Tidal river

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A tidal river is a river whose flow and level are caused by tides. A section of a larger river affected by the tides is a tidal reach, but it may sometimes be considered a tidal river if it had been given a separate and another title name.


Generally, tidal rivers are short rivers with relatively low discharge rates but high overall discharge, which generally implies a shallow river with a large coastal mouth. In some cases, high tides impound downstream flowing freshwater, reversing the flow and increasing the water level of the lower section of river, forming large estuaries. High tides can be noticed as far as 100 kilometres (62 mi) upstream. Oregon's Coquille River is one such stream for which that effect can be noticed.


The area of a tidal river can be difficult to define. The term "tidal river" generally encompasses the area upriver of the maximum limit of salinity intrusion and downriver of tidal water level fluctuations. [1] This classification is based on both tidal trends and salinity. By this definition, a tidal river will be affected by tides, surges, and sea level variation, though its water may not have a high salinity content. If that is the case, this section of river can be known as a "tidal freshwater river" or a "river reach." [1] In terms of tides, tidal rivers are classified as microtidal (<2 m), mesotidal (2-4 m), and macrotidal (>4 m). [2] Areas of brackish water seaward of the tidal river section are often called estuaries. A phenomenon commonly associated with tidal rivers is a tidal bore, where a wall of water travels upriver during a flood tide. [1]

Freshwater tidal rivers discharge large amounts of sediment and nutrients into the ocean. [3] This is a necessary influx for the global water balance. Rivers contribute about 95% of sediment entering the ocean. [4] Discharge estimates from freshwater tidal rivers are important for informing water resource management and climate analyses. These discharge amounts can be estimated using tidal statistics. [3] Some challenges to estimating discharge amounts include reversing tidal flow, compensation flow for Stokes drift, spring-neap water storage effects, lateral circulation, and multiple distributaries or ebb and flood channels. [3]


Tidal rivers face threats due to climate change and other human-caused impacts. In tidal rivers' deltas, mineral and water extraction, reduced sediment input, and floodplain engineering are causing the sinking of deltas. This, combined with rising sea levels, is causing tidal rivers to become deeper, which amplifies the tidal motion and increases the extent of salt intrusion. [5] Increasing salinity in tidal rivers could have a detrimental impact on freshwater organisms and alter tidal river ecosystems significantly. [6] The increasing effect of deltaic subsidence, which is due to the removal of gas, oil, and water from deltas, will also increase the risk of flooding. [5]

Tidal river examples

Rio de la Plata

The Rio de la Plata is a tidal river on the border between Uruguay and Argentina. It is classified as microtidal, as its tidal range is less than 1 meter. This river is significant mostly due to its size, as more than one tidal wavelength can be accommodated in this river's estuary. Similarly to most tidal rivers, saltwater does not extend far up the river, due to its large volume of freshwater discharge. [7]

Amazon River

The Amazon River has the highest flow, largest volume of sediment discharge, and largest drainage basin of any river in the world. Because of its large flow volume, saltwater never enters the mouth of the Amazon River, [7] and the limit of salinity is 150 km seaward of the river mouth. [8] The Amazon River is classified as macrotidal, as its tidal range is 4 to 8 meters at the mouth of the river. [7] During low-flow periods, this river's tidal area may extend over 1,000 km into the Amazon depression. [8]

The tidal behaviour of a river is an important consideration in riverboat navigation. For major rivers, such as the Saint Lawrence River (and the associated Saint Lawrence Seaway), publications such as an atlas of surface currents (or tidal currents) may be available, based on sophisticated hydrodynamic models, subject to empirical validation.[ citation needed ]


See also

Related Research Articles

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An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.

<span class="mw-page-title-main">River delta</span> Silt deposition landform at the mouth of a river

A river delta is a landform shaped like a triangle, created by the deposition of sediment that is carried by a river and enters slower-moving or stagnant water. This occurs at a river mouth, when it enters an ocean, sea, estuary, lake, reservoir, or another river that cannot carry away the supplied sediment. It is so named because its triangle shape resembles the uppercase Greek letter delta, Δ. The size and shape of a delta are controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment. The size, geometry, and location of the receiving basin also plays an important role in delta evolution.

