngeni River rises in the uMngeni Vlei area, an ancient wetland surrounded by a rim of hills, at the west end of the catchment 1 760m above sea level. From its source, the uMngeni River flows eastward through rolling pastoral landscape and is joined by the Lions River before flowing into Midmar Dam, a popular sport and recreation resource. The river channel is narrow, overhung by grassed banks, fine-leaved shrubs and occasional trees. River habitats here are predominantly riffles and pools. Cascades are common due to hard dolerite rock. The main land use in this region is forestry and agriculture.
Tributaries
| Tributary | Comment |
|---|---|
| Molweni River | |
| uMbilo River | |
| uMhlatuzana River | |
| uMsunduze River |
Geophysical
Topography
The area from the Drakensberg mountains in the west at an altitude of over 3 000 m drops to sea level in the east over a relatively short distance of approximately 150 km. The area is rugged. Very steep slopes characterise the river valley in the inland area.
Below Midmar Dam, the uMngeni plunges over the 111m high Howick Falls and into the uMngeni Valley where remnants of Midland forest can be seen. The Karkloof tributary then joins before the uMngeni flows into Albert Falls Dam.
Approaching Albert Falls, the uMngeni meanders through grassed farmland, where the riverbanks are treeless or invaded by woody alien plants. Below the falls, the river traverses deeply dissected terrain, which forms the western edge of the Valley of a Thousand Hills. At Nagle Dam, the river is still at a high elevation (400m above sea level) despite being a mere 50km from the coast.
From Inanda Dam, the uMngeni River flows from the Thousand Hills with a gentle gradient for 24 km before it flows out to sea just north of Durban. This part of the river is extensively modified, with riparian vegetation and direction of flow significantly altered to accommodate human settlement and activities. Historically the uMngeni River entered Durban bay over a large swampy plain where Durban is now situated.
Geology
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.
| Age | Formation Group | Lithology |
|---|---|---|
| Quaternary (Holocene- Pleistocene) | Alluvium Sibayi and Isipingo Formations. Unconsolidated coastal dunes, including ‘Bereatype red sand’ | Unconsolidated sands and clays. Unconsolidated sands and clayey sands |
| Intrusive | Karoo Dolerite | Dolerite |
| Karoo (Jurassic Permian) | Drakensberg Group Beaufort Group Ecca Group Dwyka Tillite FormationNatal Group | Basalts, sandstones, shales & mudstones Sandstones, shales & mudstones Sandstones, shales & mudstones Tillite (diamictite) |
| Siburian-Ordovician | Natal Group | Sandstones, quartzites & shales |
| Late-Proterozoic | Natal Structural and Metamorphic Province (‘Basement’) | Granites, gneisses, amphibolite & schists |
Soils
Soil cover throughout the area is generally shallow and is strongly parent-material related due mainly to the prevailing topographic conditions. Soils are mainly of sandy types developed on quartzose rocks, or clayey soils developed on argillaceous and basic igneous and metamorphic rocks. Deeper transported soils are present as colluvium on lower slopes, with alluvium occurring in valley bottoms and estuaries at the coast. The soils of the coastal dunes are deep. Pressure on this resource is evident from the high levels of soil erosion, arising from over-grazing, intensive cultivation, informal farming and settlements, timber plantations, and sand winning.
Hydrology and Geohydrology
Hydrology
This is the main water catchment for the KwaZulu-Natal (KZN) Midlands, providing high quality potable water right down to Kloof and the Inanda valley Water releases from Albert Falls Dam are particularly “unnatural” as flows tend to be highest in the dry winter months, and low during summer. This flow pattern can disrupt ecological processes. At Nagle Dam, bypass gates can be used to mimic a more natural flow pattern.
Ecology
Overview
The Mgeni is referred to as a ‘working river’, the area is highly developed and highly utilised with the implication that the socio-economic impact of improving these rivers from an ecological point would be huge.
Value of conserving rare and endangered habitats and species, e.g. wattled crane of the uMngeni vlei and the endemic mountain malachite dragonfly. Also present is the rare plant Hydrostachys which grows on waterfalls in this area.
Aquatic Animals
Fresh water zooplankton species and crab, fish and prawn larvae have been reported in the water column of the Mgeni. Amongst the benthic animals, were polychaetes (Ceratonereis and Desdemona), which are extremely tolerant to low oxygen levels. Only the top 2 cm of the sediment was productive , below that the sediment was anaerobic. This explains the absence of amphipods, isopods and tanaids, which are sensitive to pollution. At the head of the Mgeni estuary, however, oligochaetes and entomostracan are present, normally indicative of fresh water. There is a rich assemblage of gastropods in high densities among the Mangroves. There are larger crustaceans, shrimps and 2 species of prawns present in the Mgeni estuary.
