River Irwell - River Irwell Catchment Area

River Irwell Catchment Area

The River Irwell catchment area extends from the moors above Bacup to the Manchester Ship Canal. The climate of the catchment area is wetter than the UK average, with rainfall of 1,456 millimetres (57.3 in) per annum compared to 1,231 millimetres (48.5 in) per annum, and the rivers quickly respond to rainfall. The topography varies considerably, with the upper reaches dominated by the Pennine moors at an altitude of between 350 metres (1,150 ft) and 450 metres (1,480 ft) AOD and the bottom of the catchment consisting of relatively flat land, which lies between 20 metres (66 ft) and 150 metres (490 ft) AOD. The watercourses are characterised by steep narrow valleys, which contributes to high rates of run-off, as does the underlying solid geology. This comprises Lower Coal Measures overlying Millstone Grit, both of which are classified as minor aquifers which will only hold relatively small amounts of water. The Millstone Grit is, in turn, underlain by limestone rocks from the lower Carboniferous period. The surface deposits comprise thick peat in the upper reaches, with glacial boulder clay and glacial sand and gravel in the lower parts. The sand and gravel are also classified as a minor aquifer, whilst the boulder clay is a non-aquifer. The higher, steeper slopes of the upper part of the catchment provide a large source of erodible material and debris which is transported downstream to the lower, flatter parts of the catchment. Where the channel is constricted, this material is then deposited and can contribute to a reduction in channel capacity.