1. Where the Rivers meet the Sea
Estuaries
Where the Rivers meet the Sea

2. Peritidal/ Estuarine system
Definition
Types
Circulation
Tides
Peritidal/ Estuarine system
Marsh
Tidal Channels
Tidal Deltas
Mudflats/ Tidal flats
Biology
“Wetlands”

3. Estuaries 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 processes
Mouths of many rivers flooded at last sea level high (last interglacial)
Boundary between fluvial and marine processes
Influenced by tides

4. Types of Estuaries
Drowned river valley
Fjord
Bar- built
Tectonic

6. Tectonic
San Francisco Bay

7. Fjord

8. Chesapeake Bay: Drowned Shoreline

9. ESTUARY
The Netherlands

10. Barrier Island

12. Value of Estuaries
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 predation
may move offshore to adult habitats

13. Estuaries 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 predation
may move offshore to adult habitats

14. TIDES tidal bulge results from moon and sun
neap (sun and moon effects cancel)
spring (compound)
diurnal; semidiurnal; mixed semidiurnal

15. W. W. Norton

16. Tidal range tides are giant shallow water wave
at coasts, funnel into bays and other indentations
water piles up in enclosed areas
microtidal- up to 2 m
mesotidal 2 – 4 m
macrotidal > 4 m

17. W. W. Norton

18. tidal current generated by horizontal movement of water flood tides
most pronounced in mesotidal
carries great volume in macrotidal
flood tides
Incoming
moves water onshore between low and high
ebb tides
Outgoing
moves water offshore
Slack
top and bottom of cycle (when high is in, or low is in)
bidirectional flow
leads to bidirectional cross- stratification with mud drapes
herring bone, mud drape, reactivation surface

19. Basic Circulation Basic plumbing
water flows in at high tide- fills up
flows out at low! - empties
(Variations on the theme)
may be completely empty or may be wet throughout cycle
Harsh environment- desiccation, daily changes in temperature and salinity

20. Classification
One 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.

21. Classification
Estuaries 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)

23. Salt Wedge
River 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 approached
Mississippi and Congo Rivers

24. Salt Wedge

25. Highly 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 water
Examples of this type of estuary are fjords, which are usually very deep and have a large salt water reservoir below the upper layer.

26. Fjord-Type Estuary

27. Slightly 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 layers
This type of estuary is widespread in temperate and subtropical climates

28. Vertically 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 approached
This type of estuary is found in regions of particularly strong tides; an example is the River Severn in England.

29. Well Mixed

30. Slightly Mixed

31. Inverse 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 approached
This 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.

33. Components of the Peritidal System
Salt marsh 1. High marsh 2. Low marsh
Tidal channels
Tidal deltas
Ebb
Flood
Tidal mudflats

34. Salt Marsh
Salt marshes are vegetated inter-tidal flats.

35. Salt Marsh Marsh divided into high and low marsh
high is region above high tide
low is region that’s flooded daily
each has distinctive vegetations and grain size
high has coarser sediments
Is 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 patens
low has finer sediments
Is dominated by the smooth cordgrass, Spartina alterniflora.
cut by channels which funnel water in and out
may completely empty or may be wet throughout cycle

36. Salt Marsh
Salt marsh profile.

39. Salt Marsh cut by tidal channels funnel water in and out
daily influx of water brings muds
build up mud flats

40. Tidal channels funnels water into single or multiple channels
currents can scour base
also meander and can cause walls to fail
lag of gravelly deposits (including shell material and mud clasts
armored mud balls
form point bars similar to those in rivers
BUT have fine material on top of sloping bar surface
alternations of sand and mud on bar deposits
tidal channel has lag;
channels generally cut down a few meters at least channel abandoned and finer sediments infill

43. Tidal flats
form in proximal areas and between channels

44. Tidal mudflats away from area of strong tidal currents
flooded at high; exposed at low
fine sediments are carried in on high tide and deposited during slack and as it retreats
commonly bioturbated
surface network of channels partially channelize flow
allows vegetation to flourish
PEAT: thick accumulations of dead vegetation
organisms eat decaying vegetation
low marsh characterized by this process of daily alternating currents
higher areas of marsh are site of accumulation of dead marsh grass (wrack)

45. Cores through tidal flat deposits of the Devonian Swan Hills Formation, Alberta, Canada
Dr. Stacy Atchley, Baylor Univ.

46. Tidal 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

47. Tidal Deltas
Ebb
Flood

49. Tidal Mudflats Flooded at high; exposed at low
Fine sediments are carried in on high tide and deposited during slack and as it retreats
commonly bioturbated
fiddler crabs, etc
Surface network of channels partially channelize flow
allows vegetation to flourish
PEAT: thick accumulations of dead vegetation
(organisms eat decaying vegitation)

50. Marine 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 intertidal
Lenticular and flaser bedding: sandstone and mudstone. Typical of tidal flats
Massive sandstone with vertical burrows

51. Biology 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.

52. Productivity of Salt Marshes
The most productive portion of the salt marsh is the low salt marsh region
inhabited by Spartina alterniflora.
little is grazed directly by animals
most of the production enters the detritus food chain.
material not consumed by detritivores accumulates in the sediment
forms peat or is exported from the marsh in tidal currents
root systems of salt marsh plants bind the sediments
stabilize the substrate
Reduces erosion of the coastal environment

53. Estuarine successions
if subsidence provides space, sediments can accumulate (otherwise reworked)
great LATERAL variability

54. Recognizing estuaries BUT delta is progradational
Looks a lot like deltas!
BUT delta is progradational
estuaries are aggradational
build up within drowned river channel
base of succession is usually an erosional surface

55. Wetlands flank estuaries since estuaries are natural harbors
and often PORTS wetlands are often polluted

56. NYC
Harold Connolly, 2002

57. NYC
Harold Connolly, 2002