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We examine the impact of baryon-dark matter relative velocities onintergalactic small-scale structure and the 21 cm signal during reionization.Streaming velocities reduced clumping in the intergalactic medium (IGM) on massscales of $\sim 10^4 - 10^8$ M$_{\odot}$. This effect produced a distinctbaryon acoustic oscillation (BAO) feature in the 21 cm power spectrum at wavenumbers $k\sim 0.1$ h/Mpc, near which forthcoming surveys will be mostsensitive. In contrast to the highly uncertain impact of streaming velocitieson star formation, the effect on clumping is better constrained because it isset mainly by cosmology and straightforward gas dynamics. We quantify thelatter using coupled radiation-hydrodynamic simulations that capture the Jeansscale of pre-reionization gas. The clumping factor of ionized gas is reduced by5-10\% in regions with RMS streaming velocities. The suppression peaks $\approx5$ Myr after a region is reionized, but disappears within 200 Myr due topressure smoothing. We model the corresponding impact on the 21 cm signal andfind that the BAO feature is most likely to appear at $\approx$ 10 \%ionization. During this phase, the feature may appear at the 1 \% (5 \%) levelat $k \sim 0.1 (0.06)$ h/Mpc with an amplitude that varies by a factor of $<10$ across a range of reionization histories. We also provide a model for thesignal originating from streaming velocity's impact on ionizing sources, whichcan vary by 4 orders of magnitude depending on highly uncertain sourceproperties. We find that the clumping signal probably dominates the source oneunless Population III star formation in $10^6 - 10^8$ M$_{\odot}$ haloscontributed significantly to the first 10\% of reionization.

A Model-Insensitive Baryon Acoustic Oscillation Feature in the 21 cm Signal from Reionization