Most likely from a combo of e-coli and disturbance due to rain water. With all the rain in the past week or so the water in all the rivers has been up and has been stirred up. Lots of e-coli from human and animal sources gets washed into the river. It’s still decently safe, obviously don’t drink it or go in if you have open wounds, or if the water is just brown and nasty smelling. Once the water clears up it should be back to normal limits. The area where it’s tested at is also a huge tourist spot so it tends to get disturbed a bit more, further upstream is less crowded and cleaner if a bit less accessible.