Refer to Ref. 1, a 1978 estuarine status: 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.
Invertebrate condition appears close to Natural. Fish are in good condition but lower down are only in Fair condition due to the impact of aliens (trout, carp and bass) and invasive indigenous catfish. For most biota the upper reaches of the uMngeni and Lions Rivers are in better condition than downstream towards Midmar Dam.
Brown trout (introduced here in 1890) are found in the upper uMngeni River while rainbow trout (used for recreational angling) are found in dams of the upper catchment. Although breeding takes place in some streams where suitable gravel beds exist, reproduction in the streams is limited and populations are generally maintained by the annual introduction of trout reared in hatcheries.
Potamonautes dentatus (left) is a river crab found in the uMngeni River, discovered here in 1992. This large species favours fast-running water and can sometimes be seen climbing the vertical cracks in the waterfall rocks at Albert Falls waterfall.
Terrestrial Animals
Refer to Ref. 1, a 1978 estuarine status: 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.
Vegetation
Refer to Ref. 1, a 1978 estuarine status: 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. Aquatic flora species spotted in the estuarine area of the Mgeni, are sea-lettuce (Ulva), during winter. Water hyacinth (Eichornia) and eelgrass (Vallisneria).
Riparian habitats are degraded by invasive trees such as wattles, gums and pines.
Hot, dry valley bushveld.
Hydrostachys polymorpha is a small fern-like plant that grows in fast currents, clinging to the rocky substrate of waterfalls and rapids. Hydrostachys is sensitive to changing temperatures (especially below dams) and excessive silt in the water. It is found fairly widely in the upper reaches of the uMngeni (especially the Karkloof tributary) and neighbouring rivers.
Climate
Temperature
The mean annual temperature ranges between 12°C and 14°C in the west to between 20°C and 22°C at the coast. Maximum temperatures are experienced in the summer months from December to February and minimum temperatures in winter in June and July.
The Water Management Area experiences a high relative humidity in summer in a similar pattern as the rainfall. There is a daily mean peak in February, ranging from 68% in the inland areas to greater than 72% for the coast and a daily mean low in July, ranging from 60% in the inland areas to greater than 68% at the coast. Potential mean annual gross evaporation (as measured by ‘A’ pan) ranges from between 1 600 mm and 1 800 mm in the west to between 1 400 mm and 1 600 mm in the coastal areas.
Rainfall
Rainfall over the area is markedly topographically-related.
Average annual rainfall varies from between 1 000 mm to 1 200 mm along the coast to greater than 1 500 mm in the north-west along the Drakensberg mountains. Rain shadows occur in the interior valley basins of the major rivers where the annual rainfall can drop to below 700 mm. The peak rainfall months are December to February in the inland areas and November to March along the coast. The rainfall is seasonal with most rain falling in summer from October to March.
Land-use
Agricultural
Sugar, fodder crops, tropical fruits, beef and cattle farming are the main agricultural activities in the catchment.
the areas used for irrigation, forestry and dryland are 95 km2 , 529 km2 and 580 km2 respectively.
Tribal and communal areas cover 632 km2 in the catchment.
The river and the many dams in the area provide for sport and recreation. Indigenous (the KZN yellowfish is a demon to catch!) and exotic fish e.g. trout in cool headwater streams and dams, are popular catches here. Midmar Dam is a hub for water sports including the famous Midmar Mile that draws thousands of swimmers each year.
Populated Places
| Town | District | Municipality | River or Tributary |
|---|---|---|---|
| Clermont | eThekwini | eThekwini | uMgeni River |
| Durban North | eThekwini | eThekwini | uMgeni River |
| Greyville | eThekwini | eThekwini | uMgeni River |
| Pinetown | eThekwini | eThekwini | uMgeni River |
| Springfield | eThekwini | eThekwini | uMgeni River |
| Molweni | eThekwini | eThekwini | uMgeni River |
| Newlands | eThekwini | eThekwini | uMgeni River |
| Westville | eThekwini | eThekwini | uMgeni River |
Water Infrastructure
Dams
| Name | Capacity (mil.cub.m) | Comment |
|---|---|---|
| Albert Falls Dam | 288.1 | |
| Inanda Dam | 241.7 | |
| Midmar Dam | 235.4 |
REFERENCES:
Ref 1: Begg, G. (1978) The estuaries of Natal. 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