Southwick racked up 216 yards, while D.J. Harper added 107 rushing yards with two scores for the Broncos (9-2, 6-1 MWC).
Kirby Moore led the Boise State receivers with four catches for 36 yards.
M.J. McPeek was held to just 4-of-9 through the air for 19 yards with two interceptions for Colorado State (3-8, 2-5).
Backup Conner Smith was inserted in the second half and gained 53 yards on 4-of-11 passing.