Here's why:
* Intransitive verbs do not take a direct object. They describe an action that is complete within itself.
* Transitive verbs require a direct object to receive the action of the verb.
In this sentence:
* "jumped" is the verb.
* "The dog" is the subject.
* "into the stream" is a prepositional phrase that describes where the dog jumped, but it doesn't receive the action of the verb.
Since there's no direct object receiving the action of "jumped," the verb is intransitive.