Science Center Objects

Located in northwestern Washington State, the Dungeness River and its tributaries drain about 200 square miles, mostly in the Olympic Mountains to the south. After emerging from the mountains, the river flows for about 11 miles northward across an area with a shallow water-table aquifer before emptying into the Strait of Juan de Fuca. Water in the Dungeness River and the shallow aquifer are closely related, although the amounts and seasonal variations of water exchange between them are uncertain or unknown.

To provide water-resource managers with information needed to manage water in the area for the competing needs of drinking water from ground water, irrigation water from the Dungeness River, and salmon habitat in the Dungeness River, the USGS is studying the interaction of the ground water and the water in the river.

WA435 - Relation Between the Dungeness River and the Shallow Aquifer in the Sequim-Dungeness Area, Clallam County, Washington - Completed FY2002

Problem - The Sequim-Dungeness area is about 65 square miles and is located in northwestern Washington State (fig. 1). The Dungeness River and its tributaries drain about 200 square miles, mostly in the Olympic Mountains to the south. After emerging from the mountains, the river flows for about 11 miles northward across the study area before emptying into the Strait of Juan de Fuca. A shallow water-table aquifer underlies the study area. It is mostly coarse deposits of alluvium and glacial outwash and its thickness averages about 100 feet. The Dungeness River and the shallow aquifer are closely related. Some reaches of the river recharge ground water and some reaches of the river receive ground-water discharge. However, this is a complex relation and the magnitudes and seasonal variations of ground-water recharge and discharge are uncertain or unknown. A better understanding of the relation between the Dungeness River and the shallow aquifer is needed to effectively manage the water resources of the study area. Competing needs are drinking water from ground water, irrigation water from the Dungeness River, and salmon habitat in the Dungeness River.

Objectives - The broad objective of this proposed study is to describe the relation between the Dungeness River and the shallow aquifer. The study will describe the annual and seasonal longitudinal distributions of flow quantities and water-level gradients between the river and the aquifer.

Relevance and Benefits - This study will improve the understanding of the relation between the Dungeness River and the shallow aquifer in the Sequim-Dungeness area of northwest Washington. This improved understanding can benefit other areas of the Pacific Northwest, because the hydrology of the Sequim-Dungeness area and the emerging social, economic, and biologic issues are similar to other areas of the Pacific Northwest. Ground-water discharge provides an important source of streamflow especially during the low-flow periods of summer and fall. In some areas, leakage from rivers provides an important source of ground-water recharge. The demand for ground water is increasing in many areas because of a rapidly growing population. At the same time, most salmon populations in Pacific Northwest rivers have been declining and the Chinook Salmon was recently placed on threatened status as part of the Endangered Species Act. Minimum levels of streamflow are needed to sustain and improve salmon populations, and there is concern that increased ground-water withdrawals will decrease streamflows below the minimum requirements.

Local, State, and Federal agencies in the Pacific Northwest need to work together to balance the demands for water from a growing population with the requirements of minimum streamflow levels for salmon. Knowledge about the relation between rivers and ground water is crucial to make informed decisions about granting new water rights, planning economic development, and managing natural resources. Methods or tools are needed to determine the relation between rivers and ground water, and this study will gain information about the value of a method that can be applied to other areas of the Pacific Northwest. In addition to the specific information for the Dungeness River, the concepts and general relations learned in this study can be applied to other areas of the Pacific Northwest.

This study is relevant and provides benefits to the national USGS goals and mission and to important issues identified in the USGS Washington Water Science Center 1999 Science Plan. The study addresses one of the nine priority issues outlined in the Strategic Directions of the Water Resources Division, 1999-2008. Issue number (8) is surface-water and ground-water interactions as related to water-resource management. This study will provide information that can be used to evaluate the effects of ground-water withdrawals on streamflows and it will estimate the longitudinal and seasonal distribution of the flows between the Dungeness River and ground water.

This study is appropriate for the role of the USGS in the Federal-State Cooperative Program because it will provide information that fits into two of the broad goals specified in WRD Memorandum 95.44. Information for goal number (2) (advancing field or analytical methodology) will be the knowledge gained on how useful it is to apply analytical methods to water-level data on ground-water/surface-water interactions. Information for goal number (3) (advancing understanding of hydrologic processes) will be the detailed understanding of the longitudinal and seasonal distribution of flows between the Dungeness River and ground water.

The USGS Washington Water Science Center 1999 Science Plan identified five important water-resource issues in the State of Washington. The District is focusing its efforts on advancing hydrologic methods related to these issues and on improving the understanding of the hydrology affecting these issues. This study will provide information on one of the five issues-- water availability. This study specifically addresses one of the subissues for water availability -- ground-water/surface-water interactions and declining ground-water levels.

Approach

1) The Dungeness River will be divided into six reaches and two seepage runs will be conducted, once in the fall and once in the spring.

2a) One gaining and one losing reach will be selected. In each reach, a surface-water gage will be installed in the river and a transect of six piezometers will be installed perpendicular to the river. Continuous recorders will be installed and water levels will be measured for 15 months. Monthly measurements will be made for water levels, specific conductance, and temperature for 15 months.

2b) A pair of piezometers and a staff gage will be installed in each of the four other seepage-run reaches and monthly measurements will be made for water levels, specific conductance, and temperature for 12 months.

3) The water-level data will be analyzed using analytical solutions for the interaction of a water-table aquifer and stream water levels.