uMgeni River EarthCache
-
Difficulty:
-
-
Terrain:
-
Size: 
(not chosen)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
From Inanda Dam, the uMgeni River flows from the Valley of a Thousand Hills for 24 km and flows out to sea at Durban. The vegetation provides habitat for aquatic and terrestrial species and serve as filters for sediment, nutrients and light.
LOWER UMGENI RESOURCE UNIT
Historically the uMgeni River entered Durban Bay over a large swampy plain where Durban city is now situated. Accordingly to Hattersley (1936) who described Durban in the 1800s: "Then as now, the beach (in Durban Bay) was the centre of amusement. The boys fishing for eels in the big pond, formed in the "dip" in West Street by the stream which originally connected the Umgeni with the bay, provided favourite sport in the days before building operations covered the swampy ground (today the dip in West St is between Field St and Gardiner St). At low spring tide, it was very nearly possible to walk on dry sand across to the Bluff, and the channels could be easily waded by those who did not fear sharks. North of the Point, the little-used ocean beach swarmed with white crabs."
UMGENI ESTUARY
The position of the estuary has changed repeatedly, moving not only north and south along the coast, but at times the uMgeni River flowed directly into Durban Bay. A 1909 Mercury Pictorial (9th September) reported that the estuary had marched "three miles" from its present position towards Durban. Concern was expressed and the estuary was manually redirected towards its current position. This attributed to the significant changes in the estuary habitat. Bridge construction, industrial and organic pollution and the damming of the river impacted most significantly on the estuary. Altered flows (due to upstream impoundment) have transformed the previously flood-dependent and permanently open estuary to a temporary open/closed system. In the absence of floods, sediment accumulates, changing the ecology of the system and thus the system’s ability to provide a certain range of estuary-based goods and services.
The uMgeni estuary thus changed from tide-dominated to river-dominated conditions. River-dominated estuaries have insufficient tidal prisms to maintain an inlet against nearshore wave and tidal action. River flow is critical to the maintenance of an outlet channel. Under drought conditions inlets may close for prolonged periods, for example, the uMgeni estuary which was formerly open for more than 90% of the year is now closed for prolonged periods since the completion of Inanda Dam, some 35 km upstream of the mouth. The role of river floods in these systems is important in eroding accumulated sediment and temporarily deepening the channel. Back-barrier response to flooding in the uMgeni estuary in 1987 took place through erosion of an average thickness of 5 m of sediment from throughout the 2.5-km estuary channel length. A mangrove-fringed island composed of laminated sands and silts and colonised by terrestrial vegetation on its higher levels was completely eroded from the centre of the channel. A total sediment volume of 1.8106 m3 was eroded from the estuary by the flood and this was evenly distributed along the channel. The tidal prism was increased temporarily by the flood. The estuary channel adjusted its morphology largely by the redistribution and vegetation-assisted stabilisation of sediment, and it appeared that additional fluvial sediment had passed through the estuary and was deposited in the sea.
Despite human impacts, the uMgeni remains an important estuary. The remaining 44ha of Beachwood mangroves in the estuary support a wide diversity of plants and animals. Studies in the early 1980s recorded 56 fish species, 13 prawn species and 11 crab species. The estuary also provides habitat for juvenile marine fish.
uMgeni Catchment statistics
uMgeni catchment size: 4416 km2, river length 225km from source to mouth
Mean annual precipitation: 410 – 1 450mm
Mean annual runoff: 72 – 680mm
Mean annual evaporation: 1 360 – 2 040mm
REFERENCES
uMngeni river and neighbouring rivers and streams - Product of the river health programme, 2002, Accessed from: (visit link) [11 November 2009]
Hattersley,AF (1936) More Annals of Natal. Shuter and Shooter, Pietermaritzburg., quoted in “uMngeni river and neighbouring Rivers and streams”
Cooper, J.A.G. 2001, Geomorphological variability among microtidal estuaries from the wave-dominated South African coast. Geomorphology 40 P 99–122
Cooper, J.A.G. 2002, The role of extreme floods in estuary-coastal behaviour: contrasts between river- and tide-dominated microtidal estuaries. Sedimentary Geology 150 P 123–137
Answer the following questions:
1) Post a picture of yourself and your GPSr with the river features in the background.
2) Describe the river mouth from your vantage point. (In terms of sand banks, vegetation, flow)
3) What alien invasive plant is disrupting the ecology in this area?
Additional Hints
(No hints available.)
Treasures
You'll collect a digital Treasure from one of these collections when you find and log this geocache:
Loading Treasures