The Tweed Estuary is a complex estuary, which discharges into the North Sea. It is a long narrow estuary, which is still largely natural and undisturbed, with its water quality classified as excellent throughout. It supports a wide range of habitats compared with other estuaries in north-east England. At its mouth there are substantial sandbanks and some areas of rocky shore. Further upstream, large areas of estuarine boulders and cobbles overlie sediment flats and extend into subtidal areas of the channel. Sheltered estuarine mud and sandflats occur away from the fast-flowing river channel. A wide range of littoral sediments occurs within the estuary. These range from exposed east-facing sandy shores at the estuary mouth, including its sheltering sand-spit, to muddy gravels where the river is actively eroding the banks. The most exposed sandy shores are subject both to wave action and, in places, the scouring action of the outflowing river; their mobile infauna (crustaceans and a few polychaetes) and ephemeral algae reflect these conditions. Species and habitat diversity rises with increasing shelter, until increasingly low-salinity estuarine conditions upstream lead to naturally low infaunal diversity, dominated by characteristic species that are tolerant of brackish-water conditions. Fish species include the rare anadromous 1102 allis shad Alosa alosa, which runs in the estuary, migratory 1106 Atlantic salmon Salmo salar, and occasional records of 1099 river lamprey Lampetra fluviatilis and 1095 sea lamprey Petromyzon marinus.
The Tweed is a long narrow estuary with a wide variety of intertidal mudflat and sandflat communities. Sandstell Point, at the mouth of the estuary, is a wide spit of clean mobile sand. This sand is subject both to wave action and, in places, the scouring action of the outflowing river, and is characterised by a mobile infauna (mainly crustaceans such as Eurydice pulchra and Bathyporeia spp. and a few polychaetes) which reflect these conditions. On the more sheltered west-facing shore of this spit, and on Calot Shad on the opposite bank, reduced mobility of the sand allows robust polychaetes such as Scolelepis squamata and Paraonis fulgens to occur with the crustaceans. Both biotopes are highly representative of the north-east of England. Further upstream at Yarrow Slake, more sheltered areas of muddy sand are characterised by polychaetes, amphipods, oligochaetes and enchytraeids that are characteristic species tolerant of brackish conditions.