When the whole thing is selected press Cntr+C a couple times to be sure it copyes it
Go back to your map and make the river to look something like this (it varies for diferent types of bridges and directions)
Cntr+v to paste the bridge and place it where you want it
Click Cntr+T for terrain layer and switch the type or brush to restangular then select dirt and map out the water tiles that are left (if there isn't any then this step is useless and you don't need to do it)