Gouritz River Estuary
- General Information and Location
- Catchment Characteristics
- Land and Estuary Uses
- Ecology
- Water Quality
- Hydrodynamics and Mouth Dynamics
- Physical Features
- Climate
- Threats
- References
General Information and Location
General
The Gourits River Estuary is situated (35°21’S; 21°53’E) approximately 27 km south of the N2 highway between Albertinia and Mossel Bay.
Location (co-ordinates)
35°21’S; 21°53’E
Dimensions
The Estuarine characteristics of the Gourits River stretch about 10km upstream from the mouth. The Gourits Estuary has a total surface area of 188ha
Catchment Characteristics
Main Rivers and Tributaries
The Gourits River is formed by the confluence of the Gamka and Olifants Rivers, south of Calitzdorp.
The Olifants River is fed by 12 tributaries, originating in the Groot Swartberge and the Kammanassieberge.
The Gamka is fed by numerous tributaries, originating in the Nuweberge.
The Buffels River, rising from the Nuweveld Mountains, joins the Groot River, along with the Touws River from the west. The Groot River then joins the Gourits.
The total river length of the Gourits River is approximately 1 045km.
It enters the Indian Ocean about 33km south-west of Mossel Bay (35º 21‘S; 21º 53’E).
The Gourits River Catchment stretches from the Karoo in the north, to the coast in the south. It is the third largest catchment in South Africa and comprises approximately 45 000 km2.
Dams
There are 10 large dams on the Gourits River of which the Floriskraal Dam (62 900 000m3) on the Buffels River, the Gamkaspoort Dam (54 300 000m3) on the Gamka River, the Kammanassie Dam (32 900 000m3) on the Kammanassie River and the Stompdrif Dam (61 200 000m3) on the Olifants River are the most important.
Geomorphology
The grain size of the sand spread along the whole sandspit and the adjacent beach in 1987 was medium and ranged from 210 to 468 microns, with a mean grain size of 342 microns.
Land and Estuary uses
Residential
The small resort of Gouritsmond is situated on the west bank of the estuary near the mouth.
The closest towns are Albertinia, Vleesbaai and Mossel Bay.
Agricultural
The main farming activities are sheep and cattle farming and fodder crops. The cattle are mostly for meat production, but a few dairy herds are also kept. The crops include Lucerne, siradellon and oats. Wheat and maize were also cultivated in the area in 1989.
Recreation
The estuary itself is used for waters ports, such as power boating, water skiing, canoeing, yachting, swimming and angling. Prawns, used for bait, are collected from the mudflats in the estuary.
Ecology
Flora
Algae
Macroscopic algae (Porphyra capensis) are not abundant in the estuary, but occur on the rocky, eastern bank at the mouth. In 1987, no macrophytes were found within the estuary.
Terrestrial vegetation
Two veld types of terrestrial vegetation were represented in the lower reaches of the Gourits River, namely the Coastal Renosterveld and the Coastal Macchia.
Eleven vegetation types have been identified, namely:
Arid scrub thicket : Dominant tree species are the Sideroxylon inerme and the Euclea racemosa. Sub-tropical thicket species include Rhus longispina and Scolopia zeyheri. Other locally conspicuous species are the white aloe and Euphorbia species.
Renosterveld : The species composition is secondary in response to the impacts of fire and stock pressure and although considered to be relatively diverse, it is dominated by Elytropappus rhinocerotis. Other dominant genera include Eriocephalus, Anthospermum, Passerina, Relhania, Aspalathus, Helichrysum, Pteronia, Selago, Felicia and Hermania.
Strandveld: The area consists of a mosaic of bushclumps, restios and shrubs, with a conspicuous graminoid and herbaceous ground layer (Ref. 1). The bushclumps are dominated by Sideroxylon inerme, Schotia afra and Pterocelastrus tricuspidatus (Ref. 1). The dominant restio species are Thamnochortus bachmannii, Chrysanthemoides monilifera and Phylica axillaries var. maritime. The herbaceous ground layer comprises mainly Themeda triandra and Agathosma muirrii
Strandveld-thicket mosaic: The dominant woody species include Euclea racemosa, Sideroxylon inerme and Rhus longispina. The dominant shrub in the area is Zygophyllum morgsana. Two aloe species occur, namely Aloe ferox and A. arborescens.
Dune scrub: The dense scrub is dominated by Euclea racemosa, Tetragonia decumbens, Trachyandra divaricata, Salvia africana-lutea and Restio Eleocharis.
Dune shrubland : Dominant shrub species include Myrica cordifolia, Chrysanthemoides monilifera and Stoebe plumosa. Common grasses are Ammophila arenaria, Ehrharta villosa and Senecio elegans.
