Coastal plain estuaries are formed when pre-existing valleys were flooded at the end of the last ice age. The Humber Estuary drains a catchment area of some 24,472 km2, around 20% of the total land surface of England. Water collected from this catchment flows to the estuary through many rivers and tributaries, the largest of these are the Aire, Derwent, Don, Ouse, Trent and Wharf.
A significant feature of the Humber is the large tidal range, this is due to its position within the North Sea basin; producing a mean spring tidal range of 5.7m at Spurn. The tidal range increases as the tide moves up the estuary; it is 7.4m at Saltend and 6.9m at Hessle which is 45km inland. The Humber is classified as a macro-tidal estuary because of these large tidal ranges.
At its widest point the Humber Estuary is 14km across and its average depth is 6.5m. It covers over 30,550 ha (75,492 acres). The Humber's muddy appearance (turbidity) is due to suspended sediment. This comes mainly from the eroding boulder clay cliffs along the Holderness coast and also river sediments. This sediment is vital for the estuary's function and every tide carries over 1,500 tonnes. It is estimated that up to 1.26 million tonnes of sediment may be present in the water in the estuary. The deposited sediments maintain the estuary's important habitats such as mudflats, sandflats and saltmarsh. The Humber supports a rich variety of habitats and species and is recognised as one of the most important estuaries in Europe for overwintering birds. It supports 9 species of international importance.
The Humber Estuary is also an important industrial area and trade gateway with an average of 40,000 ship movements per year. Its ports and wharves handle 14% of the UK's international trade. It is the country's largest port complex. Industries along the estuary include chemical works, oil refinery complexes and power stations.
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