I believe that you need to look at what water district that lake is in. For example at Richland Chambers it was built for water to supply Ft. Worth. It's either Tarrant county water district or Ft. Worth, don't remember for sure which one. Hope this helps some.
I think Fork was built to supply water for Dallas, could be wrong though.
I found a site that might answer your questions, or at least you can shoot them a e-mail and get a reply. Sabine River Authority at www.sra.dst.tx.us