With the help of levers and pedals, Interior Secretary Bruce Babbitt pulled a 16,000-pound chunk from the face of the Matilija Dam yesterday, a symbolic step toward dismantling the 20-story structure in southern California. Babbitt has been calling attention to the need to take down obsolete dams in the country for the last couple of years, and the Matilija would be one of the largest to come down. Environmentalists and community leaders say the dam is blocking endangered steelhead trout from reaching spawning grounds and keeping needed sand from reaching downstream beaches.