Background/Question/Methods Purple sea urchins, Strongylocentrotus purpuratus, have been studied at Sunset Bay, Oregon, since 1963 and size-frequency data have been gathered since that time sometimes on an annual basis and sometimes with gaps of five or more years. When size data were gathered in 1995 sea urchins at all sites in the bay were typical . The sites were not visited again until 2004 when an examination of one site called the Boulder Field indicated that sea urchins were missing and this was confirmed by a more thorough search in 2007. Sea urchins had disappeared from the Boulder Field but not from other sites in Sunset Bay so the puzzle is what removed all of the urchins. Results/Conclusions Several possibilities for loss of sea urchins are collecting by humans, elevated temperatures during low tide periods, and freshwater runoff from Big Creek, a small stream that enters the bay. Other reasons for the sudden loss are possible. Of sites in the bay where sea urchins have been studied, the Boulder Field is the closest to where Big Creek enters. Evidence will be presented that freshwater runoff is the most reasonable cause of loss based on historical intense rain events coupled with extreme low tides and associated with land-use in the Big Creek watershed. Recovery of sea urchins in the Boulder Field at Sunset Bay is expected to be very slow because after a heavy settlement event in 1963 major settlement did not occur again until 1993.