The Grant County Public Utility District announced the closure Thursday. The district owns the shoreline.
Utility officials say people have been drawn to the area and are even driving on the newly exposed shoreline, which is south of Vantage.
From now on, only authorized personnel will be allowed access to the shoreline below the high-water mark of the reservoir.
The utility is posting signs and is actively patrolling the shoreline. They say law enforcement agencies will be notified whenever anyone is observed trespassing on PUD lands.
The utility is also trying to determine the best way to repair a 65-foot long crack in a spillway of the dam that was discovered last week.