"it may have rained as recently as a couple days before the landing"
Where did you hear this? I'd like to read the article.
I doesn't look like it rained recently in the photo of the pebbles, but I guess it could have rained and material flowed in another river bed. On the other hand at -180C a crust could develop rather quick?
Check out SpaceRef article to see if that helps you make some since.
The fan distribution river system looks promising as evidence of flowing material but the when is the question.