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Eerste River Estuary

General Information and location

General

The Eerste River obtained its name, when the settlers ventured out on the Cape Flats. It was the first “Eerste” river encountered on their way towards the Hottentots Holland Mountains. After passing through the coastal dunes near Macassar, the Eerste River forms a small, elongated lagoon in the slack of the backshore area of the beach.

Location (co-ordinates)
The Eerste River mouth is situated at 34°05’S, 18°46’E, approximately 36km south east of Cape Town.

Dimensions
In 1982 (Ref.1.) the extend of the tidal reach was estimated between 2 and 3 km from the mouth. The widest section in the estuary is opposite the Macassar waste water work, which is < 80 m (measured on GoogleEarth). From a distance < 1 km from the mouth, the width of the estuary is < 20 m.

Catchment characteristics

Main Rivers and tributaries

The main river is the relatively short (40km) Eerste River The Eerste River’s major tributary is the Kuils River, which is approximately 30 km long before it joins the Eerste River, approximately 4 km from its mouth. Other minor tributaries of the Eerste River are the Blouklip, Bonte, Jonkershoek, Klippies, Krom, Plankenburg, Sanddrif and Veldwagters Rivers.

Geomorphology

The sediments in the mouth area of the Eerste River estuary can be divided into 3 zones:

  • Fine sand occurred at the relatively flat foreshore.
  • Coarse sand on a relatively steep intertidal beach face.
  • Medium sand in the lagoon area, overlain with grey muds at the bottom of the lagoon.

Land and Estuary uses

Residential

The Macassar township area is a residential area is situated on towards the north west of the estuary, outside of the estuarine area. The Strand residential area is on the eastern side of the Estuary mouth.

Industrial

The area east of the estuary is owned by AECI and is a restricted area, as is the channel of the estuary north of the AECI boundary fence on the West bank. The area west of the estuary is occupied by the Macassar Sewage Works.

Ecology

Flora


Phytoplankton/Diatoms
A thin scum of diatoms and blue-green algae was apparent on the northern bank of the estuary in December 1981 (Ref. 1). A small diatom bloom was noticed in the estuary opposite the sewage outfall in 1979. The high nutrient levels introduced by the sewage works, favour eutrophication (the process by which a body of water is greatly enriched by the natural or artificial addition of nutrients. This may result in both beneficial and adverse –smothering by dominant plant types – effects)


Algae
Within the estuary Enteromorpha and Cladophora were recorded and fragments of Ulva were obtained in D-net drags. Filamentous algae were not abundant or predominant in this estuary in 1981.


Aquatic Vegetation
No aquatic angiosperms were found in the estuary in 1981.


Semi-aquatic Vegetation
During 1981, four groups of semi-aquatic vegetation were represented in the Eerste River Estuary, namely:

  • Intertidal Fringe Vegetation comprising “gansgras” (Cotula coronopifolia) and Scipus maritimus.
  • Phragmites australis Reeds, comprising Phragmites australis and Scirpus littoralis.
  • Scirpus maritimus Sedgeland.

Dune Marsh Area, comprising Paspalum vaginatum, bulrush (Typha capensis) and numerous European weeds.

Terrestrial vegetation

Three types of vegetation were identified in 1981:

  • Fore Dunes, with “steekriet” (Eragrostis cyperoides),  “klappiesbrak” (Tetragonia decumbens), Heteroptilis suffruticosa, Chrysanthemoides monilifera, sea weed (Agropyron distichum), sea pumpkin (Arctotheca populifolia), Senecio elegans and other dune plants.
  • Rhus laevigata/Senecio halimifolius Moist Shrubland, with “Rooikrans” (Acacia cyclops), tabakbos (Senecio halimifolius), “duine taaibos” (Rhus laevigata) and saltbush (Atriplex vestita).
  • Acacia Dominated Dune Shrubland, with Pelargonium capitum, “duine taaibos” (Rhus laevigata), “blombos” (Metalasia muricata), “pypgras” (Ehrharta villosa), “melkhout” (Sideroxylon inerme) and the aliens ”Rooikrans” (Acacia cyclops) and Port Jackson (A. saligna).

