Help with encirclement

[mrman0123]

Why are the purple units in the center not encircled at this point?

Encirclement.jpg (378.54 KiB) Viewed 641 times

[Wagner0445]

The units in the center are supplied through the river in the center. Minor rivers don't block supply. To know if a river is minor or not click on the river hex and read if there stands minor or major river.