Tidal rivers

Fig. 1. Floodplain of the Scheldt tidal river at low water, downstream of Gentbrugge. Photo credit Waterwegen en Zeekanaal NV^[1]

Rivers that debouch into a coastal sea are subject to tidal motion. Even small tides can have a significant impact on hydrosedimentary and ecological processes, especially in the case of small river discharges. The limit of tidal wave propagation in a river is much further upstream than the limit of seawater intrusion (except in the case of a seaward weir or dam). The intermediate section between these two limits is called the tidal river. Tidal rivers have no fixed spatial boundaries. Both the seawater intrusion limit (the location where the salinity is near zero) and the upstream limit of tidal wave propagation (the location where river discharge is not significantly modulated by the tide) vary with river discharge and tidal amplitude. The seawater intrusion limit can vary by ten to one hundred kilometers and the tidal influence limit can vary by more than 100 kilometers in some cases (Table 1).

Table 1. Examples of variations in the seawater and tidal intrusion limits^{[2] [3][4][5]}

The following is an introduction to the geomorphological and ecological characteristics of tidal rivers. These features are localized and do not move up and down the river as a function of runoff and tide. Therefore, we consider as a tidal river the zone situated between the seawater and tidal intrusion limits under average runoff and tidal conditions. The hydrodynamics is covered in a separate article River tides.

Geomorphological characteristics

Fig. 2. Cross bedding of tidal river dunes. Panel a: Cross bed dipping (grain avalanches) to the right in unidirectional river flow. Panel b. Herringbone cross bedding. The flood flow has eroded the top of the (ebb flow) river dune, creating a flood cap with cross bed dipping to the left. Redrawn after Dalrymple and Choi (2007^[3]).

Upstream of the tidal intrusion limit, river discharge is typically confined within a single channel, often fed by several tributaries. The river channel is strongly meandering with point bars at the inner bends. The average river width varies only slightly, with the exception of (possibly strong) local variations related to landscape features. The tidal river geomorphology changes downstream, where river discharge is modulated by the tide. The river width gradually increases while the sinuosity decreases^[3]. The bed grain size also decreases, but is generally large enough (greater than about 0.15 mm) to allow for the development of river dunes and bed ripples. These bedforms migrate in a downstream direction and therefore display a gently sloping stoss side and a leeward side that dips strongly according to the so-called angle of repose, see Fig. 2a. Further downstream, the river enters a zone where the discharge is mostly bidirectional, the tidal discharge being greater than the average river discharge. Bedforms here have a cross-stratified internal structure generated by the alternating currents, see Fig. 2b. Near the seawater intrusion limit, fine sediments tend to accumulate and form a so-called turbidity maximum. The underlying processes are described in the article Estuarine turbidity maximum.

Ecosystem characteristics

The floodplains along the tidal river are (partly) submerged with a much higher frequency than the floodplains of the upstream river. These floodplains often have a complex geometry including hammocks, hummocks, levees, crevasse channels, surface depressions and areas of high and low vegetated floodplain platform^[6]. The floodplain waters are shallow and levels fluctuate daily, seasonally or annually due to tides, flooding, evapotranspiration, groundwater recharge, or seepage losses. While the vegetation of seasonal wetlands is dynamic, the vegetation of tidal freshwater wetlands tends to be more stable. Certain aquatic vascular plants (macrophytes) are well adapted to the variable water levels and achieve high rates of primary productivity in spite of the ever changing environment. The vegetation comprises evergreen emergent aquatic macrophytes, chiefly graminoids such as rushes, reeds, grasses and sedges, and shrubs and other herbaceous species such as broad-leaved emergent macrophytes, floating-leaved and submergent species, and nonvascular plants such as brown mosses, liverworts, and macroscopic algae^[7]. Submerged macrophytes can form extensive beds in the lower parts of the floodplain. Floodplain wetlands usually support high diversity and primary production, especially in the case of tropical and semitropical floodplains where seasonal inundation often is prolonged and occurs at high temperatures. However, species diversity is generally not as high as in the marine part of the system^[3]. Insect larvae make abundant burrows in the sediment bed, but the degree of bioturbation is less than in the marine environment^[8]. Freshwater tidal wetlands permanently remove dissolved inorganic nitrogen (DIN) from riverine and estuarine waters via burial and denitrification, especially through the reduction of NO₃⁻ to gaseous N₂^[9]. Many features of tidal freshwater wetlands favor denitrification, such as high active surface area, shallow depth to anaerobic zone, and high availability of organic matter.

