Divers and helicopters were seen scouring the lake until evening, but no trace of Rivera was found, and the search was suspended "until first light" on Thursday.
Buschow said search and rescue teams from neighbouring counties would join on Thursday, to provide more divers and air units.
"This is a big reservoir. It's deep. These kinds of things happen," he added. "We don't know all the circumstances. Investigators are working on that, getting as much information as possible."