|Possible small oil spill found on Google Maps imagery of Alaska? Image and analysis by Michael Cote. Read more on his blog.|
I think this dark patch is most likely wind shadow: a calm patch of water on the downwind side of a topographic (or human-made) obstruction that appears darker than surrounding, wind-rippled, sun-glinty water. Moving upstream and downstream, you encounter many dark streaks and patches on the river that indicate a strong, uniform wind blowing from the northeast. But oilfield infrastructure is often grimy and oily, and it wouldn't be surprising if there was some light oily sheen emanating from this infrastructure and enhancing the water-flattening effect of wind shadow on the downwind side of this facility.
Nevertheless, it's great to see people using these tools to adopt a patch of planet Earth to watch over, and investigate things for themselves. If you feel like trying a bit of skytruthing, check out the imagery in Google Maps and Google Earth -- and the millions of free Landsat satellite images, while you're at it! -- and let us know if you see something interesting, something mysterious, something you'd like us to take a look at!