The Puyallup Tribe of Indians is interested in better understanding the water resources of the lower Puyallup River Basin in order to ensure sufficient water to meet Tribal and hatchery needs and make future water-resource decisions. The U.S. Geological Survey, in cooperation with the Puyallup Tribe, conducted a study to identify trends in streamflow in the lower Puyallup River Basin and to compare streamflows in the Puyallup River with regulatory minimum instream flows. Daily mean streamflow, monthly mean streamflow for October, and annual mean streamflow records from 1980 through 2001 for two gaging stations on the lower Puyallup River and one each on Clarks Creek and Swan Creek in the lower Puyallup River Basin were analyzed for temporal trends. Daily mean streamflow records were divided into data sets for the wet period (November through June) and the dry period (July through October) for analysis. Annual precipitation records from three National Weather Service stations and ground-water-level records from five wells in the lower Puyallup River Basin were analyzed to determine possible relations with streamflow. Daily mean streamflow, daily minimum streamflow, and unit-streamflow records for the Puyallup River for 1991 and 1992 were evaluated for the instream-flow analysis.

Significant temporal trends were not identified in daily mean streamflow records from the Puyallup River, Clarks Creek, or Swan Creek for the period of analysis. Trend analysis of monthly mean streamflow records for October at two gaging stations on the Puyallup River also indicated no significant trends for the period of analysis. Temporal trends were not evident in precipitation data from weather stations in the basin. A trend of decreasing depth to ground water with time (1995 through 1997) was identified in one well (20N/04E-34G01). This well is drilled to about 550 feet below land surface, and variations in water levels at this depth likely do not affect streamflow in the Puyallup River. Data limitations prevented the evaluation of possible correlations between streamflow in the Puyallup River and water use and land use in the study basin.

Daily mean, daily minimum, and unit-streamflow values were evaluated to determine how each measure of streamflow compared with instream-flow values. The occurrence of excursions (streamflow below the instream-flow value) was greatest when unit-streamflow values were compared with instream-flow values. The use of daily mean streamflow records may underestimate the occurrence of excursions under certain streamflow conditions.

The unit-streamflow hydrograph for the Puyallup River at Puyallup exhibits a distinct, regular pattern. The hydrograph closely mimics the hydrograph at Lake Tapps Diversion, on the White River, a tributary of the Puyallup River, which is the outflow from a power plant, suggesting that the power-plant outflow affects streamflow in the Puyallup River. Streamflow entering Lake Tapps through the White River Canal does not exhibit the same pattern as the Puyallup River or diversion. The influence of the White River Canal on streamflow in the Puyallup River appears to be obscured by operation of the Lake Tapps Diversion.