I remember seeing the video on youtube but after a quick search I did not find it again.
Does your video show the cross going both directions? If the video was "flipped" then the cross returning will look the same when the river begins to flow the proper direction again. Look for minor details that should not be repeated on the return trip.
NOT saying this is what happened,
But in Adobe Premiere you could cut the cross out in a layer, layer out the water, change the lighting and make it look like that with the cross in the same direction.
HOWEVER, this does NOT mean this is what happened. I have too heard of this and I have witnessed many things that are not of this world in the Orthodox church. The given nature of what happened, I would not doubt it at all. It sounds "miraculous", and not one of those "Mary on my grilled cheese" miracles.