shallow bay of the North Sea, 15 mi (24 km) long and 12 mi wide, between the counties of Lincolnshire and Norfolk, England. It once extended as far inland as Peterborough and Cambridge but was largely filled in by silt, brought chiefly by rivers but partly washed in by coastal currents. Land was reclaimed by artificial drainage at several points, and seawalls were built to protect the low coastal lands. At low tide the river waters reach the sea through shallow creeks between banks of sand and mud. The two main channels, Boston and Lynn deeps, provide anchorage for small vessels trading to Boston and King's Lynn.