Solved! Go to Solution.
vop.setVisibleLayers(visible);
var vop = new esri.layers.ArcGISDynamicMapServiceLayer( "http://dswcapps.dcr.virginia.gov/ArcGIS/rest/services/dnh/vop/MapServer", { id: 'vop', opacity: 0.45 } );
map.getLayer("vop").setVisibleLayers(visible);
var landsResults, riversResutls;
vop.setVisibleLayers(visible);
var vop = new esri.layers.ArcGISDynamicMapServiceLayer( "http://dswcapps.dcr.virginia.gov/ArcGIS/rest/services/dnh/vop/MapServer", { id: 'vop', opacity: 0.45 } );
map.getLayer("vop").setVisibleLayers(visible);