WHILE ON A CORPORATE FLT THRU A CANYON IN MARGINAL WX CONDS, THE HELICOPTER COLLIDED WITH THE STATIC PORTION OF A POWER TRANSMISSION LINE & CRASHED. THE ACDNT OCCURRED WHERE A 2,554 FT SPAN OF THE POWER LINE CROSSED THE SOUTH PLATTE RIVER. THE POWER LINE (WITH LESS THAN 150 FT MEAN HEIGHT ABOVE THE TERRAIN) WAS NOT MARKED, NOR WAS IT REQUIRED TO BE MARKED, ALTHOUGH SPHERICAL MARKERS HAD ALREADY BEEN SCHEDULED FOR INSTLN ON THIS SPAN. ACCORDING TO LOCAL MEDIA PLTS, THE PLT OF N99TV WAS FAMILIAR WITH THE AREA & WAS AWARE OF THE POWER LINE. THE NEAREST WITNESS (ABT 1-1/2 MI FROM THE CRASH SITE) SAW THE HELICOPTER PASS OVERHEAD BEFORE IT CRASHED. ACCORDING TO THIS WITNESS, THERE WAS REDUCED VISIBILITY IN THE AREA WITH LIGHT RAIN. HIKERS IN THE AREA RPRTD RAIN, HAIL, HIGH WINDS & REDUCED VISIBILITY WITH MTN TOPS OBSCURED.