Police erected a roadblock on Interstate 91 and other roads on Saturday morning and told fans headed to the two-day festival at the Newport State Airport to turn around.
"Because of the heavy rains, parking inside the festival site has basically become impossible, and they're concerned for peoples' safety," said Adam Lewis, a spokesman for the concert promoter, Great Northeast Productions, said Saturday.
All ticket holders denied entry to the site will get their money back, the promoter said in a statement.
The promise of a refund was no comfort to fans stopped on Interstate 91 Saturday, many of whom abandoned their vehicles and headed out on foot for the concert site, about a dozen miles from the nearest exit.
"There is no way I am not going to try to get in," said Erika Sander of Blodgett, Ore., who flew to Manchester, N.H., and rented a car to drive to the show. She left the car on the side of the road and started walking to the concert Saturday.
"There's no way I'm going to be defeated," Sander said.
A law enforcement official who asked not to be named said tow trucks had been sent to the highway to remove the abandoned cars.
The Coventry Phish concert Saturday and Sunday is the band's last, and 70,000 people bought tickets to attend. Many thousands more were expected to show up without tickets.
By Friday, Interstate 91 was backed up for 15 miles with fans in cars trying to get to the site. Other roads were also clogged. Heavy rains Thursday and Friday turned the campgrounds into quagmires, and local residents with tractors were busy pulling stuck cars from the mud.
Police said they decided to stop concertgoers from arriving because the ground was too waterlogged for driving or for camping.
Saturday, the weather had turned fair, with sun and patches of blue sky. But more rain was expected Sunday as Hurricane Charley passed by to the south.