I did a "find places" for Rainbow Falls in MN using TopoQuest -

nothing found - so it is not a named place on any USGS Topo

may have just been a made up name for a place someone found attractive - may have seen a rainbow there under the right light conditions


just doing "find" Rainbow comes up with a few hits.  A Rainbow Island in Mille Lacs Lake;  A rainbow Lake that I can't see the map of, A Rainbow Park in WIllmar MN,  A Rainbow Village Shopping Center, and a Rainbow Terrace Mobile Home Park.  No Rainbow Creek or stream or river - nada.