They drained the water.

For building across a river you could wait until a dry season to build your bridge when the water level would be low or you could sink logs into the ground to make a sort of dam around your construction site which you could then drain(either using pumps or just buckets depending on technology) then you build your support pillar before breaking down the dam you built.

Mostly, it involved building in the safest and easiest place instead of the most convenient for travel.