Quite a lot can be said about the Onyx river and the Vanda lake, just read through the Wikipedia articles, amazing stuff.

Allegedly the Onyx river is Antarctica's longest river (I believe we don't know much about the river systems located under the glaciers, I think there exists much bigger rivers under the ice), it takes melt-water from the glaciers and runs into Lake Vanda. It seems to never freeze, but the water flow is very variable, and sometimes it stops short from going to Lake Vanda.

Lake Vanda is extremely salty, it has three layers that doesn't mix, and the deepest layer is warm! The center of the lake is always covered by clear ice, but at the edges it typically melts once a year.

As far as I understand there is no exit river from Lake Vanda - all the water there either evaporates or disappear into the ground?