Draw the storage area on the Geometry editor just upstream of the reach.
Go to the Edit Menu and select “Move Points/Objects”. With your mouse pointer, grab the end of the blue streamline by left clicking on it and drag it into the storage area you just created and release it. At that point, the RAS GUI should ask if you want to connect the reach to the storage area. To remove the connection, do the same thing but drag the centerline out of the storage area.