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One of the earliest mapped courses of the River Nene, from Peterborough to The Wash, was around Whittlesey Mere, through Benwick, March and Upwell, eventually joining The Well Stream at Outwell which flowed to the then coastal town of Wisbech. Over the centuries a straight channel was dug through the marshes north of Wisbech to the current outfall north of Sutton Bridge and the land on either side reclaimed. There are numerous visible sections of the old coastal banks on both sides of the River Nene north of Wisbech, see Link.
One of the earliest attempts to improve the flow of water from Peterborough to the coast and reduce flooding in the Fens was made by Bishop Morton who had a straight channel dug in the late 15th century from Peterborough to Wisbech known as Morton’s Leam, see Link In the early 1700s a new and much larger straight channel was dug from Peterborough to Wisbech called Smith's Leam, parallel to and just north of Morton’s Leam, to improve the flow of water from Peterborough to the sea. Morton’s Leam from Guyhirn to Wisbech was incorporated into Smith’s Leam and the remainder of Morton’s Leam from Peterborough to Guyhirn became the main artery into and out of the Nene Washes which were created between Guyhirn and Peterborough.
Smith’s Leam is now known as the River Nene and the original meandering river from Peterborough to Marmont Priory Lock in Upwell is now known as The River Nene (Old course), see Link.