NOVELTY - The hydro turbine assembly has a water turbine (50) including runner blades (52) and a hub (54). Each runner blade has a blade region (63) and a base (64). The base includes blade mounting surfaces (68,70). Recesses (66) around the hub individually seat the bases of the runner blades. The blade mounting surfaces seat onto hub mounting surfaces (56,58) in the recesses. Both mounting surfaces are stepped.    USE - Hydro turbine assembly for electricity generation.    ADVANTAGE - Adding stepped mounting surfaces to a tapered hub increases degree of freedom in arranging the runner blades and the pattern of mounting bolts which secure the blades to the hubs. The stepped mounting surfaces also allow the base to better conform to a long blade root, and allows for relatively large area of surface contact between each blade and each recess which permits the bolt pattern to be optimized to sustain high load forces. Reducing the hub diameter may allow for larger water passage through the throat. The mounting system using the stepped mounting surfaces may be used on new water turbines or retrofitted on existing water turbines, and aid in transporting the hub and blades to a dam for installation. The blades may be shipped separately from the hub to reduce shipping difficulties which would otherwise occur in shipping a complete water turbine with all blades attached. The mechanical arrangement of mounting surfaces stepped along the axial direction provides greater freedom in designing the blade shape and bolt pattern. Increasing the water flow passage cross-sectional area without increasing the throat diameter yields significant power increase by the water turbine without incurring high costs associated with expanding the throat diameter.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method to install a replacement hydro turbine.    DESCRIPTION OF DRAWING(S) - The drawing shows the partially cross-sectional side view of a water turbine with stepped mounting surfaces on the runner blades and on the recesses in the hub.    Water turbine (50)    Runner blades (52)    Hub (54)    Hub mounting surfaces (56,58)    Blade region (63)    Base (64)    Recesses (66)    Blade mounting surfaces (68,70)