Reach the other side of the bridge to find iron deposits. To repair the bridge you will need 1 Wooden Plank, 4 Flitch and 8 Nails. To craft the Wooden Plank you'll need to unlock the Woodworking technology first.
Once you repair the bridge, you will be able to cross it to reach the other side of the river. There, you can find a lot of iron deposits. Each deposit gives you 6 pieces of iron. Remember that the deposits won't appear again over a certain period of time.
Iron can be sold to the Blacksmith for about 9 bronze coins a piece. However, you can turn Iron Ore into Iron Ingots - this increases their price to 60 bronze coins.
Iron can be turned into Nails, Simple iron parts and Complex iron parts. These items are necessary in future repairs and upgrades.
Unlimited supply of iron
In Graveyard Keeper there is a terrain called Quarry. It can be found further to the west from the Forest near the River.
To get to this location, you need to first get through a blocked passage between rocks. In order to do this, you will need 2 wooden planks, 4 flitch and 4 nails.
You will discover an abandoned shack there - where you will be able to build - as well as a deposit of stone, coal and iron. You will be mining whole blocks of those resources, not pieces. Because of that, it is generally a good idea to construct a storage for a given type of resource (like, for instance, iron ore stockpile) and place them there.