State law sets guidelines on how many years specific types of records must be retained by public agencies. Emails that aren't considered "substantive" can be deleted upon receipt, said Peter Scheer, executive director of the nonprofit First Amendment Coalition advocacy group. Mistakes can occur, depending on whether the person making the decision knows the rules, he said.

"When you have ordinary staff deleting or destroying email based on their own understanding of the law, you are inevitably going to be destroying lots of public records," he said.

David Schonbrunn, a plaintiff in an environmentalist lawsuit that challenged the high-speed rail project's proposed route over the Pacheco Pass in Santa Clara County, said he suspects the rail authority routinely destroys emails that would provide useful insights into its decisionmaking.

Schonbrunn said part of the lawsuit turned on whether the rail authority had altered a computerized "travel mode demand model" that had been used to plot the best route to link the Bay Area with the Central Valley.

The plaintiffs believed the model had been tweaked to make the Pacheco Pass route seem more attractive than an alternative route over the Altamont Pass, he said.

During pretrial investigation, the plaintiffs found evidence that the model had indeed been changed, said Schonbrunn, a paralegal who worked on the case.

"But because the email was destroyed, we were unable to get somebody giving the instructions for that to be done," he said.

"An awful lot of business is transacted using emails, and when those are deleted, you're losing the entire history."