Downstream, the tidal river ecosystem is subject to variable brackish-water conditions during low-runoff periods. Organisms are also exposed to high turbidity in the water column and must cope with frequent sediment disturbance (deposition or erosion), periodic exposure to the atmosphere and the associated temperature changes on the intertidal flats. Few organisms are adapted to live in this hostile environment. The number of species present (i.e. species diversity) is generally lowest in areas with salinity levels of about 1–5 ppt, with species diversity increasing towards the sea and upstream towards fresh water^[3]. The organisms that do live within the fluvial–marine transition are generally those that are adapted to life in salt water and display behaviours that protect them from these harsh and highly variable conditions^[10]. They are mainly opportunists, able to quickly colonize surfaces when conditions are suitable^[11]. They have a fast reproduction rate and usually occur in large numbers (high abundancy), commonly in near mono-specific communities^[12]. Most organisms live within the sediment rather than on the surface, employing various feeding strategies due to the variable nature and location of food resources. A variety of detritus feeders (including oysters, clams, lobsters, and crabs) and various insect larvae, annelid worms, and molluscs are tolerant of these stressed conditions. They represent impoverished marine communities, rather than mixtures of freshwater and marine biota ^[13]. Organisms tend to be smaller in size than the same species would be in fully marine settings^[14].

Channel incision

Tidal rivers are favorite sites for urban and industrial development as they provide easy waterway connectivity between the sea and the hinterland. The availability of fresh water is also a reason why large port cities have sprung up on the tidal river rather than on the saline estuary. European examples are Hamburg (Elbe), Antwerp (Scheldt), Rouen (Seine), Nantes (Loire), Bordeaux (Gironde) and London (Thames). The navigability of the tidal river is an important economic asset for these cities. In many cases this has led to engineering interventions to improve navigability by channelizing the river. In many tidal rivers the fairway has been constricted by groins during the past century, e.g. in the Rhine (Waal branch), Elbe and Loire. The resulting increase in current velocities, especially during periods of peak runoff, has led to erosion of the river bed and incision of the river channel (cutting down of the bed). In the Loire, for example, the river bed was lowered several meters, see Fig. 3, with various adverse consequences. The water level at low runoff has fallen as much as the average riverbed incision, resulting in a net decrease in water depth at sites with erosion-resistant soil layers. In addition, bridge piers have been undermined, groundwater levels have been lowered, side arms have been disconnected, inflow structures have been lost, and habitat quality has deteriorated, affecting the ecological status of valuable floodplains. Similar consequences of river channeling have also been reported for other rivers, for example the Danube^[15] and Italian rivers^[16]. Incision of river channels can also be the result of other interventions in the river system, in particular dam construction (by retaining sediment in reservoirs) and sand extraction, which disrupts the continuity of sediment transport. These interventions generally take place higher upstream, so that downstream incision of the riverbed only occurs in the long term. Reservoirs can also reduce flood peaks downstream, potentially reducing the effects of sediment supply starvation, causing channel narrowing, or allowing fine sediments to accumulate on the bed^[17]. The adverse effects of river bed incision can be mitigated by sand nourishments, a measure that is intended for the Waal^[18]. A restoration program for the tidal river Loire consists of removing and lowering the groynes and remobilizing the sand that has accumulated in the intermediate dead zones^[19].

Fig. 3. Channel incision Loire tidal river. Left panel: Groins in the Loire tidal river near Anetz. Photo credit GIP Loire Estuaire. Right panel: Lowering of the river bed along the thalweg due to channel constriction. Redrawn from VNF (2019^[19])

Related articles

River tides

Estuarine turbidity maximum

Morphology of estuaries

References