estuaries

estuaries An estuary is a transitional zone between a river and the sea as represented by a shallow marine shelf or platform. Dominated by tidal currents and by interactions between sediment-laden salt and river waters, it is a generally narrowing, elongated inlet which reaches across a coastal alluvial plain or inward along a river valley as far as the upper limit of tidal rise. Estuaries experience strong spatio-temporal gradients in many important environmental factors, and consequently are affected by a rich variety of processes while displaying an exceptional range of morphological, sedimentological, and biological responses. Although among the most productive of biological systems, estuaries are for these same reasons stressful environments for organisms. Under appropriate conditions—a rising sea level seems particularly favourable— estuarine deposits have become preserved in the rock record, where they may now retain fossil fuels. Over the last few centuries in particular, estuaries have provided the people of many of the world's cities and towns with their nearest experience of a natural habitat. Yet the quality and functioning of estuaries can be irreparably damaged by poorly judged land-claim, ill-regulated industrial development, and inadequate waste management. Contaminants and pollutants become concentrated and trapped in estuaries, because of the natural processes operating there. Estuarine salt marshes form part of the natural system of flood defence, but require careful management if they are to remain effective.

Setting

The character of an estuary as a coastal landscape feature largely depends on the more recent geological history of its surroundings. Today's estuaries occupy landscapes influenced by major oscillations of global sea level, as well as by slow vertical crustal movements of tectonic origin. At higher latitudes, glacio-hydro-isostatic adjustments and direct glacial action also played their parts. All estuaries as we see them today represent a lengthy integration of the actions of numerous processes.

As global sea level most recently moved upward, many estuaries occupy drowned river valleys, some narrow and bold but others hardly noticeable. Good examples of boldly framed estuaries are provided by the Columbia River, on the tectonically active North American West Coast, and by the Severn in south-west Britain and the Gironde in France. By contrast, estuaries on the North American East Coast fill many-branched drowned valleys of subdued relief. Here, modern sedimentation is adding to a thick, offlapping sequence of later Mesozoic and Cenozoic coastal-plain and shallow-marine deposits. Estuaries in drowned valleys are to varying degrees constrained or ‘rock-bound’. Because of resistant hills and valley sides, their shape in plan departs to some degree from the funnel-like form otherwise developed, and there is little room for the normal marginal environments.

Estuaries of alluvial type, unconstrained by bedrock features, arise where sediment is sufficiently abundant for wide valley-flats and coastal plains to have formed. Here, the inner estuary is a sequence of tidal meanders, cuspate giving way seaward to sinuous, which lead to an outer part with the shape of an approximately exponentially expanding funnel, as in the Alligator Rivers of the Northern Territory, Australia, and the Ord River estuary, Western Australia. Each of these presents the full set of estuarine environments permitted by the prevailing tidal and climatic regimes.

Hydraulics

That an estuary is a place where river and sea water mix is the main determinant of its morphological, sedimentary, and biological features. The mixing is neither uniform nor steady. It is non-uniform because the ratio of salt to fresh water increases as the estuary grows in cross-section seaward. The mixing is unsteady, because of variations in the height and strength of the tide, especially on semidiurnal, diurnal, and spring–neap scales, and because of seasonal and storm-related changes of river discharge. Thus different mixing states can exist in an estuary at any one moment over space and at different times at the same place. Averaged tidally, however, estuaries are of three mixing types (Fig. 1): salt-wedged, partially mixed, and well-mixed.

Salt-wedge estuaries typically are microtidal, the characteristic tidal range being below 2 m. In a salt-wedge estuary, the river water, because of its lower density, rides over a wedge of salt water which moves short distances up and down the estuary in response to the weak flood and ebb of the tide. The stream of river water erodes the sharp top of the salt wedge or halocline, and there is a consequent upward mixing into it. Replacement sea water enters the estuary as a weak landward current. Normally, relatively fresh water spreads far and wide over the sea from the mouth of a salt-wedge estuary, with offshore waves accomplishing the final mixing.

A partially mixed estuary normally has a mesotidal regime, typified by a range of 2–4 m. The tidal currents are now quite strong and the whole water mass in the estuary moves first landward and then seaward with a tidal periodicity. Because of strong shear the currents are very turbulent. The halocline is now a broad vertical zone through which salt water mixes upward as fresh water mixes downward. The tidally averaged inflow of salt water to replace what is mixed out of the bottom layer is relatively vigorous and there is a correspondingly much enhanced discharge of brackish water through the surface layer. A seaward increase in the salinity of both the surface and bottom layers is assured by this mechanism of mixing.

