By Sally Westwood

Common Buzzard

 

Deben Estuary and changing tides

The River Deben is thought to commence west of Ulveston Hall, near Mickfield, alternatively the Deben may stem from the south in Bedingfield¹. The river flows along gradually turning into an estuary between Melton and Woodbridge, and enters the North Sea at Harwich Haven, between Bawdsey and Felixstowe Ferry. A significant feature of the Deben Estuary is the changing tides. The tide is described as “the ocean’s response to the gravitational pull of the Moon and the Sun”² but more specifically, “it is the variation in this pull over the surface of the Earth that makes tides”³. When tide is rising and coming in, the North Sea washes heavy sediment⁴ into the Deben which forms intertidal flats around the area of the mouth of the estuary at Felixstowe Ferry. This produces sand and shingle beaches. Finer silt is driven along the estuary into the river to form the mudflats we see at the sides of the river near Woodbridge, and more extensively further along the river at Melton, Wilford Bridge and beyond. The tide rises and falls twice a day⁵. High water on the River Deben occurs at intervals of approximately twelve hours, and the tide starts ebbing to low tide about six hours following high tide. High tide occurs one hour later each day. The height of the tide is affected by both the strength and direction of wind and atmospheric pressure⁶.

 

Impact of the tides on waders

The change in tides has a considerable influence on the activities of the birds on the River Deben. The tides determine when birds have access to food⁷. At high tide waders stop feeding and settle in a roosting place, usually with flocks of other waders. Birds roost at the same place each year. They tend to select remote roosting sites, such as spits, that permit full view and provide safety. When there are large areas of mud flats, birds tend to roost on the seaward edge of the flats. If the tides are unusually high, with spring tides, and the mud flats are totally immersed, waders may fly to adjacent fields which are bare after ploughing or recent harvesting. Sea water mixes with fresh water in estuaries. The location where salt water mixes with fresh water moves, and this depends on the amount of fresh water flowing along and the current state of the tide⁸. Since salt water is heavier than fresh water it enters the estuary below the inflowing river water, on the surface of the water. The fauna and flora present in an estuarine area is determined by the salinity of the water, particularly invertebrate animals. 

 

Greenshank

 

Vegetation on the Deben

Three species of Eelgrass⁹ are submerged by the tide or exposed briefly at low tide. Green Seaweed¹⁰ and Eelgrass are food for wintering Wigeon¹¹ and Brent Geese¹². Brown Seaweeds, such as Wracks live around rocks. Wracks are not eaten by birds, instead they are eaten by invertebrates, and these in turn are eaten by particular waders. Estuarine vegetation is a patchwork of different maritime plants, which have adapted in different ways to submergence and drainage of water. Cord-grass¹³ and Sea Aster¹⁴ are found on the lowest zones on the mud flats. Cord-grass has been planted on many estuaries on the East Coast. It has long roots and vast underground stems which help to bind and trap silt. Salt Marsh Grass¹⁵ covers many salt marshes, and spreads over the marshes like a lawn. If livestock and geese are seen grazing vigorously on the salt marshes at low tide, it usually means salt marsh grass is present. If the grazing is light, Sea Aster, particularly the type with rayless flowers, and low lying, bushes of green-grey Sea Purslane¹⁶ tend to be present in the marsh. In high salt marshes, that are less likely to be submerged by the tide, a variety of attractive plants appear at particular times of the year. White Scurvy Grass¹⁷ which is an edible cress appears in May. Sea Spurrey¹⁸ and Pink Thrift¹⁹ appear in June. This is followed by Blue Sea Lavenders²⁰ in late July and August. Other less noticeable plants also appear on the high salt marshes, such as Sea Plantain²¹, Sea Milkwort²² and Sea Arrow-grass²³. Salt marsh plants such as Sea Aster, Orache²⁴ and Marsh Samphire²⁵ may produce vast crops of seeds as winter feed for Reed Buntings²⁶, finches, such as Twite²⁷ and Skylarks²⁸. These small birds attract Marsh Harriers²⁹, Sparrowhawks³⁰, Common Buzzards³¹, Merlins³² and Short-eared Owls³³.

 

Molluscs and fish in the Deben

Birds on the Deben are attracted to the availability of their main food, although their main food may vary at different times of the year. It is not surprising that wildfowl and waders are attracted to the Deben, since mollusc that cope with the changing salinities of the water are abundant in the inter-tidal mud, and these are the main food of many species of birds on the Deben. Redshanks³⁴ and Dunlins³⁵ feed in muddy areas of the river and prefer to eat Corophium, a burrowing shrimp and Hydrobia, which is a tiny snail. Both of these creatures can be found in vast numbers in a small area, for example, 24,000 has been observed in a square metre³⁶. Shellducks³⁷ also eat these snails as a main food. Bar-tailed Godwits³⁸ Eurasian Curlews³⁹ and Grey Plovers⁴⁰ eat Ragworms⁴¹ which are found in abundance on inner mud flats. Oystercatchers⁴² feed on cockles on sand banks at the edge of the mud flats. Ruddy Turnstones⁴³ search rocky muddy areas for snails and crustaceans. Avocets⁴⁴ use their upturned bill to find shrimps in liquid mud and shallow water. Greenshanks⁴⁵ chase shrimps and fish in brackish pools. During the winter Grey Herons⁴⁶, Great Cormorants⁴⁷ and Little Grebes⁴⁸ eat flounders. In the summer time flounders are eaten by passing Osprey⁴⁹, terns and a variety of gulls. Another reason why the Deben is attractive to waders and wildfowl is because the shore tends not to freeze over during winter.

