[Sacajaweau]

Maybe it's for transporting goods. The river is navigable, yes??

[cajungirl]

Maybe the sewer system! Hahah

[Cboldt]

Maybe it's for transporting goods. The river is navigable, yes??

Tunnel "to" a river is strange. Under river, sure. But for transport, may as well use surface. No size limits, cheaper to build, etc.

The waste system angle is a possibility. So is electrical power transmission.