The western side of the Peak District is drained by the Etherow, Goyt, and Tame, which are tributaries of the River Mersey. The north east is drained by tributaries of the River Don. Of the tributaries of the River Trent that drain the south and east, the River Derwent is the most prominent. It rises on Bleaklow just east of Glossop and flows through the Upper Derwent Valley where it is constrained by the Howden, Derwent and Ladybower Reservoirs. The reservoirs of the Upper Derwent Valley were built from the early to mid-20th century to supply drinking water to the East Midlands and South Yorkshire.