They are a source of the water that all of us (people, animals, plants) need to survive.
(They; are; a source of the water) (all of us; need; to survive) Context(all of us need,List([36, 76))):(all of us; need to survive to survive; ) (They; are a source of; the water)
all of us need,List([36, 76))
They all of us all of us They
are need need to survive to survive are a source of
a source of the water
to survive
the water