The Little Don River also known as the Porter, is a tributary of the River Don in South Yorkshire, England. Arising on the Langsett Moors in the northern Peak District, the Little Don River feeds the Langsett and Underbank Reservoirs. It runs through the town of Stocksbridge before joining the River Don. A section of the river has been designated as a site of special scientific interest since 1977. The site is of considerable importance as it is a proposed reference example for various subdivisions of the Carboniferous. It is one of 35 sites of special scientific interest in South Yorkshire. Course The Little Don rises as a large number of streams flowing northwards from Howden Edge, close to the contour, on Rushbed Moor and Harden Moor, which unite and then flow eastwards. It is soon joined by Loftwshaw Brook, which similarly starts as a number of streams on Rushbed Moor, but slightly further to the west, and follows a more northerly route before joining the Little Don. It is joined by Mickleden Beck and Haslingshaw, both of which flow northwards and join the right bank. Long Moor Beck flows eastwards and joins as the Little Don enters Swinden Plantation, and the moorland scenery gives way to woodland. The river loops to the north, where it is joined by Great Grain, and passes over a weir to enter Langsett Reservoir. The reservoir was built as a join venture by the corporations of Sheffield, Doncaster, Rotherham and Barnsley. Sheffield Corporation bought the site in 1898 (or 1889 - sources vary) from the Pilkington family, and also purchased a number of local farms which were then abandoned, in order to prevent contamination of the water supply. Work started on constructing the dam, and the dams of Midhope and Underbank Reservoirs, further downstream, simultaneously, and Langsett Reservoir was completed in 1904. It was officially opened on 17 October, and held the largest volume of water of the dams constructed in the Sheffield area. Transport of materials for the construction was aided by the Sheffield Corporation Railway, which ran along the north bank of the river from Stocksbridge. The dam crossed the valley at the eastern end of the reservoir, and Midhope Cliff Lane ran along the top of it. A treatment works and filters were constructed below the dam, and a spillway to maintain flow in the river ran from the northern edge