<span class="mw-page-title-main">Salt marsh</span> Coastal ecosystem between land and open saltwater that is regularly flooded

A salt marsh, saltmarsh or salting, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. It is dominated by dense stands of salt-tolerant plants such as herbs, grasses, or low shrubs. These plants are terrestrial in origin and are essential to the stability of the salt marsh in trapping and binding sediments. Salt marshes play a large role in the aquatic food web and the delivery of nutrients to coastal waters. They also support terrestrial animals and provide coastal protection.

<span class="mw-page-title-main">Barrier island</span> Coastal dune landform that forms by wave and tidal action parallel to the mainland coast

Barrier islands are a coastal landform, a type of dune system and sand island, where an area of sand has been formed by wave and tidal action parallel to the mainland coast. They usually occur in chains, consisting of anything from a few islands to more than a dozen. They are subject to change during storms and other action, but absorb energy and protect the coastlines and create areas of protected waters where wetlands may flourish. A barrier chain may extend for hundreds of kilometers, with islands periodically separated by tidal inlets. The largest barrier island in the world is Padre Island of Texas, United States, at 113 miles (182 km) long. Sometimes an important inlet may close permanently, transforming an island into a peninsula, thus creating a barrier peninsula, often including a beach, barrier beach. Though many are long and narrow, the length and width of barriers and overall morphology of barrier coasts are related to parameters including tidal range, wave energy, sediment supply, sea-level trends, and basement controls. The amount of vegetation on the barrier has a large impact on the height and evolution of the island.

<span class="mw-page-title-main">Narragansett Bay</span> Bay in the state of Rhode Island

Narragansett Bay is a bay and estuary on the north side of Rhode Island Sound covering 147 square miles (380 km2), 120.5 square miles (312 km2) of which is in Rhode Island. The bay forms New England's largest estuary, which functions as an expansive natural harbor and includes a small archipelago. Small parts of the bay extend into Massachusetts.

<span class="mw-page-title-main">Coos Bay</span> Estuary in Oregon, United States

Coos Bay is an estuary where the Coos River enters the Pacific Ocean, the estuary is approximately 12 miles long and up to two miles wide. It is the largest estuary completely within Oregon state lines. The Coos Bay watershed covers an area of about 600 square miles and is located in northern Coos County, Oregon, in the United States. The Coos River, which begins in the Oregon Coast Range, enters the bay from the east. From Coos River, the bay forms a sharp loop northward before arching back to the south and out to the Pacific Ocean. Haynes Inlet enters the top of this loop. South Slough branches off from the bay directly before its entrance into the Pacific Ocean. The bay was formed when sea levels rose over 20,000 years ago at the end of the Last Glacial Maximum, flooding the mouth of the Coos River. Coos Bay is Oregon's most important coastal industrial center and international shipping port, with close ties to San Francisco, the Columbia River, Puget Sound and other major ports of the Pacific rim.

<span class="mw-page-title-main">Tidal creek</span> Inlet or estuary that is affected by ebb and flow of ocean tides

A tidal creek or tidal channel is a narrow inlet or estuary that is affected by the ebb and flow of ocean tides. Thus, it has variable salinity and electrical conductivity over the tidal cycle, and flushes salts from inland soils. Tidal creeks are characterized by slow water velocity, resulting in buildup of fine, organic sediment in wetlands. Creeks may often be a dry to muddy channel with little or no flow at low tide, but with significant depth of water at high tide. Due to the temporal variability of water quality parameters within the tidally influenced zone, there are unique biota associated with tidal creeks which are often specialised to such zones. Nutrients and organic matter are delivered downstream to habitats normally lacking these, while the creeks also provide access to inland habitat for salt-water organisms.

<span class="mw-page-title-main">Fly River</span> River in Papua New Guinea

The Fly River is the third longest river in the island of New Guinea, after the Sepik River and Mamberamo River, with a total length of 1,060 km (660 mi). It is the largest by volume of discharge in Oceania, the largest in the world without a single dam in its catchment, and overall the 20th-largest primary river in the world by discharge volume. It is located in the southwest of Papua New Guinea and in the South Papua province of Indonesia. It rises in the Victor Emanuel Range arm of the Star Mountains, and crosses the south-western lowlands before flowing into the Gulf of Papua in a large delta. The Fly-Strickland River system has a total length of 1,220 km (760 mi), making it the longest river system of an island in the world. The 824 km (512 mi) Strickland River is the longest and largest tributary of Fly River, making it the farthest distance source of the Fly River.

