When available, published flow records provide the most accurate data for designing culverts and bridge openings. This is because the values are based on actual measured flows and not calculated flows. The streamflows are measured at a gaging site for several years. A statistical analysis (typically Log Pearson Type III) is then performed on the measured flows to predict the recurrence intervals.
The USGS maintains a large majority of the gaging sites throughout Washington State. A list of all of the USGS gages that have adequate data to develop the recurrence intervals and their corresponding flows is provided in Appendix 2-1. In addition to these values, the OSC Hydraulics Branch maintains records of daily flows and peak flows for all of the current USGS gages. Also, average daily flow values for all current and discontinued USGS gages are available through the Internet on the USGS homepage (note that these are average daily values and not peak values).
Some local agencies also maintain streamflow gages. Typically, these are on smaller streams than the USGS gages. While the data obtained from these gages is usually of high enough quality to use for design purposes, the data is not always readily available. If the designer thinks that there is a possibility that a local agency has flow records for a particular stream then the engineering department of the local agency should be contacted. The OSC Hydraulics Branch does not maintain a list of active local agency streamflow gages.