How does David McCullough use imagery in The Johnstown Flood?

Imagery:

"'From seven o'clock on the water rose. People who were glad they 'didn't live downtown' began to wish they didn't live in town at all. On the water crept, and on, up one street and out the other... Eighteen inches an hour the Stony Creek rose for a time, and the Conemaugh about as rapidly.' "On the street below his window the current, coming across from the Stony Creek, was rushing at an estimated six miles an hour."