Fauna
Zooplankton
Zooplankton samples taken on 1 December 1981 revealed that zooplankton were very sparse and mainly dominated by Harpacticoid copepods (3 spp.), Ostracoda and larval fish (Liza richardsoni)


Aquatic Invertebrates
There were no invertebrates on the hard substrata in the estuary in 1981. On the soft substrata, the ocean crab (Hymenosoma orbiculare), the shrimp (Palaemon pacificus), the sand prawn (Callianassa kraussi) and interstitial meiofauna, such as nematodes, Harpacticoid copepods, flatworms, Polychaeta, Oligochaeta, as well as eight Mollusc species were reported.

Insects
In 1981, insects collected included chironoid midges and larvae, dragonflies, kelp flies, staphylinid beetles, other beetles, house flies, water boatmen, ceratopogonid larvae and dead may-fly nymphs.


Fish
In 1981, fish species recorded were sand gobies (Psammpgobius knysnaensis), sole (Heteromycteris capensis), juvenile and post-larval stages of Liza richardsoni, and “leervis” (Lichia amia).


Amphibians and Reptiles
12 frog species and 11 snake spp. 8 lizard spp. and 4 tortoise spp. were recorded in 1981.


Birds
33 bird species were recorded in 1981, with the sandwich tern being the most abundant.


Mammals
Three bat species, one shrew, one baboon, a southern seal elephant, grysbok, Cape greater gerbil, Verreaux’s Rat, black and white Dormouse, Cape dune Mole rat, Cape clawless otter, Cape wild cat, serval and leopard were recorded by Stuart et al in 1980.

Water Quality

Physio-chemical Characteristics
An assessment in 1981 was as follows:
Surface water temperatures in the Eerste River estuary varies from 10°C in June to 26°C in December. Surface salinities in winter were 0 ppt (parts per thousand) throughout at LWST on the 7th of June 1982, with the river water running out strongly. At HWST waves flooded over the sandbar and salinities ranged from 22 ppt in the mouth, to 1ppt at the surface, 350m upstream. All this, compared to sea surface salinities measured in the surf, 100m west of the mouth were 25 ppt.


Dissolved oxygen (DO) concentrations in summer (1 December 1981) ranged from 6.8 ppm (parts per million) to 10.8 ppm in the surface water of the Eerste River estuary, compared to DO values of 4.0 ppm to 8.5 ppm in the bottom waters. In winter the surface DO levels ranged from 8.5 ppm to 10.4 ppm. DO levels in the sewage outfall were 9,4 ppm on the surface and 8.5 ppm at the bottom.


Water transparency measured with a Secchi disk, ranged from 0,75m in the lagoon to 0,3m in the upper reaches of the estuary.
pH values were measured in the surface waters of the Eerste River estuary and ranged from 8.0 to 8.7 closer to the sewage outfall.
All of the nutrient values were considerably increased at samples measured from the sewage outfall.

Pollution
The Eerste River estuary receives treated sewage effluent directly from the adjacent Macassar sewage works, serving Somerset West and Strand. It also receives sewage effluents indirectly from the Zandvliet sewage works (near the head of the esturay) and from various sewage works upstream on the Eerste and Kuils Rivers respectively.
Effluents from various industries, including wineries are also discharged in the Kuils River and Eerste River.
Studies on the pollution of the Eerste and Kuils Rivers indicated that pollutants such as oil, fats, soaps and nutrients were present (Ref. 4). It is also expected that pesticides and herbicides, usually present in water draining from agricultural areas, are polluting the Eerste River water.

Hydrodynamics and Mouth Dynamics

Hydrodynamics
In 1982, when the estuary was open, it was only slightly tidal, because of its high elevation of the sill at the mouth (2m above LWOST). Originally the lagoon extended to both sides of the river onto the beach area. The eastern arm was cut off by the construction of a causeway in 1977, which left a confined stagnant water body on its eastern side. This stagnant water body developed into a marshy area. At present only the western arm of the lagoon is functioning.
During a survey on 11 May 1982, during LWST, the water flow through the meander-shaped mouth channel, was measured as 3 mł/s.
The tidal range between Mean Low Water Spring and Mean High Water Spring in False Bay is approximately 1.48 m.

