Just weeks before the Lower Mississippi River Museum and Riverfront Interpretive Center on Washington and Levee streets is set to open, U.S. Army Corps of Engineers employees were busy at work this morning pouring and forming concrete to finish a scale replica of nearly 70 miles of the Mississippi River. The center is just yards from the Yazoo Diversion Canal, in top of photo at right. Research hydraulic engineer Howard Park of the Corps said the model will be about 200 feet in length and topped with a playground surfacing so children can walk on it. The center is scheduled to open Aug. 24.