Presentation on theme: "Where the Rivers meet the Sea"— Presentation transcript:
1Where the Rivers meet the Sea EstuariesWhere the Rivers meet the Sea
2Peritidal/ Estuarine system DefinitionTypesCirculationTidesPeritidal/ Estuarine systemMarshTidal ChannelsTidal DeltasMudflats/ Tidal flatsBiology“Wetlands”
3Estuaries Where the Rivers meet the Sea Semi- enclosed body of water where there is a mix of river and seawater, and mix of fluvial and marine processesMouths of many rivers flooded at last sea level high (last interglacial)Boundary between fluvial and marine processesInfluenced by tides
4Types of EstuariesDrowned river valleyFjordBar- builtTectonic
12Value of EstuariesGreatly influenced by river discharge and tidal mixing.Environmental conditions fluctuate widely.Biomass is high because estuaries have high nutrient levels.Species diversity is low because fewer species can cope with the wide fluctuations in temperature, salinity etc.“nursery grounds”young stages develop in the estuary(abundant food; low predationmay move offshore to adult habitats
13Estuaries greatly influenced by river discharge and tidal mixing. Environmental conditions fluctuate widely.Biomass is high because estuaries have high nutrient levels.Species diversity is low because fewer species can cope with the wide fluctuations in temperature, salinity etc.“nursery grounds”young stages develop in the estuary(abundant food; low predationmay move offshore to adult habitats
14TIDES tidal bulge results from moon and sun neap (sun and moon effects cancel)spring (compound)diurnal; semidiurnal; mixed semidiurnal
18tidal current generated by horizontal movement of water flood tides most pronounced in mesotidalcarries great volume in macrotidalflood tidesIncomingmoves water onshore between low and highebb tidesOutgoingmoves water offshoreSlacktop and bottom of cycle (when high is in, or low is in)bidirectional flowleads to bidirectional cross- stratification with mud drapesherring bone, mud drape, reactivation surface
19Basic Circulation Basic plumbing water flows in at high tide- fills upflows out at low! - empties(Variations on the theme)may be completely empty or may be wet throughout cycleHarsh environment- desiccation, daily changes in temperature and salinity
20ClassificationOne way of quantifying is by comparing the volume R of freshwater that enters from the river during one tidal period, with the volume V of water brought into the estuary by the tide and removed over each tidal cycle.R is sometimes called the river volume, while V is known as the tidal volume.
21ClassificationEstuaries can be grouped into classes, according to their circulation properties and the associated steady state salinity distribution. The most important estuary types are 1. salt wedge estuary (R>>V) 2. highly stratified estuary (R>V) 3. slightly stratified estuary (R<V) 4. vertically mixed estuary (R<<V) 5. inverse or reverse estuary (R=0)
23Salt WedgeRiver volume R is very much larger than the tidal volume V, or there are no tides at all.The fresh water flows out over the sea water in a thin layer.All mixing is restricted to the thin transition layer between the fresh water at the top and the "wedge" of salt water underneath.Vertical salinity profiles therefore show zero salinity at the surface and oceanic salinity near the bottom all along the estuary.The depth of the interface decreases slowly as the outer end of the estuary is approachedMississippi and Congo Rivers
25Highly Stratified Estuary River volume R is comparable to but still larger than tidal volume V.Strong velocity shear at the interface produces internal wave motion at the transition between the two layers.The waves break and "topple over" in the upper layer, causing entrainment of salt water upward.Entrainment is a one-way process, so no fresh water is mixed downward.This results in a salinity increase for the upper layer, while the salinity in the lower layer remains unchanged, provided the lower layer volume is significantly larger than the river volume R and can sustain an unlimited supply of salt waterExamples of this type of estuary are fjords, which are usually very deep and have a large salt water reservoir below the upper layer.
27Slightly Stratified Estuary River volume R is small compared to tidal volume V.The tidal flow is turbulent through the entire water column (the turbulence induced mainly at the bottom).As a result, salt water is stirred into the upper layer and fresh water into the lower layer.Salinity therefore changes along the estuary axis not only in the upper layer but in both layersThis type of estuary is widespread in temperate and subtropical climates
28Vertically Mixed Estuary River volume R is insignificant compared with tidal volume V.Tidal mixing dominates the entire estuary.Locally it achieves complete mixing of the water column between surface and bottom, erasing all vertical stratification.As a result, vertical salinity profiles show uniform salinity but a salinity increase from station to station as the outer end of the estuary is approachedThis type of estuary is found in regions of particularly strong tides; an example is the River Severn in England.