Acacia cyclops thicket: Scattered clumps of the exotic species, Acacia cyclops occur. It can spread rapidly and should be eradicated.
Saltmarsh : The saltmarsh vegetation is not particularly well developed. Sarcocornia pillansiae is dominant on both banks at the mouth. Other species include Chenolea diffusa, Sporobolus virginicus, Juncus acutus, J. kraussii and Phragmites australis. Cotula coronipifolia and Triglochin species occur at the lower tidal levels, forming saltmarsh lawns.
Dune thicket : A dune thicket of up to 3m occurred on the western bank of the estuary. The most conspicuous species are Sideroxylon inerme, Euclea racemosa, Pterocelastrus tricuspidatus and Clausena anisate.
Limestone fynbos: The most common protea species are Protea repens, P. susannae, and P. lanceolata, as well as Leucospermum praecox and Leucadendron galpinii. Heavy infestations of the exotic Acacia cyclops were evident in 1989.
Secondary grassland: This secondary grassland use to be fynbos, but was converted by agriculture.
Fauna:
Aquatic Invertebrates
On the western bank, the aquatic invertebrates are dominated by the mud prawn, Upogebia, as well as at Steentjies-se-Gat. Bloodworms (Arenicola) were rare. Some Callianassa kraussii were also observed on the east bank, about 6,5km from the mouth
Fish
Fresh water fish species recorded in 1943, included Cape galaxies (Galaxias zebratus), kurpur (Sandelia capensis), moggel (Labeo umbratus), and 3 barber species (Barbus asper, B. tenius and B. anoplus)
In 1957 Van Wyk (in litt.) reported the following marine fish species: Harders, kabeljou, white streenbras, elf and possibly leervis. Sea barbells (Galeichthys sp.) occur in the river.
Amphibians and Reptiles
Amongst the amphibians and reptiles, 11 species of amphibians, three species of tortoises, 26 species of snakes and 12 species of lizards, none of which are rare or endangered, were reported by several authors between 1943 and 1984.
Birds
The birds at the Gourits Estuary have been studied by several scientists. Summers et al. recorded 12 species (among a total bird count of 292 birds) of waders at the estuary mouth in 1976. In 1981 Underhill and Cooper reported a total of 1250 50 of 78 different species.
Mammals
Few mammal species records are available. Only three species have actually been recorded, namely the Cape dassie, the Cape porcupine and the Cape mole-rat. A further 77 species may occur in this area, of which 8 species are rare or vulnerable.
Water Quality
Physio-chemical Characteristics
During a survey in 1987 (Ref.1), the top and bottom water samples were taken at 4 stations. Readings were taken for Temperature, Dissolved oxygen, salinity, and water transparency. It was concluded that the water was well oxygenated with dissolved oxygen values just below saturation. Salinity and temperature values indicted fairly good mixing at most stations. Water transparency varied between 1.1 and 2 m (Secchi disk measurements), with the water colour being varying intensities of green.
Pollution
Interesting: References to pollution incidents. An article in “Die Burger” of 16 July 1982, which reported on the death of fish in the Gourits River, most probably caused by vinegar like substance resulting in oxygen depletion. Thousands of dead moggels (Labeo umbratus) were found as far upstream as Herbertsdale (100km inland). A press release from the CDNEC on 19 August 1987, which reported the death of 15 blue cranes through poisoning on the banks of the Gourits River near Herbertsdale
Hydrodynamics and Mouth dynamics
Hydrodynamics
The tidal lag in the estuary was in 1987 approximately 2 hours at low tide , at the head of the estuary Saline water seemed to penetrate up to 2 km upstream of the road bridge at “Die Eiland” during high tide. This point is the absolute upper limit of estuarine conditions as confirmed by the vegetation.
Mouth dynamics
The mouth of the Estuary is normally about 18m wide but during the rainy season and at high tide, it may be wider. During the floods in August 1986, the mouth was approximately 40m wide In the past the mouth must have been closed for various lengths of time . During times of low flow, the sandspit is build up to a state of near closure . The mouth is opened during floods, when the sediments are washed out to sea, forming an offshore delta, from where the sand is returned to the sandspit during calmer conditions.
Although the Gourits River mouth occasionally forms a double spit, as was the situation in 1984, the mouth as a rule consists of an extensive sandspit on the western side with rocks lining the eastern shore of the mouth.
Physical Features
Topography
Generally, the topography at the mouth is described as a flat featureless topography.
Obstruction
In 1981 no man-made obstructions were present in the estuary, which restrict the flow of water.
A low-level bridge across the river, about 8 km upstream of “Die Eiland”, was washed away during the floods in 1981. It was replaced by a new, better constructed bridge, with a higher elevation, further upstream in 1983.
