What do you need for this design?

• A few rock blocks.
  
• A few windmills.
  
• A few mechanisms, levers, shafts.
  
• Pipe sections, Corkscrews (of wood) for Pumps.
  
• Materials ready for blocking off the river.

This is how the River Drainer looks like:

Remember,
MAKE SURE TO MAKE IT AT THE RIVER SOURCE FACING THE ENTERING WATER DIRECTION.
Check the latter part of the page for learning how to find what river source is.

Moving to the underwater part:

• You make the pumps facing the river flow inlet side.
  
• You make the pumps from one direction, wait until one pump is completed and then go for the next.
  
• Give power at the end to avoid deaths and problems.
  
• One pump requires 10 power, one windmill gives around 40.
  
• Make the retractable bridge so when it retracts, only then the pumps work.
  
• Making one pump removing water in the opposite direction shall make sure the area behind the giant pump stack will clear water in one region.
  
• That enables water shallow enough to make a wall as shown in images.
  
• Once the wall is made, the river will drain. Slowly, but it will drain entirely.
  
• There are many designs for this which may or may not be updated in this but yeah.
  
• Make the designs and layouts as per your requirement.
  
• have raised bridge to like control the inlet flow any time.
  
• Be sure to start the pumps again to remove the wall.

This is quite an easy project to make step by step. Skipping steps can greatly escalate the situation causing lot of deaths depending upon the mistakes. A very dangerous flooding design project if done the right way. You legit throw a river into its face so you can redirect it into a flooding room. Unless it is winter and the river freezes ofc.

Lastly, if you complete this, congratulations on completion of your first mini-project. You stopped a river in dwarf fortress! how great is that!!! Keep it up! Good luck.