If you search Google with 'historia canal seco xerta la senia' you will find a few bits of information about it.

or: http://territori.scot.cat/cat/notices/canal_xerta_la_sEnia_533.php

From what I have been told it was one of the last of Francos projects before his death. It was never completed and was abandoned. Over the years attempts have been made to revive it with little success in agreements between the various mayors.  A few years ago they finally reached an agreement and the current works were started; planned in phases over various years.

It seems (due to lack of money most likely) to have ground to a halt. I'm sure it will be re activated at some time.  The idea is to move irregation water from Xerta to La Senia. Many locals believe it is a part of the currently abandoned plan to move water to the Valencia province.  There have been many protests about this over the years.

There is also a plan to build a new road to bypass Tortosa, Roquetes and Jesus.  One of the proposed routes (and possibly the most likely as it has the least impact) follows the canal so be careful or you may end up with a dual carriageway passing your house.  Again, due to lack of money, this doesn't seem to be progressing but all crisis end at some point....

I'm sure there are people out there who have more information about the canal..

Nigel