They will spread - but usually along the river catchment where they started. It would take a while for them to establish on other river systems.
However, due to the delays in legal releases, there are also quite a few illegal releases. That's not always ideal, but is probably the quickest way that they are spreading just at the moment.
Looks like it will be warm but with some rain, so I'm planning to spend a while reading in the gazebo while it is raining. There is definitely something enjoyable about being outside, but out of the rain and not having to do anything.