Query: These tidal floods are often just a foot or two deep, but they can stop traffic, swamp basements, damage cars, kill lawns and forests, and poison wells with salt.

Relevant passages:
Salt marsh <=SEP=> A salt marsh or saltmarsh , 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 salt water 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 .

River Parrett <=SEP=> The River Parrett flows through the counties of Dorset and Somerset in South West England , from its source in the Thorney Mills springs in the hills around Chedington in Dorset . Flowing northwest through Somerset and the Somerset Levels to its mouth at Burnham-on-Sea , into the Bridgwater Bay nature reserve on the Bristol Channel , the Parrett and its tributaries drain an area of 1700 km2 -- about 50 per cent of Somerset 's land area , with a population of 300,000 .   The Parrett 's main tributaries include the Rivers Tone , Isle , and Yeo , and the River Cary via the King 's Sedgemoor Drain . The 37 mi long river is tidal for 19 mi up to Oath . The fall of the river between Langport and Bridgwater is only 1 ft/mi , so it is prone to frequent flooding in winter and during high tides . Many approaches have been tried since at least the medieval period to reduce the incidence and effect of floods and to drain the surrounding fields .   In Anglo-Saxon times the river formed a boundary between Wessex and Dumnonia . It later served the Port of Bridgwater , and enabled cargoes to be transported inland . The arrival of the railways led to a decline in commercial shipping , and the only working docks are at Dunball . Man 's influence on the river has left a legacy of bridges and industrial artefacts . The Parrett along with its connected waterways and network of drains supports an ecosystem that includes several rare species of flora and fauna . The River Parrett Trail has been established along the banks of the river .

Salt pannes and pools <=SEP=> Salt pannes and pools are water retaining depressions located within salt and brackish marshes . Pools tend to retain water during the summer months between high tides , whereas pannes generally do not . Salt pannes generally start when a mat of organic debris ( known as wrack ) is deposited upon existing vegetation , killing it . This creates a slight depression in the surrounding vegetation which retains water for varying periods of time . Upon successive cycles of inundation and evaporation the panne develops an increased salinity greater than that of the larger body of water . This increased salinity dictates the type of flora and fauna able to grow within the panne . Salt pools are also secondary formations , though the exact mechanism ( s ) of formation are not well understood ; some have predicted they will increase in size and abundance in the future due to rising sea levels .   Salt pannes and pools are unique microhabitats dominated by various species of halophytes , benthic plants and varying estuarine marine life that vary considerably in composition due to a variety of factors :  Substrate type : affects the ability of the depression to hold water .  depth and diameter : affect water temperature and evaporation rate in the depression . A shallow and wide pool will evaporate at a greater rate than a pool of the same volume of water which is deeper and has a smaller surface area . Evaporation rate also affects salinity , the higher the evaporation rate the higher the salinity , with rates as high as a third greater than ocean water .  location within the intertidal zone , whether high marsh or low marsh and distance from the mean low tide mark which affects the length and duration of inundation until the depression is subject to evaporation as well as length of time until the rising tide replenishes the water volume .   These factors affect the types of species which can survive within the various types of salt pannes and pools .   Variants of salt pannes and pools :   Low salt marsh   Low salt marsh panne   Usually devoid of vegetation , that may be present include smooth cordgrass ( Spartina alterniflora ) , marine algae such as knotted wrack ( Ascophyllum nodosum ) and rockweeds ( Fucus spp . ) . The substrate is typically soft , silty mud .   High salt marsh   Arrow-grass ( forb ) panne  Briefly flooded , very shallow with a moderate amount of vegetation usually dominated by Arrow grass ( Triglochin maritimum ) , with the deeper sections possibly remaining unvegetated .   Smooth cord-grass ( short form ) panne  Shallow anaerobic depressions with poor drainage , poor water quality due to low nutrient levels and high concentrations of sulfides and similar compounds which inhibit plant growth . Short form ( 6-12 '' tall ) smooth cord-grass ( Spartina alterniflora ) is the dominant plant species . Typically found on the high salt marsh , but can occasionally be found on the upper margins of low salt marsh .   Salt marsh mosquito panne   Minimal vegetation often found on the upper half of the high salt marsh . It is typically deeper than forb and smooth cord-grass pannes . Usually flooded by the higher of the two spring tides , retains water for 2 -- 3 weeks later until drying out . The female eastern salt marsh mosquito ( Aedes sollicitans ) lays eggs on the exposed surface . The eggs lay dormant until the next time the panne floods .   Widgeon grass ( Ruppia maritima ) - marsh minnow deepwater pool  Pools on the high salt marsh that are semi-permanently and permanently flooded . They are able to sustain populations of Sheephead minnow ( Cyprinodon variegatus variegatus ) , Mummichogs , ( Fundulus heteroclitus ) , and other species of small fish which may become trapped in the pools and benthic species of vegetation . Occasioanally can be found at the upper edge of the low salt marsh .

Wetland <=SEP=> A wetland is a land area that is saturated with water , either permanently or seasonally , such that it takes on the characteristics of a distinct ecosystem . The primary factor that distinguishes wetlands from other land forms or water bodies is the characteristic vegetation of aquatic plants , adapted to the unique hydric soil . Wetlands play a number of roles in the environment , principally water purification , flood control , carbon sink and shoreline stability . Wetlands are also considered the most biologically diverse of all ecosystems , serving as home to a wide range of plant and animal life .   Wetlands occur naturally on every continent except Antarctica , the largest including the Amazon River basin , the West Siberian Plain , and the Pantanal in South America . The water found in wetlands can be freshwater , brackish , or saltwater . The main wetland types include swamps , marshes , bogs , and fens ; and sub-types include mangrove , carr , pocosin , and varzea .   The UN Millennium Ecosystem Assessment determined that environmental degradation is more prominent within wetland systems than any other ecosystem on Earth . International conservation efforts are being used in conjunction with the development of rapid assessment tools to inform people about wetland issues .   Constructed wetlands can be used to treat municipal and industrial wastewater as well as stormwater runoff . They may also play a role in water-sensitive urban design .

River Thames <=SEP=> The River Thames ( -LSB- tɛmz -RSB- ) is a river that flows through southern England , most notably through London . At 215 mi , it is the longest river entirely in England and the second longest in the United Kingdom , after the River Severn . It also flows through Oxford ( where it is called Isis ) , Reading , Henley-on-Thames and Windsor . The lower reaches of the river are called the Tideway , derived from its long tidal reach up to Teddington Lock . It rises at Thames Head in Gloucestershire , and flows into the North Sea via the Thames Estuary . The Thames drains the whole of Greater London .   Its tidal section , reaching up to Teddington Lock , includes most of its London stretch and has a rise and fall of 7 m. Running through some of the driest parts of mainland Britain and heavily abstracted for drinking water , the Thames ' discharge is low considering its length and breadth : the Severn has a discharge almost twice as large on average despite having a smaller drainage basin . In Scotland , the Tay achieves more than double the average discharge from a drainage basin that is 60 % smaller .   Along its course are 45 navigation locks with accompanying weirs . Its catchment area covers a large part of South Eastern and a small part of Western England and the river is fed by 38 named tributaries . The river contains over 80 islands . With its waters varying from freshwater to almost seawater , the Thames supports a variety of wildlife and has a number of adjoining Sites of Special Scientific Interest , with the largest being in the remaining parts of the North Kent Marshes and covering 5449 ha .   In 2010 , the Thames won the largest environmental award in the world -- the $ 350,000 International Riverprize .