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Interesting, but not conclusive.
On the same day/time as the 1991 incident water reportedly flowed backwards as far north and west as Whiskey Still Sink which is connected hydrologically to Indian via isotope trace studies (Wisenbaker, NSS News, Oct 1992). This suggests that whatever happened that day was upstream of Whiskey Still.
A localized rock collapse or sandslide at Indian can't really account for that.