Passage.
"Passageway" is one word.
The one word synonym for "underground passageway" is "tunnel".
A valve is a device which controls or directs the flow of a substance; liquid or gaseous, from a high pressure area to a low pressure area by opening, closing or obstructing the substance's passageway.
Ramp
There is no root word of river. It itself is a root word.
the root word is'confirminglyishness'
Strait
Root word is usual.
the root word is decor and that is the root word of decorate
The root word is gift.
end is the root word
Tri is the root word of intricate. The root word means three.