Low-flow statistics for approximately 2,800 ungaged bridge locations were determined using drainage-area ratios from 312 streamflow-gaging stations. Regulation and diversion of streamflow can significantly modify low-flow discharges. Regulation is defined for this website as a stream with an upstream flood-control reservoir(s) which controls 10 percent or more of the contributing basin. If regulation began while a gaging station was in operation, records were analyzed for pre-regulation, post-regulation, and entire period of record streamflow conditions. In the case of multiple upstream reservoirs controlling the streamflow, the year in which the reservoir was built that makes the cumulative controlled area equal to 10 percent determines the break in record. While statistics were computed for pre-regulation, post-regulation and the entire period of record for gages with upstream regulation, only the post-regulation conditions are transferred to bridges downstream of a flood-control reservoir. Below are instructions to transfer a different set of statistics to a bridge site.

1. Determine the drainage area (DA) ratio:
DAsite / DAgage = DAratio
2. Multiply the calculated ratio times the low-flow statistic at the gage:
DAratio * gage statistic