It had drifted from the shoal of dolphins spotted in the river a few days ago, they said.
"Yesterday, a dolphin was found stranded in backwaters of the sea towards river....It's carcass was found today," Principal Chief Conservator of Forest C N Pandey told PTI.
He said the authorities had tried hard to ensure that it goes back to sea.
"But, sometimes these large fish tend to deviate and drop in shallow waters when sea recedes, and finds it difficult to go back," Pandey said.
Dolphins in Gujarat are frequently spotted in Gulf of Kutch off Jamanagar coast, and their numbers is growing, he added.
It is estimated that Gujarat is home to around 50 to 100 dolphins, though no official census has been conducted so far, the officer pointed out.