Overstaying a visa, a common way of being in the US illegally, is a civil issue.
However, that's not the case for border jumpers, as imprisonment is clearly part of the potential punishment.
8 U.S. Code 1325 - Improper entry by alien
Any alien who
(1) enters or attempts to enter the United States at any time or place other than as designated by immigration officers, or
(2) eludes examination or inspection by immigration officers, or
(3) attempts to enter or obtains entry to the United States by a willfully false or misleading representation or the willful concealment of a material fact, shall, for the first commission of any such offense, be fined under title 18 or imprisoned not more than 6 months, or both, and, for a subsequent commission of any such offense, be fined under title 18, or imprisoned not more than 2 years, or both.
Did you bring your child or any other aliens with you when you jumped to border? Criminal.
8 U.S. Code 1324 - Bringing in and harboring certain aliens
(a) Criminal penalties
(1)
(A) Any person who—
(i) knowing that a person is an alien, brings to or attempts to bring to the United States in any manner whatsoever such person at a place other than a designated port of entry or place other than as designated by the Commissioner, regardless of whether such alien has received prior official authorization to come to, enter, or reside in the United States and regardless of any future official action which may be taken with respect to such alien;
(ii) knowing or in reckless disregard of the fact that an alien has come to, entered, or remains in the United States in violation of law, transports, or moves or attempts to transport or move such alien within the United States by means of transportation or otherwise, in furtherance of such violation of law;
(iii) knowing or in reckless disregard of the fact that an alien has come to, entered, or remains in the United States in violation of law, conceals, harbors, or shields from detection, or attempts to conceal, harbor, or shield from detection, such alien in any place, including any building or any means of transportation;
(iv) encourages or induces an alien to come to, enter, or reside in the United States, knowing or in reckless disregard of the fact that such coming to, entry, or residence is or will be in violation of law; or
shall be punished as provided in subparagraph (B).
(B) A person who violates subparagraph (A) shall, for each alien in respect to whom such a violation occurs—
(i) in the case of a violation of subparagraph (A)(i) or (v)(I) or in the case of a violation of subparagraph (A)(ii), (iii), or (iv) in which the offense was done for the purpose of commercial advantage or private financial gain, be fined under title 18, imprisoned not more than 10 years, or both;
(ii) in the case of a violation of subparagraph (A)(ii), (iii), (iv), or (v)(II), be fined under title 18, imprisoned not more than 5 years, or both;
(iii) in the case of a violation of subparagraph (A)(i), (ii), (iii), (iv), or (v) during and in relation to which the person causes serious bodily injury (as defined in section 1365 of title 18) to, or places in jeopardy the life of, any person, be fined under title 18, imprisoned not more than 20 years, or both; and
(iv) in the case of a violation of subparagraph (A)(i), (ii), (iii), (iv), or (v) resulting in the death of any person, be punished by death or imprisoned for any term of years or for life, fined under title 18, or both.
And once an illegal gets here, assuming they've avoided the criminal penalties of these and many other statutes, their whole life here is one long string of criminal actions, starting with that first job:
US Code says [Title 18, Part I, Chapter 47, Section 1015] "Whoever knowingly makes any false statement or claim that he is, or at any time has been, a citizen or national of the United States, with the intent to obtain on behalf of himself, or any other person, any Federal or State benefit or service, or to engage unlawfully in employment in the United States...Shall be fined under this title or imprisoned not more than five years, or both."
