If it's a set bridge piece, you'll have to adjust the terrain to fit, or add more pieces onto the end of it until it reaches the other side. It's quite fiddly. Otherwise, you can place individual bridge 'tiles' one by one, which is what I do. It takes forever, but you can adjust each bit as you go.
"I love finding threads like these, it's like walking outside and finding the city's been bombed while you were sleeping." - Blighboy