<span class="mw-page-title-main">Tidal marsh</span> Marsh subject to tidal change in water

A tidal marsh is a marsh found along rivers, coasts and estuaries which floods and drains by the tidal movement of the adjacent estuary, sea or ocean. Tidal marshes experience many overlapping persistent cycles, including diurnal and semi-diurnal tides, day-night temperature fluctuations, spring-neap tides, seasonal vegetation growth and decay, upland runoff, decadal climate variations, and centennial to millennial trends in sea level and climate.

<span class="mw-page-title-main">Head of tide</span> Farthest point upstream where a river is affected by tidal fluctuations

Head of tide, tidal limit or tidehead is the farthest point upstream where a river is affected by tidal fluctuations, or where the fluctuations are less than a certain amount. The river section influenced by tides and marine forces but without salinity is a tidal river, while downstream areas are brackish and termed estuaries.

<span class="mw-page-title-main">River mouth</span> End of a river where it flows into a larger body of water

A river mouth is where a river flows into a larger body of water, such as another river, a lake/reservoir, a bay/gulf, a sea, or an ocean. At the river mouth, sediments are often deposited due to the slowing of the current, reducing the carrying capacity of the water. The water from a river can enter the receiving body in a variety of different ways. The motion of a river is influenced by the relative density of the river compared to the receiving water, the rotation of the Earth, and any ambient motion in the receiving water, such as tides or seiches.

A mouth bar is an element of a deltaic system, which refers to the typically mid-channel deposition of the sediment transported by the river channel at the river mouth.

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A river plume is a freshened water mass that is formed in the sea as a result of mixing of river discharge and saline seawater. River plumes are formed in coastal sea areas at many regions in the World. River plumes generally occupy wide-but-shallow sea surface layers bounded by sharp density gradients. The area of a river plume is 3-5 orders of magnitude greater than its depth; therefore, even small rivers with discharge rates ~1–10 m/s form river plumes with horizontal spatial extents ~10–100 m. Areas of river plumes formed by the largest rivers are ~100–1000 km2. Despite the relatively small volume of total freshwater runoff to the World Ocean, river plumes occupy up to 21% of shelf areas of the ocean, i.e., several million square kilometers.

<span class="mw-page-title-main">Beaches in estuaries and bays</span> Type of beaches

Beaches in estuaries and bays (BEBs) refer to beaches that exist inside estuaries or bays and therefore are partially or fully sheltered from ocean wind waves, which are a typical source of energy to build beaches. Beaches located inside harbours and lagoons are also considered BEBs. BEBs can be unvegetated or partially unvegetated and can be made of sand, gravel or shells. As a consequence of the sheltering, the importance of other sources of wave energy, including locally generated wind waves and infragravity waves, may be more important for BEBs than for those beaches on the open coast. Boat wakes, currents driven by tides, and river inflow can also be important for BEBs. When BEBs receive insufficient wave energy, they can become inactive, and stabilised by vegetation; this may occur through both natural processes and human action. BEBs exist in all latitudes from beaches located in fjords and drowned river valleys (rias) in high latitudes to beaches located in the equatorial zone like, for example, the Amazon estuarine beaches.

<span class="mw-page-title-main">Sedimentation enhancing strategy</span> Environmental management projects aiming to restore land-building processes in deltas

Sedimentation enhancing strategies are environmental management projects aiming to restore and facilitate land-building processes in deltas. Sediment availability and deposition are important because deltas naturally subside and therefore need sediment accumulation to maintain their elevation, particularly considering increasing rates of sea-level rise. Sedimentation enhancing strategies aim to increase sedimentation on the delta plain primarily by restoring the exchange of water and sediments between rivers and low-lying delta plains. Sedimentation enhancing strategies can be applied to encourage land elevation gain to offset sea-level rise. Interest in sedimentation enhancing strategies has recently increased due to their ability to raise land elevation, which is important for the long-term sustainability of deltas.

<span class="mw-page-title-main">Coastal hydrogeology</span> Branch of hydrogeology

Coastal Hydrogeology is a branch of Hydrogeology that focuses on the movement and the chemical properties of groundwater in coastal areas. Coastal Hydrogeology studies the interaction between fresh groundwater and seawater, including seawater intrusion, sea level induced groundwater level fluctuation, submarine groundwater discharge, human activities and groundwater management in coastal areas.


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