A tidal range in excess of 4 m gives an estuary a macrotidal or hypertidal regime and invariably makes it well-mixed. The vigorous tidal currents are sufficient to preclude any vertical salinity stratification, creating an effectively vertically homogeneous water mass. Only a gradual longitudinal salinity gradient can be sustained. Some lateral variations in salinity and water velocity arise, however, and horizontal circulations tend to develop on a tidal scale. There can be a landward drift of water along one bank but a seaward drift along the other; an axial inward drift balanced by an outward drift along each bank; or in the largest estuaries a complex set of large horizontal gyres.

Sediment supply and dispersal

Estuaries receive sediment from many sources. Some bed-material is supplied by rivers, but tracer studies show that a part, and in some cases nearly all, of the sand originates offshore. Although the river catchments are the primary providers of most suspended fine sediment, which becomes flocculated in the saline environment, in many estuaries there is a vigorous recycling of mud because of alternating erosion and deposition at the bed and banks. Well-mixed estuaries are particularly turbid. The suspended mud in the hypertidal Severn Estuary, for example, in mass equals many times the annual fluvial supply. Some mud reaches estuaries from seaward, having been scoured from neighbouring coastal cliffs or the sea bed. Cliffs within rock-bound estuaries can be local sources of coarse debris, which can form beaches.

A complex interaction between the tidal flow and sediment erosion–deposition creates in partially and well-mixed estuaries a turbidity maximum (Fig. 1). Here the greatest concentration of suspended mud can be one to two orders of magnitude more than in most of the estuary. Because the turbidity maximum is a dynamic phenomenon, it varies in strength and position on tidal periodicities and also with river discharge, as research in the Seine Estuary, France, has demonstrated.

Sedimentary processes, environments, and facies

Sediment erosion and deposition roughly balance on a tidal scale in estuaries, implying that the tidal streams have much the same speed everywhere. This ensures that estuaries, which are blind-ended, must progressively widen and deepen seaward in order for the tidal flows to meet the requirements of hydraulic continuity. Because the ‘tidal wave’ is reflected in an estuary, on account of the blind end, high and low tide are times of slack or sluggish water, whereas the tidal streams peak in speed at about mid-tide. The vertical distribution of sedimentary environments and facies in an estuary is controlled by this pattern of currents.

Suspended mud settles out from the low-velocity waters associated with high tide, creating tidal marshes and mudflats traversed by dendritic to net-like creek-systems in the upper part of the intertidal zone on the margins of estuaries. Tidal marshes trap silt brought in by the tides high enough to drown them, but also obtain sediment, which may be preserved as peat, from the salt-tolerant plants (halophytes) growing there. The salt marshes of high and middle latitudes, populated by herbaceous and some shrubby plants, are replaced in the tropics by mangals, where shrubs and long-lived trees, particularly species of mangrove, predominate. Tidal marshes support dense population of invertebrates. The lower limit of halophyte growth, about the level of neap high tides, is controlled by predation on seeds and seedlings as well as by the salt tolerance of adult plants. Mudflats lie lower in the upper intertidal zone than marshes and are smooth, almost level surfaces across which tidal creeks meander. Lacking macroscopic plants, but with many algae, they support huge, low-diversity populations of filter- and surface-feeding invertebrates, preyed on particularly by birds.

Sandy or gravelly beaches replace tidal marshes and mudflats in the upper intertidal zone in the outer parts of large estuaries and at the foot of bedrock cliffs, wherever the fetch at high tide and exposure can allow powerful wind-waves. On exposed coasts, the wind can blow sand inland from the beaches to form belts of supratidal aeolian dunes, so long as a sufficiently protective vegetation cover is unable to form. Together with salt marshes, these dunes also contribute to the natural flood defences of estuaries, protecting low-lying alluvial areas to landward. Strong mid-tide flows create unstable sand shoals, sand flats, and channels in the subtidal and low intertidal zones of estuaries, for example, the Severn Estuary and Bay of Fundy. Tidal ripples and dunes decorate these shoals, which internally are mainly cross-stratified, with some parallel laminations formed by peak currents. Few organisms can tolerate these shifting substrates, but transported shells and waterlogged wood and plant debris are common.

Slack water occurs again around low tide. Not only do gravels arise subtidally, but mud can accumulate permanently at these depths, especially where the turbidity is high. Associated bodies of flud mud—partly settled but not truly deposited—feature in the subtidal zones of many partially and well-mixed estuaries.

John Allen

Bibliography

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Dalrymple, R. W.,, Zaitlin, B. A.,, and and Boyd, D. (1992) Estuarine facies models: conceptual basis and stratigraphic implications. Journal of Sedimentary Petrology, 62, 1030–46.
Dyer, K. R. (1994) Estuarine sediment transport and deposition. In ( Pye, K. (ed.) Sediment transport and depositional processes, pp. 193–218. Blackwell Scientific Publications, Oxford.
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