There's still a lot to fight about when it comes to determining how much fresh water is needed for a healthy San Francisco bay and delta, but research by a UC Davis scientist has advanced the controversy far enough so that final federal regulations protecting the bay ecology could be issued Dec. 15. Central to the forthcoming rules is a simple measurement, known as the "X2" marker. Scientists have long suspected that fresh-water diversions were harmful to certain fish in the bay. Yet, skeptical agricultural and urban water users did not want to lose any of their share of river water to the fish. So UC Davis limnologist Alan Jassby and his colleagues analyzed available data on all organisms ranging from the microscopic phytoplankton to the striped bass and calculated one simple indicator of habitat health -- an estuarine transition zone, or the X2 marker, where the near-bottom salinity measures 2 percent. On a map, the X2 salinity marker would resemble a wavy topographic line that ebbs and flows with tides and weather, usually somewhere east of Carquinez Strait. Besides offering a precise and easily measured index for monthly water-flow regulation, the research "shows for the first time a clear, large and across-the-board effect of fresh-water flow on all populations," Jassby says.