The effort to protect Oroville Dam entered a critical phase Monday when engineers shut off water flowing out of the damaged main spillway, giving officials their first unobstructed view of the eroded concrete chute since a crisis prompted mass evacuations earlier this month. For the next five to seven days, geologists and engineers will have an unhindered view of the concrete spillway, which on Monday was revealed to have severely deteriorated on its lower half during the last two weeks of use.