Portuguese term (edited): Nascentes e Olhos d\'água

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They are synonymous, referring to the spring at its source. But you can also use synonyms in English. I will try to see if there are some subtle differences.

Unfortunately, "spring" in English can also refer to the water from the spring flowing in a channel.

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Note added at 5 hrs (2010-02-21 01:37:43 GMT)
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If anything, the spring(head) is smaller; the fountainhead is the ultimate source of a river.

Definitions:

springhead definition
You can also say "source," but it can mean a lot of other things, as can "spring."

spring·head (-hed′)

noun
a source or fountainhead

Collins:
springhead
n. the source of a spring
http://dictionary.reverso.net/english-definition/springhead

fountainhead
a spring that is the source of a river; an abundant source of knowledge, etc
en.wiktionary.org/wiki/fountainhead

This is how I would translate it.

nascente = source (river, stream)

olho d'água = spring