Someone discovered the problem in the Pixley National Wildlife Refuge on Monday morning.
Park service officials say the horses were pastured there for the winter, but are normally used by staff in the Sequoia and Kings Canyon Parks.
Authorities won't say how long the animals had gone without water. They will only say they're taking the issue seriously and are calling it a tragic loss.
A team of investigators is trying to figure out what happened, and why.
"It's literally like losing several members of your family," Dana Dierkes said. "These animals were a vital part of our operation they were really part of our team our staff is heartbroken."
Congressman Nunes is also investigating what happened. Friday night his staff says someone should be held responsible.