Name: Roots of Power
Circles: Earth / Mentalism or Biomancy
Description: Enchanted city produces +1 mana point for each tile of forest in its area of influence. (this spell has no upkeep cost)

I see there is one more <Blank> of Power spell proposed (for Rivers).

So Veins (mountains), Roots (forests) and Streams (river) of Power are the 3 proposed spells.

What I think would make a better spell is one that lets the caster choose the terrain type to draw power from.

Name: Natural Power
Circles: Earth / Biomancy
Description: Enchant a city to draw +X power (mana) from a terrain type chosen at the time of casting.

The effect is rather narrow to devote several spells to, but having one spell serve all terrain is doable to me.