Navigation through Lincoln and on to Boston was hampered by the shallow draught available under the medieval High Bridge, just below Brayford Pool. Sir Joseph Banks, when proposing the construction of the Horncastle Canal knew that the problem needed addressing, and William Jessop was commissioned to survey the Witham and the Foss Dyke in 1791, with particular reference to the junction between the two. He proposed a bypass to the south of the city, using the Sincil Dyke, or excavating the channel beneath the bridge, although he recognized that this solution was not ideal, due to the narrow width of the bridge hole, and the lack of a towing path through it. However, the city was keen to keep trade within its limits, and improvements to High Bridge, which were completed by 1795, were authorised as part of the Horncastle Canal act of 1792.