Canadian Press: Every spring, wardens at Elk Island National Park just east of Edmonton patrol the shores of Astotin Lake to clean up debris exposed by receding water. The junk that shows up on the ever-lengthening beach provides a kind of index for just how long it has been since water levels were that low. “This year, I found a tire from a Model A car,” says Clayton Szafron. “It’s kind of disconcerting.” Astotin isn’t alone. The verdant parkland between Edmonton and Saskatoon was once …