![]() While the vegetation of seasonal wetlands is dynamic, the vegetation of tidal freshwater wetlands tends to be more stable. The floodplain waters are shallow and levels fluctuate daily, seasonally or annually due to tides, flooding, evapotranspiration, groundwater recharge, or seepage losses. These floodplains often have a complex geometry including hammocks, hummocks, levees, crevasse channels, surface depressions and areas of high and low vegetated floodplain platform. The floodplains along the tidal river are (partly) submerged with a much higher frequency than the floodplains of the upstream river. The underlying processes are described in the article Estuarine turbidity maximum. Near the seawater intrusion limit, fine sediments tend to accumulate and form a so-called turbidity maximum. Bedforms here have a cross-stratified internal structure generated by the alternating currents, see Fig. ![]() Further downstream, the river enters a zone where the discharge is mostly bidirectional, the tidal discharge being greater than the average river discharge. 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. 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. The river width gradually increases while the sinuosity decreases. The tidal river geomorphology changes downstream, where river discharge is modulated by the tide. ![]() The average river width varies only slightly, with the exception of (possibly strong) local variations related to landscape features. The river channel is strongly meandering with point bars at the inner bends. Upstream of the tidal intrusion limit, river discharge is typically confined within a single channel, often fed by several tributaries. Redrawn after Dalrymple and Choi (2007 ). The flood flow has eroded the top of the (ebb flow) river dune, creating a flood cap with cross bed dipping to the left. Panel a: Cross bed dipping (grain avalanches) to the right in unidirectional river flow. The hydrodynamics is covered in a separate article River tides.įig. Therefore, we consider as a tidal river the zone situated between the seawater and tidal intrusion limits under average runoff and tidal conditions. These features are localized and do not move up and down the river as a function of runoff and tide. The following is an introduction to the geomorphological and ecological characteristics of tidal rivers. Examples of variations in the seawater and tidal intrusion limits Tidal river 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). 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. Tidal rivers have no fixed spatial boundaries. The intermediate section between these two limits is called the tidal river. 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). Even small tides can have a significant impact on hydrosedimentary and ecological processes, especially in the case of small river discharges. Rivers that debouch into a coastal sea are subject to tidal motion. Floodplain of the Scheldt tidal river at low water, downstream of Gentbrugge.
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