Drainage from the western slopes flows via Sharp Knott Gill and Fother Gill to Cogra Moss. This is a reservoir sitting in the deep valley between Blake Fell and Knock Murton, a reed rimmed waterbody held back by a substantial dam at the western end. Also known as Arlecdon Reservoir, it has a depth of around 30 feet. The waters from this flank of the fell join the River Marron and ultimately the Derwent. The eastern face of Blake Fell flows either to Loweswater or to its outfall, ensuring that all of the drainage eventually runs to the River Derwent.