The peak-flow special conditions were identified or verified using criteria customized for each special condition. For peak flows having a peak-flow code 3 or 9 in NWIS, the goal of the verification process was to determine which special condition formed the basis for the code, disambiguating cases where multiple meanings were possible for a single code or where different code combinations were historically assigned for a single special condition. The verification process did not seek to confirm the accuracy of the original identification of the special condition.
The bases for peak-flow codes 3 or 9 fell into two general categories, sudden release of water and snowmelt. For peaks having snowmelt as the basis for the code, the peak-flow special condition was always snowmelt. For peaks affected by a sudden release of water, peak-flow special conditions included the sudden draining of lakes or ponds dammed by various materials, the natural switching of flow paths in indeterminate drainage basins, and ground subsidence from detonation. Because the term glacial lake outburst flood (GLOF) is a general term that can apply to many types of outburst flood mechanisms, the special conditions for sudden releases in glacierized basins were instead specified by dam material or process. In particular, this dataset differentiated glacier (ice) dammed lakes from lakes dammed by other materials because ice dams often reformed and generated recurring outbursts, whereas other dams in this dataset generated a single outburst.
Verification methods prioritized documented comments regarding the special conditions. Peaks having no documentation that occurred at sites having other glacier dammed lake outbursts were verified as glacier dammed lake outbursts, where possible, using a hydrograph of daily mean flow or plot of peak-flow data that showed flow magnitude and timing consistent with the other peaks generated by the same condition at the site. A large number of peaks having no documentation were verified as snowmelt because they occurred in April, May, or June, a common spring snowmelt period, and no other special condition could be determined. A small number of peaks having no documentation that occurred in a winter month were also considered within a snowmelt period (possibly also associated with rainfall) after consideration of site location and other peaks at the site. For one site where no documentation was available to verify the basis for code assignment, but snowmelt could be ruled out because the peak month of occurrence was outside a likely snowmelt period, the likely special condition for several peaks was inferred as glacial dammed lake outburst from other peaks at the site. For peaks having no available information to verify the basis for code assignment, the basis was noted as unverified.