You should have some soil layers near the river, or are there no trees or anything here? Sand, soil, peat, clay should all be "growable" areas. You can also splash some river water onto stone if all else fails. Stone + water = mud.
You should look at how to control water. Put a floodgate 1 square away from the river (leave 1 square of ground in between). link it to a lever which is safe somewhere else (it can be in a master control room for the whole fortress, levers work at a distance), test the floodgate by flipping the lever on and off. then get a miner to channel out the 1 square behind the floodgate. You now have a water tap with an on/off switch that can divert river water anywhere, though you should build a floor over the channel for security reasons. Note, that if you make a pipe too long horizontally, the water pressure drops and it won't go all the way. you can use gravity or pumps to get it to go further.
If you have a big map and far flung resources, you can also set up either temporary or long-term "outputs" which produce resources. i.e. if only have soil near the river then farm near the river, and work out how to pump water closer to the fortress later. This gives you time to work out how to shift needed resources closer to the fortress later (e.g. running water system and magma pumping system). You can also set up automated or manual minecart shipping systems to get produced materials around the place.
A good optimization for far flung production, is to place bedrooms and dining area for the farmers right next to the farm, and make a farmer's burrow for them to live and work in. They won't often wander all the way back to the main fortress then, and will hang out near their job. Make sure you use underground tunnels to connect up the areas, and those outpost don't even need to connect to the surface for added security.