DerGroBe
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Okay, here's a simple method for placing bridges: place the bridge in the desired location; then adjust the z-coordinate (Object Properties => Pos). By decreasing the z-coordinate, you can lower the bridge deck and its base to the desired height.
Indeed, it turns out to be that simple. Thank you very much for your help; otherwise, I would have struggled for a long time trying to adjust the terrain to fit my bridge.