Publication Summary

Siebert Creek in Clallam County was divided into seven reaches to investigate the interaction of groundwater and streamflow. Six of the reaches were determined to be gaining reaches with groundwater discharging to the stream. Reach 1, below Old Olympic Highway, receives the greatest contribution (per channel mile) from groundwater. Reach 2, between Highway 101 and Old Olympic Highway, is a losing reach with streamflow recharging the groundwater.

Siebert Creek drains a small watershed in Clallam County on the northern coast of the Olympic Peninsula. During the summer of 2003, the flow in Siebert Creek was primarily groundwater discharge. The changes in streamflow that occurred between its headwaters and its mouth reflect this groundwater interaction.

To investigate groundwater/surface water interactions, I divided Siebert Creek into seven reaches, five on the mainstem with the east and west forks each comprising an additional reach. I established a streamflow-measurement station at the start of each reach and measured streamflow several times between May and October, 2003. An increase in streamflow indicated groundwater was discharging to the stream, while a decrease in streamflow indicated groundwater recharge.

Six of the seven reaches were "gaining" reaches with a positive groundwater discharge. Reach 2 (RM 1.3 to 2.5) was a "losing" reach with streamflow decreasing as water was lost to groundwater (groundwater recharge). With the exception of Reach 2, streamflow increased from upstream to downstream as groundwater entered the stream. Total groundwater discharge from May through September averaged 4.2 cfs (8.3 acre-ft./day). On an area-inch basis, total groundwater contributions to streamflow were relatively consistent from the headwaters through Reach 3 varying between 0.83 and 1.43 inches. Reach 2 lost 1.14 inches of streamflow to groundwater, and Reach 1 (RM 0.6 to 1.3) gained 6.72 inches of groundwater. On an acre-ft./channel mile basis, total groundwater contributions to streamflow were similar from the headwaters through Reach 4 varying between 57 and 90 acre-ft./mile. A large increase in groundwater contribution per mile of channel occurred in Reach 3 where streamflow gained 149 acre-ft./mile. This changed to a streamflow loss (-51 acre-ft./mile) in Reach 2. The greatest groundwater contribution to streamflow occurred in Reach 1 where streamflow increased by 408 acre-ft./mile of channel.