Again, during the recent Tournament I discovered beside a major river as a secured flank, additional Deep Water terrain pieces ('lakes' or 'major rivers')are possible.
So does it also mean you could have potentially 2 major rivers (1 on each flank), first the compulsory (if secured flank) and a second as an option (provided no 3/4 is rolled) ?
No. The Deep Water as a secure flank is the only one that can go down a whole flank, the others are other terrain piece sized. Don't over-interpret the descriptions.