(Portland, OR)  --  State boating officials are warning residents to beware of hazards in local waterways over the Labor Day holiday weekend.  The Oregon State Marine Board says lower water levels have exposed shoals, gravel bars and stumps that could damage the bottoms of boats and cause serious accidents.  Officials say there are also more logs, branches and debris being pushed downriver following the summer's massive windstorms.  Officials are urging boaters to assess the water conditions where they are before heading out and to go at a slow pace and wear a lifejacket.