Surprisingly, there are a number of possible causes we're familiar with that actually produce water colors like this.
For instance, biological activity is often offered as a cause of discolouration of water bodies. However, in this instance, bacteria wasn't the cause. Intrusion of colour causing bacteria is usually the result of a decrease in oxygen concentration of a water body. But, as this is a river, and by definition is free moving, a significant decrease in oxygen levels on this scale is unlikely. I would also think an algal bloom is an unlikely cause, the main microorganisms that cause large discolourations or “red tide” are predominantly marine based, and of course, this is fresh water.
Alternatively, another possible explanation for flowing red waters is Acid Mine Drainage. AMD, can occur when water flows over or through sulphur-bearing materials forming solutions of net acidity. It is mainly associated with abandoned coal mines and currently active mining. An iron percipitate is formed and is the cause of a red/orange discolouration.
In this case however, most likely the cause was a different result of mining. Upstream of this site there is a mine that has disturbed reddish-colored ground, most likely a reddish clay layer. The clay is red for the same reason as acid mine drainage; iron oxidation, and an appropriately placed storm or flood likely washed much of that material downstream, creating this interesting view.
-Jean
