IRS considers cryptocurrency as property, and exchanging one for another is a taxable event.

See "Q16. Will I recognize a gain or loss if I exchange my virtual currency for other property?" [1]:

   A16.  Yes.  If you exchange virtual currency held as a capital asset for other property, including for goods or for another virtual currency, you will recognize a capital gain or loss.  For more information on capital gains and capital losses, see Publication 544, Sales and Other Dispositions of Assets.

[1]: https://www.irs.gov/individuals/international-taxpayers/freq...