Chapter 6 Tide-Dominated Estuaries and Tidal Rivers

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Summary Physical and biological processes in nearly all estuaries are influenced by tides. The degree of influence is governed by estuarine morphology, tide range, water and sediment discharge, winds, and shelf processes. Tide dominated estuaries are those in which tidal currents play the dominant role in the fate of river-borne sediments, resulting in appreciable upstream transport of bedload sediment and, in extreme cases, little or no density-driven circulation. Tidal rivers, which have many of the same morphologic and sedimentologic features, are estuaries that occur in the lower reaches of large rivers where the penetration of tide extends farther than, and is decoupled from, the upstream penetration of salt. Here, subaqueous deltaic sedimentation is common. Most tide dominated estuaries and tidal rivers have a funnel shape, bidirectional sediment transport, mutually-evasive transport pathways, a tide- or density-induced turbidity maximum, and extensive regions of fine-grained sediment deposition, often in the form of fluid mud. Bottom sediments range from mud to gravel. As tides move upstream through smaller cross-sectional areas, the tidal currents become progressively more asymmetric in both speed and direction. In many cases, this leads to net landward transport of the bedload sediments. Characteristic bedforms include tidal sand ridges, large sand waves, and megaripples; characteristic sedimentary structures include cross bedding, tidal bedding, reactivation surfaces, and flaser, wavy, and lenticular bedding. Tidal flats, mangrove swamp, or marsh grass usually form the margins of the estuaries. Examples of tide-dominated estuaries and tidal rivers can be found in a wide variety of settings: for example, in the Rio de la Plata and Amazon tidal rivers, where respective tide ranges are less than 1 m and 4–8 m, much of the sedimentation occurs in the form of subaqueous deltas that have built over and around transgressive sands; in the Gironde, where tide range is range 4 m, a highly transitory turbidity maximum characterizes the estuary yet 60% of the suspended sediment leaves the estuary and accumulates on the shelf; in the Severn, where tide range is 8 m, sedimentation rates are so low that exposed bedrock covers extensive sections of the estuary; and, in the Cobequid Bay-Salmon River, where tide range can exceed 12 m, extensive progradation is occurring from sands derived seaward of the estuary.