 

Great Cormorant

 

Common breeding birds on the salt marshes and banks of the Deben

Less accessible areas of salt marsh have small colonies of breeding Black-headed Gulls⁵⁰, these are occasionally joined by terns. On the river bank, the most common breeding birds are Meadow Pipits⁵¹, Reed Buntings⁵² and Skylarks⁵³. In the streams running parallel to the bank Sedge Warblers⁵⁴ and Reed Warblers⁵⁵ are also common breeding birds. Birds that nest on the high salt marshes are at risk from flooding from spring tides. Canada Geese⁵⁶ tend to create their nests at regular intervals on the heaped mudbank, running some distance from Melton towards Wilford Bridge. The mudbank has a fence of wooden posts that forms a central line between the river edge and the flooded area of marsh behind. This mudbank sometimes becomes submerged in May and June during spring tides. Wildfowl moult their plumage in late summer and lose their ability to fly for several weeks. They need the estuary to obtain food that is in good supply and to feel safe from predators. Canada Geese, for example tend to stay at the far side of the river in late summer, when they are in moult to take advantage of sheltered water, mud flats and sand banks.

 

Risks to the river and its birds

Various factors may have a negative impact on the state of the river, including water recycling plants; pollution from the surrounding land⁵⁷ and management practices involving low water flows and abstraction⁵⁸.

 

Sally Westwood

Sally Westwood is a Psychologist, and works as a researcher. She has taught in higher education as an English Language teacher. Her interests are varied. She has written many articles for Bird Magazines and Journals. She is also a professional bird photographer. Birds are her passion. She loves to draw and paint birds. She lives on a boat on the Deben and loves to sit and be with the birds, simply watching and photographing birds, in their daily activities.

 

Footnotes

1. Pye, A. S. (2021) River Deben from source to sea. Available from:  https://www.geograph.org.uk/article/River-Deben-from-source-to-sea [Accessed 26 April 2024].
2. Bowers, D. G. & Roberts, E. M. (2019)
3. As above.
4. Doeke, E. (2019) Intertidal Deposits: River Mouths, Tidal Flats and Coastal Lagoons. CRC Press.
5. As above.
6. White, J. (2017) The Science and Spirit of the Ocean. Trinity University Press.
7. Cady, M, & Hume, R. (1998). The Complete Book of British Birds. AA/RSPB.
8. Doeke (2019), as above.
9. Zostera
10. Enteromorpha
11. Anas Penelope       
12. Branta bernicla
13. Spartina maritima
14. Tripolium pannonicum
15. Puccinellia
16. Atriplex portulacoides
17. Cochlearia officinalis
18. Spergularia media
19. Armeria maritima 
20. Limonium sinuatum
21. Plantago maritima
22. Lysimachia maritima
23. Triglochin maritima
24. Atriplex hortensis
25. Salicornia europaea
26. Emberiza schoeniclus
27. Carduelis flavirostris
28. Alauda arvensis
29. Circus aeruginosus
30. Accipiter nisus
31. Buteo buteo
32. Falco columbarius
33. Asio flammeus
34. Tringa totanus
35. Calidris alpina
36. Cady & Hume (1998), as above
37. Tadorna tadorna
38. Limosa limosa
39. Numenius arquata
40. Pluvialis squatarola
41. Nereididae
42. Haematous
43. Arenaria interpres
44. Recurvirostra avosetta
45. Tringa nebularia
46. Ardea cinerea
47. Phalacrocorax carbo
48. Tachybaptus ruficollis
49. Pandion haliaetus
50. Larus ridibundus
51. Anthus pratensis
52. Emberiza schoeniclus
53. Alauda arvensis
54. Acrocephalus schoenobaenus
55. Acrocephalus scirpaceus
56. Branta canadensis
57. Essex and Suffolk Rivers Trust. (2024) Recovering the Deben from source to sea. Available from: https://www.essexsuffolkriverstrust.org/recovering-the-deben [Accessed 26 April 2024]. ⁵⁷ For further information on pollution see the Deben Climate Centre report: Deben Climate Centre. (2024) Report on the Quality in the River Deben February 2024. Available from: https://www.woodbridgeclimateactioncentre.org/post/deben-climate-centre-report-details-high-pollution-levels-in-the-river-deben [Accessed 26 April 2024].
58. For further information on saltmarsh and mudflats see: Suffolk Biodiversity Information Service. (n.d.) Coastal Saltmarsh and Intertidal Mudflats. Available from: https://www.suffolkbis.org.uk/sites/default/files/images/habitat/Coastal saltmarsh and intertidal mudflats factsheet.pdf [Accessed 22 April 2024].