Courtesy tboardAt 7:30 on Saturday morning, the 570-foot-tall, 5770-ton smokestack of the High Bridge power plant will come crashing down. Xcel Energy’s new gas-fired plant is complete, and the old coal-burning plant, built in 1923, is being torn down. If you want to watch, try the bluff across the river. (Traffic will be stopped on the High Bridge, Randolph Avenue, and Shepard Road.) And be on time: the stack is expected to fall in about 10 seconds. Even the dust cloud should dissipate quickly.