zummy,
Yes, there are freshwater jellyfish in Melvern. I first saw them in the early 1990's when I was working at the lake. I don't have any info on the current status of them at the lake though. I have not personally seen them in any other lake in Kansas but an internet search produced a list of other lakes that are said to have them (looks like we may have looked at the same website with locations and dates of sightings).
Here's an informative link from Missouri DOC.
http://mdc.mo.gov/conmag/2000/07/missouri-jellyfish
Interesting screenshots!
Hope this helps you out a little.
Craig