I pray to the clickbait gods that it is something more interesting than rust, but, alas, our punishment is eternal and the reason is just what you thought it was.
I don't think the point is that it's rust, but what's causing the rust to form...
Runoff from mining? Activist groups I'm sympathetic to have been screaming for years about certain types of mining specifically open pit mining obliterating watersheds. At a guess I would say either new mining interests are being developed around the relevant rivers in Alaska or old mining interests are being improperly decommissioned. Either way id bet my guess is right or close to right.
Why is there rust in a river? Why now? Is it bad? What can be done?
Would “Why there is rust in Alaska’s rivers” be more acceptable to you?
This is more a commentary on how bad things have gotten, but I’d say this is like a 3/10 on the Clickbait’o’meter.