Good follow-up information. The picture of that grocery cart agrees with what I saw earlier this year on a lake up in Wisconsin -- total coverage of submerged objects it quite possible.
Very likely the infestation in Powell is from some fisherman bringing a contaminated boat in from Lake Mead or Havasu, or some other Western body of water that is already infested.
With regard to the Grand directly, the commercial GC operators who end their trips at South Cove on Lake Mead have been taking strict decontamination procedures, since their boats often turn around from Mead and go to Lees Ferry in a day or two.
However, one might argue that the battle appears to already have been lost for Lake Powell. And given the proximity of these postive tests to the dam, the probability of them going down into the Grand is evident, although of unknown timeframe and final outcome. Hopefully they won't like the rapids as noted in the last post, but the Grand has lots of slow-moving stretches where they could prosper.