jam vs dam up

jam is a noun but dam up is not a noun.

Word	Noun	Adjective	Verb	Adverb
jam	Yes	No	Yes	No
dam up	No	No	Yes	No

As verbs, dam up is a hyponym of jam; that is, dam up is a word with a more specific, narrower meaning than jam:

jam (noun)	dam up (noun)
deliberate radiation or reflection of electromagnetic energy for the purpose of disrupting enemy use of electronic devices or systems
preserve of crushed fruit
a dense crowd of people
informal terms for a difficult situation

jam (verb)	dam up (verb)
interfere with or prevent the reception of signals	obstruct with, or as if with, a dam
crowd or pack to capacity
get stuck and immobilized
block passage through
crush or bruise
press tightly together or cram
push down forcibly