Mouth dynamics
It 1978 (Ref.3.) it was reported that the estuary may take many forms, from a more widened outlet channel to a lagoon several hectares in extent, held by a wind-and-wave-built sandbar. The mouth may be greatly out of alignment with the river or may be closed entirely except at high spring tide. The dynamics of the Eerste River mouth are governed by seasonal variations in river flow and wave energy. The incidence of relatively high wave energy causes the build-up of a high beach bar. The lagoon breaches the bar during winter rains at a site where the bar is low. Strong meandering of this overflow channel indicates a rather attenuated pattern of discharge.
In 1980 (Ref.2.) it was reported that the Eerste River Estuary mouth is closed during summer and breaks open after the first winter rains in about May. However in 1982 the mouth appeared to remain open for most of the year. Due to inflows from numerous wastewater works this open mouth condition remains until present.

Physical Features

Bathymetry
During a survey in 1981, the bathymetry of the estuary was established. The estuary was relatively shallow, with the deepest measurement taken at 1km from the mouth, measuring 1,8m. The water depth at the mouth was 0,6m at the time of the survey, on 1 December 1981.

Obstructions
In 1981, a high concrete bridge crossed the Eerste River 3km from the mouth, carrying the road to the Macassar Sewage works, but this had sufficient clearance for minimal obstruction of floodwaters. This bridge was beyond the observed limit of tidal penetration. In 1981, a small temporary wooden bridge crossed the estuary channel near the observed tidal limit, 1,5km from the mouth. As any major flood would carry this temporary crossing away it was not considered to be an obstruction.
Outcrops of rocks from the Malmesbury Group near the sewage outfall in the channel are a natural obstruction and collect driftwood.
In 1981 a short section of the fence on the west bank of the estuary, 900m from the mouth, at the boundary of the then Kentron controlled area, had flood debris adhering to it indicating some degree of obstruction.
In 1981 the then Kentron boundary fence, embankment and road causeway on the east side of the estuary were a complete obstruction, totally isolating the wetland area, which was formerly the eastern area of the Eerste River estuary.
The Macassar Sewage Works has been constructed in part of the original estuarine flood plain.

Climate

General
Generally the climate in this area is mild. In winter, when the weather can be very unpredictable, there are still many sunny days, with high wind at times. The weather in the summer is dry and hot, although the heat is tempered by sea breezes. The best weather occurs in the early autumn, when the days are balmy days and the evenings warm.
Temperatures in summer between 23°C – 35°C: winter will be 9°C – 15°C (Ref. The mean annual temperature is a mild 20 °C.


Rainfall
The mean annual rainfall of 634 mm, mainly between April and September.

References:

Ref 1: Grindley, J.R. (1981). Estuaries of the Cape. Part II: Synopses of available information on individual systems Report No. 16: Eerste (CSW 6). CSIR Research Report 415, 51pp.

Ref 2: Glass, A. (1980). Geology, morphology, sediment cover and sand movement of False Bay. In: The future management of False Bay. Proceedings of a seminar held on 11 June 1980 in Cape Town: B. Gasson (Ed.) Dept. Urban and Regional Planning, University of Cape Town, pp.15-25.

Ref 3: Harrison, A.C. (1978). Sea-run rainbow trout of the Eerste River. Piscator 103: 68-76.

Ref 4: Bartlett, P.D. and Hennig, H. (1982). Pollution monitoring surveys of the Eerste River Estuary. CSIR Report T/SEA 8209. 13pp.

Ref.: Stuart, C. T. et al (1980). Preliminary distribution maps of mammals of the Cape Province (excluding Cetacea). CPA Dept. Nature and Environmental Conservation. Unpublished Research Report: Mammals. 176pp.

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