MGENI RIVER: Topography, geology and soils

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Map of the MGENI River

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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.(Ref.2.)
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. (www.csir.co.za/rhp/state_of_rivers/state_of_umngeni_02/umngeni.html#lumng)
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. (www.csir.co.za/rhp/state_of_rivers/state_of_umngeni_02/umngeni.html#lumng)
    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. Ref.1.
The relevant geological formations occurring in the area are listed below (Ref.2.)
  Age   Formation or group   Lithology
Quaternary (Holocene- Pleistocene)  

 

Intrusive 

Karoo (Jurassic- Permian)

Siburian-Ordovician

Late-Proterozoic

Alluvium Sibayi and Isipingo Formations. Unconsolidated coastal dunes, including ‘Bereatype red sand’

 

Karoo dolerite 

Drakensberg Group Beaufort Group Ecca Group Dwyka Tillite FormationNatal Group 

Natal Group 

Natal Structural and Metamorphic Province (‘Basement’)

 Unconsolidated sands and clays. Unconsolidated sands and clayey sands.

 

Dolerite

Basalts, sandstones, shales & mudstones Sandstones, shales & mudstones Sandstones, shales & mudstones Tillite (diamictite)

Sandstones, quartzites & shales

Granites, gneisses, amphibolites & 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. (Ref.2)
  References                                      

Ref 1. Begg, G. (1978)  The estuaries of Natal.

Ref.2.  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.

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