The Nine Mile hydroelectric facility, located on the Spokane River in northeastern Washington, was built in 1908 and has been operated by Avista since 1925. In the mid-1990s, long-term sedimentation was beginning to restrict operation of the facility and causing damage to the turbines. NHC led a program of field investigations and physical modelling studies to design a unique sediment bypass tunnel adjacent to the powerhouse. The tunnel was constructed in 1998 and immediately improved operational conditions of the four generating units. With new life breathed into this century-old facility, Avista began planning and implementing project upgrades, which included replacing the timber flashboards with an automated spill gate and upgrading two of the generating units. These upgrades would again increase sedimentation, and NHC used two-dimensional numerical modelling and physical modelling to predict how the changed operation would affect sedimentation and evaluate the performance of alternative sediment bypass facilities under the predicted future conditions.