There is some weird irony in the fact that the most notorious investigative matter of Amanda Marshall's prosecutorial career has been that of a politician brought down primarily because of a relationship, and it's now possible that her career as a public official will end because of a relationship. [UPDATE 3/20: The Oregonian's Bryan Denson reports that there wasn't any romantic relationship, but rather too much texting and emailing, possibly about the Assistant U.S. Attorney's personal life.]
But irony aside, one might wonder what will be the likely impact of Marshall's leave on the ongoing investigation of ex-Governor Kitzhaber and Cylvia Hayes.
As I see it, the likely impact is pretty much ... zero.
U.S. Attorneys are frequently tapped to become U.S. District Judges, which is not surprising, since the candidates for both positions are usually selected from a pool recommended by the Senator(s) of that State, assuming congruent political party affiliation with the President. (Or, they may become state judges, which is what happened with Karin Immergut, who served as the U.S. Attorney in Oregon from 2003 to 2009.)
Yet, when such U.S. Attorneys leave to become judges, the work of their offices does not come crashing to a halt. That's because the day-to-day work of the investigation is handled by Assistant U.S. Attorneys, and there's typically a Chief of the Criminal Division who would supervise those AUSAs. (In a large office, like that in L.A., the Criminal Division is further divided into separate units such as Major Frauds, Narcotics, and so on - providing even more levels of supervision.) That's not to say that the U.S. Attorney is just a figurehead position, and of course, in a high profile matter like the Kitzhaber investigation, we'd expect that the U.S. Attorney is involved in major decisions. But in the absence of a U.S. Attorney, it's not as if the remaining structure in place would be incapable of proceeding onward.