The Natural Estuary
Estuaries are unique environments of constant change. At the boundaries of the land and the sea, the Medway, like all estuaries, forms a complex ecosystem. The mixing of fresh and sea waters and tidal movement create changing levels of salinity and nutrients providing a fertile environment that can support large populations of animals, particularly invertebrates, fish and birds. A mosaic of habitat types exists within the Medway estuary with inter-tidal mud flats, saltmarshes, coastal grazing marsh, ditches and seawalls.
Mudflats are intertidal habitat comprising soft sediment exposed at low tide. They are not vegetated, except for the presence of eelgrass (Britain's only flowering marine plant) which occurs in sheltered sections of the estuary. Mudflats are highly productive ecosystems enabling them to support large numbers of invertebrates, waders and wildfowl. A square metre of mud contains the energy equivalent of sixteen chocolate bars. The mudflats of the estuary are the most extensive in Kent.
Saltmarshes are formed on mudflats that are high enough to be colonised by specialist plants capable of being frequently submerged in salt water. The plant roots and stems help bind the mud and surface vegetation, which slows down the flow of water, encouraging more silt to be deposited. This process enables the saltmarsh to maintain its height.
Within the estuary there are a number of islands, including Fowley, Burntwick, Nor Marsh and Greenborough. These islands are now dominated by saltmarsh species and provide important roosts for birds at high tide and undisturbed breeding grounds for wildfowl.
Grazing marsh is an area of periodically flooded pasture or meadow with ditches containing brackish or fresh water. The majority of grazing marshes around the estuary is semi-natural in origin; the habitat is the result of containment, drainage and grazing. Grazing marsh provides important high tide roosts for waders and wildfowl. In the spring, the grazing marshes provide habitat for significant proportions of breeding birds; the North Kent Marshes are the third most important area in England for breeding waders.
Ditches represent a key landscape and conservation feature of the grazing marshes. The ditch network is calculated to be over two thousand kilometres in length and is used to control water levels within the marshes and to provide wet fences for the management of livestock. With sympathetic management, ditches can provided important habitat for some species.
Large expanses of grassland are contained on the clay sea walls built for flood defence. Many plants of interest occur along the Medway and Swale Estuary sea walls. The seaward wall of the embankment often supports plants typical of the upper zone of saltmarsh: these upper saltmarsh zones are less common naturally because of the presence of the sea walls. Where clay has been extracted for sea wall construction, 'borrow-dykes' have been created. These provide important reedbed habitats.
Estuarine waters are neither river nor sea water but changing mixtures of the two. Levels of salinity occur in zones ranging from 5 to 30%; zones vary from estuary to estuary, but are a common feature of all estuaries. These sheltered waters provide a sheltered habitat for fish to feed, spawn and grow, and a haven for migrating fish.