Fine Print

They drive on a dirt road alongside the dried-out river name. The road is in pretty poor condition and they get shaken thoroughly. Mary sits in the front with Jack, showing him the way.
She appears to be having a blast: Faster! Faster!
I don't think Annie would appreciate that.
Aww.
They hear Annie say thanks from behind Jack.
somezthing along the lines of "You good back there?"

After a bit less than ten minutes of driving, Mary points to a structure extending into the river bed and says: That's it.
Jack pulls over and parks the truck, and they get out. They walk through the tall grass towards the structure. A bridge further upstream catches Jack's attention.
Is that a road or a railway?
Mary says: Railway – why?
What's the condition like?
Not sure, it's been a while... We used to play there as kids, we even got a railcar to move once.
Sounds like serviceable enough to me. Do you know where it goes?
There's a train station a bit that way, with a bunch of other ruins in the area.
And the other direction?
I don't know. We followed it pretty far, the only thing we found is a switch.
Okay. Would be incredibly useful if that ended up somewhere in Ivydale.

They reach the structure. It's the entrance to a tunnel with a few walls that pertrude into the river and two weers? translate: Stauwehre / Wehre, one for controlling the inflow? tanslate: Zufluss, one for a bypass can be adjusted with large screws . The floor of the tunnel sits significantly beneath the river bed and, while the river bed is completely dry, there is still some water inside.
TBD:
- look at the large buried tank / branching point
- go upstream to see where the river has gone