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We identify ten -- seven for the first time -- elements of cold halosubstructure (ECHOS) in the volume within 17.5 kpc of the Sun in the inner haloof the Milky Way. Our result is based on the observed spatial and radialvelocity distribution of metal-poor main sequence turnoff (MPMSTO) stars in 137Sloan Extension for Galactic Understanding and Exploration (SEGUE) lines ofsight. We point out that the observed radial velocity distribution isconsistent with a smooth stellar component of the Milky Way's inner halooverall, but disagrees significantly at the radial velocities that correspondto our detections. We show that all of our detections are statisticallysignificant and that we expect no false positives. We also use our detectionsand completeness estimates to infer a formal upper limit of 0.34 +/- 0.02 onthe fraction of the MPMSTO population in the inner halo that belong to ECHOS.Our detections and completeness calculations suggest that there is asignificant population of low fractional overdensity ECHOS in the inner halo,and we predict that 1/3 of the inner halo (by volume) harbors ECHOS with MPMSTOstar number densities n ~ 15 kpc^-3. ECHOS are likely older than known surfacebrightness substructure, so our detections provide us with a direct measure ofthe accretion history of the Milky Way in a region and time interval that hasyet to be fully explored. In concert with previous studies, our result suggeststhat the level of merger activity has been roughly constant over the past fewGyr and that there has been no accretion of single stellar systems more massivethan a few percent of a Milky Way mass in that interval. (abridged)

Insight Into the Formation of the Milky Way Through Cold Halo Substructure. I. The ECHOS of Milky Way Formation