When the Environmental Protection Agency made headlines last week with an announcement that 34 rivers might be seriously contaminated with toxic chemicals, they were headlines it could have done without.

After receiving "a whole lot of phone calls," according to an aide, EPA deputy administrator John Hernandez is writing 23 governors and 34 mayors around the country to reassure them that the rivers were not necessarily as bad as EPA was saying.

The announcement had said that the rivers might be so polluted that controls planned for 1984 might not be enough to deal with potential problems. The letter, according to EPA press chief Byron Nelson, "says the numbers were just calculations and didn't involve any monitoring. We really don't know what's in there."

But the Natural Resources Defense Council, which made the EPA list public, points out that the rivers issue is one of several that EPA has asked the courts to take from its shoulders. "The list does not mean just nothing," said the NRDC's James Banks. "They're putting up a smokescreen."

The rivers were listed in partial compliance with a 1976 settlement of four NRDC lawsuits over the adequacy of EPA's pollution control rules. "The list was caveated to death. It was a tabletop analysis, highly preliminary, based on very little actual collection data," said Steven Schatzow, director of EPA's Office of Water Regulations and Standards.

EPA sources said that, in fact, the list had been thrown together to comply with the court order and meant little. For example, the Lake River, listed as being in Portland, Ore., is actually a brook in Washington state.

"It's a legitimate list of streams with problems, but the degree of problems is really yet to be determined," said Nelson. "We're not backing off the list . . . and we're not abandoning the streams."

But EPA complained in its petition to the court that budget cuts make it hard enough to comply with existing law, much less do anything extra to take care of rivers like the ones listed.