Abstract
The Gamtoos is a shallow flood-tidal estuary located on the south coast of South Africa. Even though it has an extensive catchment area, dams limit runoff and mean freshwater inflow is estimated at less than 1 m3 s−1, and the flood tidal deltas constrict and at times even close the mouth. The results presented here derive from an intensive measurement program carried out over a 3-wk period at the end of 1992, immediately after good rains in the Gamtoos catchment region. Freshwater inflow increased to more than 10 m3 s−1, driving the salt wedge downstream and resulting in intense haloclines in the mid-estuary region. The program monitored the return to more average estuarine structures, and even though tidal exchange was restricted, marked differences occurred in stratification at neap and spring tides; tidal exchanges provided the dominant mixing forces. It is found that the shallower upper reaches of the estuary are flushed with relatively small increases in freshwater inflow, though a balance exists with the tidal exchanges through the constricted mouth. The variation in the position of the salt wedge and in the salinity stratification can have substantial implications for biota.
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Literature Cited
Aubrey, D. G. andP. E. Speer. 1985. A study of non-linear tidal propagation in shallow inlet/estuarine systems. Part I: Observations.Estuarine, Coastal and Shelf Science 21:185–205.
Garvine, R. W., R. K. McCarthy, andK-C. Wong. 1992. The axial salinity distribution in the Delaware estuary and its weak response to river discharge.Estuarine, Coastal and Shelf Science 35:157–165.
Goschen, W. S. andE. H. Schumann. 1995. Upwelling and the occurrence of cold water around Cape Recife, Algoa Bay, South Africa.South African Journal of Marine Science 16:57–67.
Heydorn, A. E. F. and K. L. Tinley. 1980. Estuaries of the Cape Part 1: Synopsis of the Cape Coast, Natural features, dynamics and utilisation. CSIR Research Report 380, Stellenbosch, South Africa.
Hilmer, T. 1990. Factors influencing the measurement of primary production in small estuaries. Ph.D. Dissertation, University of Port Elizabeth, Port Elizabeth, South Africa.
Jenkins, G. M. andD. G. Watts. 1968. Spectral analysis and its applications. Holden-Day, Inc., San Francisco, California.
Jerling, H. L. andT. H. Wooldridge. 1994. The mesozooplankton of a freshwater-starved estuary, p. 301–306.In K. R. Dyer and R. J. Orth (eds.), Changes in Fluxes in Estuaries. Olsen & Olsen, Fredensborg, Denmark.
Largier, J. L. 1986. Structure and mixing in the Palmiet estuary, South Africa.South African Journal of Marine Science 4:139–152.
Largier, J. L., C. J. Hearn, and D. B. Chadwick. 1995. Density structures in ‘low inflow estuaries’,In Proceedings of the 7th International Conference on the Physics of Estuaries and Coastal Seas. (in press).
Largier, J. L. andJ. H. Slinger. 1991. Circulation in highly stratified Southern African estuaries.Southern African Journal of Aquatic Sciences 17:103–115.
MacKay, H. M. andE. H. Schumann. 1990. Mixing and circulation in the Sundays river estuary, South Africa.Estuarine, Coastal and Shelf Science 31:203–216.
MacKay, H. M. andE. H. Schumann. 1991. Tidal and longperiod water level variations in the Sundays river estuary, South Africa.South African Journal of Science 87:597–600.
Pearce, M. W. andE. H. Schumann. 1994. Water quality of an agricultural drainage system in the Gamtoos floodplain, p. 563–570.In Proceedings of a Conference on 50 Years of Water Engineering in South Africa: A Tribute to Prof Des Midgley. South African Institution of Civil Engineers, Johannesburg, South Africa.
Reddering, J. S. V. 1988a. Prediction of the effects of reduced river discharge on the estuaries of the south-eastern Cape Province, South Africa.South African Journal of Science 84:726–730.
Reddering, J. S. V. 1988b. Coastal and catchment basin controls on estuary morphology of the south-eastern Cape coast.South African Journal of Science 84:154–157.
Reddering, J. S. V. andN. G. Scarr. 1990. Sediment dispersal in the Gamtoos estuary. Institute for Coastal Research Report Number 24, University of Port Elizabeth, Port Elizabeth, South Africa.
Schroeder, W. W., S. P. Dinnel, andW. J. Wiseman. 1990. Salinity stratification in a river-dominated estuary.Estuaries 13:145–154.
Schumann, E. H. andK. H. Brink. 1990. Coastal-trapped waves off the coast of South Africa: Generation, propagation and current structures.Journal of Physical Oceanography 20:1206–1218.
Schumann, E. H. andL. DeMeillon. 1993. Hydrology of the St Francis Bay marina, South Africa.Transactions of the Royal Society of South Africa 48:323–337.
Schumann, E. H., G. J. B. Ross, andW. S. Goschen. 1988. Cold water events in Algoa Bay and Cape south coast, South Africa, in March/April, 1987.South African Journal of Science 84:579–584.
Tyson, P. D. 1986. Climatic Change and Variability in Southern Africa. Oxford University Press, Cape Town, South Africa.
Uncles, R. J. andJ. A. Stephens. 1990. Computed and observed currents, elevations, and salinity in a branching estuary.Estuaries 13:133–144.
Whitfield, A. K. andT. H. Wooldridge. 1994. Changes in freshwater supplies to southern African estuaries: Some theoretical and practical considerations, p. 41–50.In K. R. Dyer and R. J. Orth (eds.), Changes in Fluxes in Estuaries. Olsen & Olsen, Fredensborg, Denmark.
Wooldridge, T. H. 1994. The effect of periodic inlet closure on recruitment in the estuarine mudprawn,Upogebia africana (Ortmann), p. 329–333.In K. R. Dyer and R. J. Orth (eds.), Changes in Fluxes in Estuaries. Olsen & Olsen, Fredensborg, Denmark.
Zhang, P. 1995. Evolution of the Gamtoos river flood plain, South Africa. M.S. Thesis, University of Port Elizabeth, Port Elizabeth, South Africa.
Zhang, P., E. H. Schumann, andR. W. Shone. 1995. Note on the effect of wind-induced storm set-up in the Gamtoos estuary, May 1992.South African Journal of Science 91:412–414.
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Schumann, E.H., Pearce, M.W. Freshwater inflow and estuarine variability in the Gamtoos Estuary, South Africa. Estuaries 20, 124–133 (1997). https://doi.org/10.2307/1352726
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DOI: https://doi.org/10.2307/1352726