First off, I do not have the experience to even qualify as an armchair amateur geologist, but I have done geology (formation mapping, mineral identification etc) in the field, and had a few college level courses... so keep in mind my observations may be skewed by inadequate knowledge.

1) I think it is clear that the 'riverbed' predates the crater... I could be wrong but I cant see any other conclusion that can be reached.

2) The ejecta blanket ends at the banks of the channel, So I agree that it seems that there was some flow in the channel after the impact. It also seems odd that the ejecta blanket is not seen on the other side of the channel.

3) BUT, I think it also seems clear that there was never a lot of flow after the crater was made. There is no evidence of downstream formations (whichever way downstream is) that you would expect had there been significant flow after its formation.

Maybe this channel was an occasional river- flowing weakly every few years, centuries, or millennia... whatever its frequency it wasnt often or sustained as indicated by the lack of downstream features.

A sense of scale would help, as would an idea of the image's place in respect to other features.

REAL geoligists are welcome to assign a grade to this post- (I hope it isnt an F-)