The people in the cars whizzing over the Twin Cities' Mississippi River bridges this morning may not realize that they're passing over a National Park site. But they are. The Mississippi National River and Recreation Area consists of seventy-two miles of the Father of Waters and a host of parks, trails, and historic sites along the riverbanks in the Twin Cities area. This weekend, the National Park Service celebrates the opening of a visitor center in downtown Saint Paul to help acquaint people with the attractions along the National River. Minnesota Public Radio's William Wilcoxen reports....