Intertidal mudflats are submerged at high tide and exposed at low tide.

They form in sheltered coastal areas, such as estuaries and inlets, but also along stretches of open coast with a gently sloping shoreline, when fine silt and clay sediments settle.

The physical structure of the intertidal flats ranges from mobile, coarse-sand beaches on wave-exposed coasts to stable, fine-sediment mudflats in estuaries and other marine inlets.

This habitat type can be divided into three broad categories - clean sands, muddy sands, and muds – although in practice there is a continuous gradation between them. Within this range, plant and animal communities vary according to the type of sediment, its stability, and the salinity of the water.

There are extensive mudflats fronting the Welsh shore and Bridgwater Bay, and large banks of clean sands in the more central parts of the estuary at Middle and Welsh Grounds. Together, these cover an area of approximately 20,300 hectares – the 4th largest area in a UK estuary. It represents approximately 7% of the total UK resource of intertidal mudflats and sandflats. 

The surface of mudflats can appear barren and devoid of vegetation. However, mats of microalgae are common, which help to bind the sediments together.

They are also rich in organic material, making them ideal habitat for hosts of filter feeding and scavenging invertebrates. The high biomass of invertebrates in the mudflats of the Severn Estuary provide an important food source for a diverse range and large numbers of fish and benthic (bottom-dwelling) predators.

Mudflats also provide a valuable feeding, roosting and resting area for a wide range of wading birds and waterfowl. As the ebb tide exposes the mudflats, thousands of birds, such as oystercatchers, curlew, and dunlin, come to feed.

Around 80,000 wintering wetland birds use its marshes and mudflats each year. This makes the Severn Estuary one of the top wetlands in the UK and internationally important. 

As more sediment is desposited on the mudflat, its height increases and the time spent submerged is reduced. Beyond a critical point, the mud is exposed long enough for pioneering salt-tolerant plants to become established, leading to the formation of saltmarsh.

Although they can exist without each other, mudflats and saltmarshes are interdependent. Where mudflats exist without saltmarshes they are more vulnerable to erosion through wave action, especially on the upper level where waves are reflected by natural cliffs or hard defences, such as seawalls. Similarly, where saltmarshes exist without mudflats, the lower edge is more vulnerable to erosion.

Mudflat wildlife

Seen something interesting?

It’s important to record wildlife sightings with South East Wales Biological Recording Centre, which stores, manages and shares wildlife records from across the region.

Follow the link below to find out how to record your sightings.