Some ideas about how to use bridges in your wargaming terrain.
Sure, the easiest way is to buy already made bridge terrain. There are different solutions - made from plastic, resin, or even wood. Or you can make the bridge yourself. But the main point in bridge terrain is not how it's made - the main thing you need to be aware of is your river banks.
If your river is a flat surface - it's the easiest case. Every bridge which has its pillars on the same level will suit you.
Sometimes such river looks natural enough. And it's really easy to imitate. Also, it can be a battle mat or something like that.
By the way, sometimes you can use bridges without pillars at all. It's perfect for sci-fi terrain when you have acid or radioactive waste instead of water.
The banks which stand up a bit (for example, made of stone) are a kind of compromise. They rise the ground level near the river and it looks deeper. Such banks make you choose the height of the pillars.
A nice idea is modular terrain with bridge section - the bank is already made and the bridge can be set up everywhere. By the way - you can easily make a bridge from icesream sticks.
You can make the whole river modular - this provides really a lot of gaming options. But in this case you need to think about modular roads - you can't have your bridge leading to nowhere.
Anf the most difficult way is to carve the river in the table. And to place the bridge on the one place. It looks the most natural way, however, it leaves fewer options. It's up to you, what to choose for your wargaming terrain!