The lower viewing platform at Multnomah Falls was closed Monday for public access while crews break up a rock overhanging the cliffs above a trail that leads to the upper falls.  The hazards posed by the rock were discovered in April after about a cubic yard of rock split off, falling onto the trail between Benson Bridge and the upper viewing platform.  A geological assessment found that unstable sections of the rock could detach at any time, and noted it would likely impact the trail below.  To address the hazards, officials hired a contractor remove the unstable, overhanging sections of the rock.  The lower viewing platform will be closed while operations are underway to prevent risks to visitors from any falling rocks in the vicinity.  The work is expected to last up to three business days, so visitors should anticipate possible closures on the lower platform through Wednesday, and the trail to the upper viewing platform may take additional time to reopen, as crews clear the rock and repair any damages.  Geologists believe the rock likely splintered as a result of natural weathering that occurs each spring in the Gorge, when snowmelt and thawing can cause rock formations to destabilize.