Publication Summary

Instream mini-piezometers and a seepage run were used to examine the surface water/groundwater interchange within the Walla Walla watershed. This study determined that the upper areas of the watershed were predominately losing (surface water discharging to groundwater) and the lower reaches were gaining (groundwater discharging to surface water). The Touchet River, Walla Walla River, and Mill Creek were losing in their upper reaches.

The Walla Walla River and Mill Creek basins are composed of coarse gravel in the upper portions and fine-grained deposits in the lower portions. This distribution of materials results in groundwater discharging to surface springs which is more significant in the western part of the basin, at the confluence of the Touchet and Walla Walla rivers.

Segments of the Walla Walla River and two of its tributaries, Mill Creek and the Touchet River, are on the 1996 federal Clean Water Act Section 303(d) list as impaired for temperature, fecal coliform bacteria, and pH. To establish load and wasteload allocations for these parameters, Total Maximum Daily Load (TMDL) studies were conducted by the Department of Ecology.

To better understand the interchange between surface water and groundwater within the Walla Walla watershed, data were collected during July through October 2002 using (1) instream mini-piezometers to define the vertical hydraulic gradient and direction of water flow at discrete points, and (2) a seepage run to provide estimates of the net gains or losses across broader river reaches.

Results of this study support previous investigations which determined that the upper areas of the watershed were predominately losing (surface water discharging to groundwater), and the lower reaches were gaining (groundwater discharging to surface water). Even though the study area is divided between two distinct geologic areas, the patterns of gains and losses were consistent during the monitoring period.

The Touchet River flows over loess-covered basalts with a thin layer of alluvium in the valleys. The Walla Walla River and Mill Creek are dominated by a series of coalescent fans. The Touchet River, Walla Walla River, and Mill Creek had negative hydraulic gradients and were losing in their upper reaches. The coalescent fans are composed of coarse gravel in the upper portions of the basin and fine-grained deposits in the lower portions. This distribution of coarse and fine materials influences the movement of groundwater and results in groundwater discharging to surface springs. Discharge to surface water is more significant in the western part of the alluvial fan area, at the confluence of the Touchet and Walla Walla rivers.

Link to EIM data for User Study ID KSIN0007