Riverbed

Riverbed
Riverbed Terrain
Terrain type	(Unknown)
Max Speed	Sprint
Can be paved	Yes
Can be plowed	Yes
Forageables	Riverbed Clay
Trees	(None)
Flora	(None)
Fauna	(None)
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Naturally occurring terrain that looks like a patch of exposed dirt and is the source of rivers. It can be dug to obtain riverbed-exclusive Riverbed Clay.

Speculation

Since Riverbed is subject to same construction/landscaping limits as water terrain (including the "exclusion zone" it's possible they were added ahead of a planned feature where rivers can dynamically flood or dry up. Some of the current riverbeds may have been intended as an alternative path for water to flow through.