congest vs clog up

congest and clog up both are verbs.

Word	Noun	Adjective	Verb	Adverb
congest	No	No	Yes	No
clog up	No	No	Yes	No

As verbs, congest and clog up are synonyms defined as:

Other synonyms of congest include back up, choke, choke off, clog, crap up, foul.

congest (verb)	clog up (verb)
become or cause to become obstructed	become or cause to become obstructed