Publication Summary

During 2005, the Department of Ecology conducted a variety of Total Maximum Daily Load (TMDL)-based field studies to assess stream temperatures, streamflows, and other environmental factors affecting the East Fork Lewis River and its tributaries. This study and the resulting report were part of that effort. The report describes the study area hydrogeologic setting, local surface water/groundwater interactions, and the temperature of incoming groundwater.

The purpose of the study was to provide information on groundwater inflow and outflow along the East Fork Lewis River. This information will be used for a temperature model needed to complete a Total Maximum Daily Load analysis.

Methods used to estimate groundwater inflow and outflow along the river included (1) seepage surveys (surface water discharge balances), (2) vertical hydraulic gradient measurements, and (3) continuous streambed temperature measurements from instream piezometers. The seepage survey was the most comprehensive method used and provided broad-scale measurements. Vertical hydraulic gradients and streambed thermal profiles measured in the lower river provided localized indications of groundwater flow direction. All monitoring activities were conducted in the summer of 2005.

Seepage surveys consisted of two, one-day flow analyses of the lower 32 miles of the East Fork Lewis River. The river was divided into 11 reaches of 0.8-5.6 miles. Streamflow measurements were taken at the upper and lower end of each reach and at the mouth of each major tributary. Because results of the August survey were more representative of baseflow conditions than results from July, analyses are primarily based on August data. Four reaches showed net streamflow gains, and two reaches showed net streamflow losses. No measurable change was seen in five reaches. However, gains and losses below the 7% error in comparative streamflow measurements could not be discerned. The total of streamflow gains was 64 cfs; total of streamflow losses was 18 cfs. Hyporheic temperature measurements indicated gaining conditions at four downstream sites. Temperature of inflowing groundwater ranged from 10.6 to 12.5 degrees C.

Groundwater temperatures were lower than surface water temperatures except at the most downstream site (river mile 1.8), where warmer, tidally influenced surface water was seeping into groundwater.

Public supply wells withdraw substantial amounts of water from the main aquifer supplying baseflow to the East Fork Lewis River. These withdrawals, which are increasing in some areas, may be changing groundwater flow to the river.

Link to EIM data for User Study ID EFLRTMDL