By Brian Todd, CNNGreat Falls, Virginia (CNN) — Several visitors just couldn’t wait for the weekend to get into Great Falls Park in northern Virginia. While it’s not illegal to access parks on foot during the shutdown, the scene at Great Falls illustrates park advocates’ safety concerns and visitors’ frustrations. You better believe I’m extremely nervous,” Ed Stierli, Mid-Atlantic senior regional director for the National Parks Conservation Association, told CNN in a phone call Friday. “The currents there are deadly.”Because of the lack of staff and visitor centers, Stierli said, the National Parks Conservation Association thinks that national parks should be completely closed during the shutdown. Earlier this week, other park advocacy groups such as the Coalition to Protect America’s National Parks and the Association of National Park Rangers criticized the decision to keep the parks partially open, calling it “dangerous” and “irresponsible.” The Association of National Park Rangers warned of an increased risk of vandalism and poaching.