back up vs silt up

back up and silt up both are verbs.

Word	Noun	Adjective	Verb	Adverb
back up	No	No	Yes	No
silt up	No	No	Yes	No

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

Other hyponyms of back up include gum up, block, choke up, lug, stuff, silt.

back up (verb)	silt up (verb)
establish as valid or genuine	become chocked with silt
become or cause to become obstructed
make a copy of (a computer file) especially for storage in another place as a security copy
move backwards from a certain position
give moral or psychological support, aid, or courage to