The statistics were computed from the full period of record for all stations with at least 3 years of record through water year 2015, which ended Sept. 30, 2015. The computed statistics include annual means, medians, minimums, maximums, and standard deviations of the daily mean values; percentages of daily values with zero flow; harmonic means adjusted and non-adjusted for days of zero flow; means, standard deviations, and skews of the logarithms of daily mean flows; and flow-duration statistics for selected exceedance percentiles between the 1 and 99 percentages.

The new statistics replace any old values except for streamgages in Hawaii, Indiana, Montana, Rhode Island, and South Carolina. Older values in those states were retained as Not Preferred values. Although the newer values were computed from longer periods of record than the older values, the older values for the noted states often were computed using periods of record for which the flow was considered natural. As the newer statistics were computed using whatever period of record was available, the newer statistics may be computed from periods that include both regulated and natural flow.  The statistics were computed using the USGS QSTATS program (https://pubs.usgs.gov/of/2008/1362/cd_links/ofr2008-1362-A4-QSTATS.pdf). It is intended that these statistics will be updated on an annual basis in the future.