The sensors have been attached to bridges, piers and other sturdy locations to better gauge how high Hurricane Irene will push water in rivers, bays and other waterways.
The federal agency says the sensors will help forecast future storms and assess damage from Irene.
USGS hydrologist Jon Dillow says several hundred additional sensors placed up and down the coast will be retrieved next week. They include about 70 on the Eastern Shore and about 10 on Maryland's western shore. The sensors record water pressure every 30 seconds.