It seems that they are building the Strandherd-Armstrong Bridge on the east bank of the Rideau River and then they will move it into position.
There are a few pictures from Dec. 1, 2011:
View from the Armstrong/River Road intersection of the west-bound half of the bridge
Detail of the construction of the ribs under the west-bound running way (one 'box' per lane)
The curve for the 'out-rigger' sidewalk is taking shape
A view from the west (Judging from the towers, there are about 8 ribs still to be added and another short tower.)
The temporary structures to support the bridge when it is slid across the river
The towers above the deck ribs will be used to position and hold the arches while the cables are strung. The towers give you a sense of how high the arches will be. They have constructed 'rails' under the bridge (visible in the 'sidewalk curve' picture above) so that it can be moved, but the towers are built up from the ground. The towers will need to be removed before the bridge can be moved. From the looks of it, during the move the ends of the arches will be on wheels on the rails and the bridge deck will be suspended by the cables from the arches. Since the land upon which the bridge is being built is higher than the mounding pads for the arches, I assume that the temporary support structures will have some form of hydraulic lift to lower the bridge into place once it has neen slid out over the river. I hope they provide advanced notice of when the move will take place. It would be interesting to watch; kind of like the Rapid Bridge Replacement of the 417 bridges.