National Park Service image from HOSP archives
The first bathhouses were crude structures of canvas and lumber, little more than tents perched over individual springs or reservoirs carved out of the rock. Later businessmen built wooden structures, but they frequently burned, collapsed because of shoddy construction, or rotted due to continued exposure to water and steam. Hot Springs Creek, which ran right through the middle of all this activity, drained its own watershed and collected the runoff of the springs. Generally it was an eyesore-dangerous at times of high water, and mere collections of stagnant pools at dry times. In 1884 the federal government put the creek into a channel, roofed over it over, and laid a road down above it. Much of it runs under Central Avenue and Bathhouse Row today. This allowed room for sidewalks and landscaping in front of the bathhouses, creating the Bathhouse Row you see today.
Here you can link to other information about the changing face of Bathhouse Row.