fort riley is a united states army installation in north central kansas. it is on the kansas river between junction city and manhattan. the fort riley military reservation covers 101,733 acres (41,170 ha) in geary and riley counties. the part of the fort that has housing development is part of the fort riley census-designated place. in 2010, 7,761 people lived there. the fort has a daytime population of nearly 25,000. the zip code is 66442.