TILLER — The U.S. Forest Service plans to thin a forest 30 miles east of Canyonville, and that has Tiller resident Stan Petrowski thinking about the clarity of his water. Fir trees will be cut in the 56,000-acre Elk Creek watershed in the Umpqua National Forest, reducing fire danger and creating timber jobs. The thinning will open space for oak trees, good for foraging wildlife. The project, which will take years to plan and illustrates the challenges facing forest managers who must balance sometimes competing objectives, also raises the possibility of erosion and downstream pollution. Six Forest Service employees accompanied …