The planned annual lowering of water levels on the Stillwater branch of the Penobscot River caused the death of thousands of river herring — and one Atlantic salmon — last week, but a state official said the impact on the thriving herring resource was minimal. Brookfield Renewable, the company that manages the Orono Dam, which is just downstream from the bridge on Main Street in that town, said it conducted a planned three-foot drawdown of the Orono impoundment on May 24 in order to make upgrades to the dam.