is mentioned in 1641. Expansion took place in the 18th century, and by 1780 there were 20 mills, with no obvious locations left where others could be built. Weirs were built with no thought for the ecology of the river, and while most were constructed to feed water into a dam at the side of the channel, the Fullwood mill impounded the whole of the flow of Mayfield Brook, as did Forge mill on the Porter Brook. Below its junction with the River Sheaf, the tunnels which carry the combined waters under Sheffield Station and the Castlegate area have been celebrated by urban explorers and include a huge cavern known as the 'Megatron'. However, a significant proportion of the Porter Brook is also heavily culverted as it meets the city. The river in the vicinity of Matilda Street was rerouted into a new straight channel between 1797 and 1808, bypassing several meanders. The channel was still open in 1832, as shown by Taylor's map of Sheffield, but when the first Ordnance Survey map of the area was published in 1853, parts of the Porter Brook had been culverted, to allow housing and industrial buildings to be built. This included a bone mill, and later a timber yard, with further sections having been culverted by 1891 to accommodate industrial expansion. Sheffield station opened in 1870, and was built over an area previously known as Sheaf Island, as it was hemmed in by the River Sheaf to the east, the Porter Brook cross link to the Sheaf to the south, and an unnamed dam, Bamford Dam and Pond Forge dam to the west. Both rivers had been diverted in the late 1700s, as the Bamford dam was built over the redundant course of the Sheaf, and around 1778 the Porter Brook was diverted along a cross link which ran due east to a new junction with the Sheaf. The Midland Railway bought the water rights in 1868, and effectively closed down Pond Forge. Both rivers were culverted to enable the construction of the station to proceed, and the cross link became the main channel of the Porter Brook when the dams were later filled in. The Porter Brook is prone to flooding within the city, because it has been hemmed in to narrow channels between buildings, and passes through culverts that are of inadequate size for storm flows. Sheffield began to research