The reservoir needs to be cleaned out to operate at full capacity and to meet seismic standards.
A number of local residents oppose the plan.
"It provides a home for a number of species. It's bio-diverse, and elderly people can walk on a flat area," said Altadena resident Lori Paul.
"We need to take half a million cubic yards of sediment from the dam," said Bob Spencer with the Los Angeles County Department of Public Works. "We have to find a place to put that sediment."
County Supervisor Mike Antonovich has called for a 30-day hold on the plan while residents try to work out a deal to save the trees.
If they can't, the county-owned canyon could be cleared in January.