Guide books call Lake 10400 'Upper Rock Creek Lake' because Rock Creek flows through it. It is not named on the USGS maps, and there do not appear to be any references to 'Lower Rock Creek Lake' despite the wording from the Rangers. It is southeast of Joe Devel Peak and southwest of The Major General.
(lat,lon) = (36.4949300, -118.2837100) in decimal degrees, NAD83. (zone,east,north) = (11S, 385107.0, 4039416.0) in decimal meters, NAD27.
Map exported from TOPO! software, used with permission.