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flow velocity, or Froude number, defined as V gD where V = the flow velocity, g = acceleration due to gravity, and D = mean depth. The magnitude of the Froude number is an indication of the magnitude of hydraulic forces that affect ice formation and breakup at a given location. At relatively low flow velocities and high concentrations of surface ice (approximately 50% coverage or higher) it is possible for the ice cover to spontaneously arch across the width of the open area of the channel and stop moving as gaps between ice floes freeze, a process known as bridging. It is generally not possible to predict where these bridging locations will be in a particular river without historical knowledge. Ice control booms and/or hydraulic control structures are often used to assure the initiation of ice cover at a specific location. At relatively low flow velocities, ice floes that continue to arrive at the leading edge of the bridging location may come to a stop adjacent to the leading edge. In such cases, the ice cover will progress upstream by juxtaposition. The maximum flow velocity at which juxtaposition will occur depends on floe geometry and channel depth. At higher flow velocities, the ice floes arriving at the leading edge of ice cover may be forced underneath the existing ice cover or underturn. If the flow velocity is not too high, these underturned floes will remain at or near the leading edge of the ice cover. In some cases, accumulations of underturned floes and frazil slush can become packed under a stationary ice cover, especially in areas where flow velocity is lower. The strength of an ice cover formed from many separate pieces of ice is directly proportional to its thickness. If the forces acting on the ice cover exceed the ability of the cover to withstand those forces, the ice cover will sometimes collapse in the longitudinal direction and become thicker, a process known as shoving. When shoving occurs, the strength of the ice cover is increased. An ice cover may repeatedly shove and thicken as formation progresses in the upstream direction. If the ice cover is treated as a "granular" material, the strength characteristics and the final thickness of the cover can be mathematically estimated. At relatively high flow velocities, the ice floes arriving at the leading edge of the ice cover may be underturned and transported under the ice cover for considerable distances. At this point, further upstream progression of the ice cover may be halted until the deposition of the floes somewhere downstream of the leading edge reduces channel conveyance enough to cause upstream water levels to rise and the flow velocities at the leading edge to be reduced. If flow velocities are high enough, the ice cover will stop progressing upstream. In this case, open water will remain upstream of the leading edge throughout the winter season. Intact stationary ice covers break up in the spring. Two ideal forms of breakup bracket the types of breakup commonly found throughout most of North America. At one extreme is -4-