Lough Allen () is a lake on the River Shannon in northeastern Connacht, Ireland. Most of the lake is in County Leitrim, with a smaller part in County Roscommon. The lake lies to the south of the River Shannon's source, near the Iron Mountains, and is the uppermost of the three main lakes on the river. The other two, Lough Ree and Lough Derg are much further to the south. Geography Lough Allen, out of which the Shannon takes its source, is nine miles long, and three miles wide. The lake is shaped like an isosceles triangle. The Shannon enters the lake at the wider northern end and leaves the lake at the narrow southern end. Other rivers that feed the lake include the Diffagher (northwest), the Yellow (northeast), the Stoney (east) and the Arigna (southwest). The R280 regional road skirts the west side of the lake, while the R207 follows the east bank, from Ballinagleragh to Drumshanbo. The R200 road is on the north side of the lake, traveling west from Dowra to Drumkeeran. Slieve Anierin lie to the east of Lough Allen. There has been speculation that notable reserves of oil and gas lie beneath the Allen basin. Ecology Between , water quality was reported to be excellent with an oligotrophic rating. The pike population is the "native Irish strain" ( meaning 'Irish Pike') not the other European Pike strain ( meaning 'strange or foreign fish'). The ecology of Lough Allen, and other Irish waterways, remain threatened by curly waterweed, zebra mussel, and freshwater clam invasive species. Prehistory Significant traces of Mesolithic inhabitation have been found around the lakeshore, with hundreds of stone tools collected. In total almost 1000 stone tools were collected during a set of surveys by Killian Driscoll, and 95% were formed on silicified dolomite, which outcrops locally. The remaining 5% were formed from flint, chert and quartz, along with the shale/mudstone and basalt ground/polished axes. The majority of the stone tools are characteristic of the Later Mesolithic, with possible evidence for the Early Mesolithic and limited evidence for Neolithic activity. The assemblage includes a number of stone axes and axe roughouts, and the roughouts represent the first recorded, by the Irish Stone Axe Project, as found in a lakeside context in Ireland, with most previously provenanced examples coming from axe quarry sites. History Ironworks Iron ore has been extracted at Slieve Anierin for millennia.