Estimates of monthly and annual mean discharge for five ungaged sites in the lower Columbia River are presented for water years 1928-65. These sites are Columbia River at Vancouver, Wash., Willamette River at mouth, Columbia River at St. Helens, Oreg., Columbia River at Longview, Wash., and Columbia River at mouth. Two tables of estimates are compiled for each site. One table lists estimates of 'observed' flow, the flows that would be measured by a gaging station. The other table lists 'adjusted' flows, the 'observed' flows adjusted for storage changes in major reservoirs arid for diversion at Grand Coulee Dam.
Two methods are used in making these estimates. One method involves a detailed summation of flows from all gaged and ungaged areas, allowance for precipitation on and evaporation from the channel water surface, and adjustment of flow to allow for travel time from gaging stations at the Dalles, Oreg. (Columbia River main stem), and at Salem Oreg. (Willamette River).
The other method is the same as the summation method except that flows from individual gaged and ungaged areas below The Dalles and Salem are not summed in detail. Instead, the entire drainage area below The Dalles is divided into three major inflow segments and the flow from each segment is obtained from a correlation with streams that are representative of the flow from that segment. Estimates of monthly mean flows are considered to be accurate within 2 or 3 percent.