During 1987 a group of riparian land owners constructed an illegal, earth weir across the river, approximately 1.5 km upstream of “Die Eiland” near the head of the estuary. The effect of this obstruction was not yet determined by 1989, but it was suspected that it could cause increased erosion of the river bed and should the weir be washed away or damaged, it could result in increased siltation in the estuary below the site of the weir. It could also alter the water chemistry and result in desalination above the weir, and increased salinity below the weir, which in turn, could result in changes in plant and animal communities. The brackish water upstream of the weir contributes to the increased mineralization of the adjacent agricultural soil.
Climate
General
Gouritsmond has a moderate climate.
Temperature varies from 6 – 25 degrees Celsius in winter and 10 – 28 degrees Celsius in summer.
Rainfall
Rainfall ranges from less than 200 mm/a to over 1 000 mm/a Gouritsmond has an average rainfall of 350mm per year. 57% of this rainfall falls from April to September.
Wind
On average for the whole year, winds are well distributed within the western, south-western and eastern sectors, with the westerly winds having the highest frequency of occurrence and highest velocity.
Threats
A fine balance should be maintained when planning future developments, such as dams, which could result in a reduction of the regular scouring of the river mouth by frequent smaller floods
Invader plants such as Acacia cyclops and Acacia saligna threaten the environment.
Other threats to the environment are wind erosion, overgrazing and over utilisation by animals and man, dune moles in cultivated lands, shortage of trace elements in soils, periodic flooding, brackish soils, poor quality, limited water for irrigation, low pH and nutrient values of soils. Wind erosion is aggravated by mineralization and salt deposition on agricultural lands and consequently results in the lack of vegetation cover.
References
Ref.1. Heydorn, H.J. (1989). Estuaries of the Cape. Part II: Synopses of available information on individual systems. Report No. 38 Gourits (CSW 25). CSIR Report 437, 66pp.
Ref.2. Pitman, M.V. et al. (1981). Surface water resources of South Africa. Vol. IV. Drainage regions E G H J K L, Western Cape Water Research Commission. Hydrological Research Unit Report 13/81. Parts 1 and 2.
Ref.3. Duvenhage, I. (1983). Getyrivieroppervlaktes van sommige getyriviere aan die Kaapse kus. Stellenbosch. NRIO unpublished report.
Ref.4. Summers, R.W., Pringle, J.S. and Cooper, J. (1976). The status of coastal waders in the South Western Cape, South Africa. Cape Town. Percy Fitzpatrick Institute of African Ornithology. 162pp.
Ref.5. Heydorn, A.E.F. and Swart, D.H. (1987). Recording of major episodic occurrences (ROMEO). Observations on the effects of episodic floods during the period of 22 to 30 August 1986 on estuaries of the southern Cape. Stellenbosch. NRIO Memorandum 8702.
Ref.6. Day, J.H. (1981). Summaries of current knowledge of 43 estuaries in southern Africa. In: Estuarine ecology with particular reference to southern Africa. J. H. Day (ed.). Cape Town. Balkema: 251-330.
Ref.7. Heydorn, A.E.F. and Tinley, K.L. (1980). Estuaries of the Cape, Part I. Synopsis of the Cape Coast. Natural features, dynamics and utilization. Stellenbosch. CSIR Research Report 380. 97pp.
Ref.8. Geustyn, Forsyth and Joubert Inc. (1987). EMSO Mossel Bay gas development project offshore environmental impact assessment study. Concise Report No. 1. Comprehensive report No. 1, Vol. 1 – Environmental appraisal and management options, Vol. 2 – Annexure to Vol. 1.
Ref.9. 1. The national water research strategy. D16 WATER MANAGEMENT AREA 16: Gouritz.
http://www.dwaf.gov.za/Documents/Policies/NWRS/Sep2004/AppendixD16Gouritz.doc.
Ref.10. Acocks, J.P.H. (1975). Veld Types of South Africa. Mem. Bot. Surv. S. Afr. 40. 128pp.
Ref.11. Barnard, K.H. (1943). Revision of the indigenous freshwater fishes of the South-western Cape region. Ann. S. Afr. Museum XXXVI, Part II: 7-262.
Ref.12. Underhill, L.G. and Cooper, J. (1983). Counts of waterbirds at coastal wetlands in South Africa: Estuaries 1979-1981. Unpublished Computer Printout. Western Cape Wader Study Group and Percy FitzPatrick Institute of African Ornithology.
Ref.13. Smithers, H.N. (1986). South African red data book – terrestrial mammals. Pretoria. South Afr. Natl. Sci. Programme Rep.. No. 125. 216pp.
Ref.14. Brink, Stokes, Marais and Moolman (1986). Loodsprojek: Kussensitiwiteite, Mosselbaai tot Breërivier. Saamgestel vir die Departement van Omgewingsake.
Ref.15. Gouritsmond. General information.