Bill Kortum, a longtime Sonoma County environmental activist, died early Saturday in Petaluma after battling prostate cancer for more than three years. In 1972, he worked to pass Proposition 20, a measure that established the California Coastal Commission, the agency that regulates land and water use along the state’s coastline. Mr. Kortum also helped establish an open space district in Sonoma County and worked to create Sonoma County Conservation Action, an organization that aims to educate the public on environmental issues. Soft-spoken but tenacious, “Bill was like a big, strong redwood tree — bending with the wind but never breaking, and always looking far ahead,” Cardo said. A veterinarian by trade, Mr.