Each line segment should have a unique ID based upon when it was created (i.e. the first node would have an ID of 1, next 2 etc). This would mean the numbering can be used as an indicator of direction. If you can work out if the line was drawn moving up or downstream you can then select layer by attributes using the ID field being > or < the ID where the intake is located. Then buffer the selected features using the appropriate buffer specifications. Not sure how complicated your dataset would be, but that might work for you.
... View more