31Inverse or Reverse Estuaries These estuaries have no fresh water input from rivers and are in a region of high evaporation.Surface salinity does not decrease from the ocean to the inner estuary, but water loss from evaporation leads to a salinity increase as the inner end of the estuary is approachedThis results in a density increase and sinking of high salinity water at the inner end.As a result, movement of water is directed inward at the surface and towards the sea at the bottom, with sinking at the inward end.
33Components of the Peritidal System Salt marsh 1. High marsh 2. Low marshTidal channelsTidal deltasEbbFloodTidal mudflats
34Salt MarshSalt marshes are vegetated inter-tidal flats.
35Salt Marsh Marsh divided into high and low marsh high is region above high tidelow is region that’s flooded dailyeach has distinctive vegetations and grain sizehigh has coarser sedimentsIs a much more terrestrial-like environment because it is flooded only at times of extreme high tide.Is characterized by a more varied plant community. Spartina patenslow has finer sedimentsIs dominated by the smooth cordgrass, Spartina alterniflora.cut by channels which funnel water in and outmay completely empty or may be wet throughout cycle
39Salt Marsh cut by tidal channels funnel water in and out daily influx of water brings mudsbuild up mud flats
40Tidal channels funnels water into single or multiple channels currents can scour basealso meander and can cause walls to faillag of gravelly deposits (including shell material and mud clastsarmored mud ballsform point bars similar to those in riversBUT have fine material on top of sloping bar surfacealternations of sand and mud on bar depositstidal channel has lag;channels generally cut down a few meters at least channel abandoned and finer sediments infill
43Tidal flatsform in proximal areas and between channels
44Tidal mudflats away from area of strong tidal currents flooded at high; exposed at lowfine sediments are carried in on high tide and deposited during slack and as it retreatscommonly bioturbatedsurface network of channels partially channelize flowallows vegetation to flourishPEAT: thick accumulations of dead vegetationorganisms eat decaying vegetationlow marsh characterized by this process of daily alternating currentshigher areas of marsh are site of accumulation of dead marsh grass (wrack)
45Cores through tidal flat deposits of the Devonian Swan Hills Formation, Alberta, Canada Dr. Stacy Atchley, Baylor Univ.
46Tidal Flat Colonization—Paleontology and sedimentology of a Late Cambrian shoreline, where soft-bodied carcasses are stranded and early crawling organisms left tracks and trails. Dr. Whitey Hagadorn , Amherst Coll
49Tidal Mudflats Flooded at high; exposed at low Fine sediments are carried in on high tide and deposited during slack and as it retreatscommonly bioturbatedfiddler crabs, etcSurface network of channels partially channelize flowallows vegetation to flourishPEAT: thick accumulations of dead vegetation(organisms eat decaying vegitation)
50Marine and transitional marine to tidal flat Parallel bedded sandstone and mudstone. Note flame structures near base and load casts near top of core Shallow subtidal to intertidalLenticular and flaser bedding: sandstone and mudstone. Typical of tidal flatsMassive sandstone with vertical burrows
51Biology of Salt Marsh Fiddler crabs, snails, mussels, birds and fish The frequency of tidal flooding plays a major role in determining the activity patterns of these animals.For example, fiddler crabs seek refuge in burrows during flood tide to escape predators.At low tide they leave their burrows and search for food.
52Productivity of Salt Marshes The most productive portion of the salt marsh is the low salt marsh regioninhabited by Spartina alterniflora.little is grazed directly by animalsmost of the production enters the detritus food chain.material not consumed by detritivores accumulates in the sedimentforms peat or is exported from the marsh in tidal currentsroot systems of salt marsh plants bind the sedimentsstabilize the substrateReduces erosion of the coastal environment
53Estuarine successions if subsidence provides space, sediments can accumulate (otherwise reworked)great LATERAL variability
54Recognizing estuaries BUT delta is progradational Looks a lot like deltas!BUT delta is progradationalestuaries are aggradationalbuild up within drowned river channelbase of succession is usually an erosional surface
55Wetlands flank estuaries since estuaries are natural harbors and often PORTS wetlands are often polluted