Oregon Water Science Center Studies

Reconnaissance of Water Resources in the Grand Ronde Area, Oregon (OR 167)

LOCATION: S. Fork Yamhill River Basin, Oregon Coast Range

PROJECT EXTENT: Polk and Yamhill Counties

TOTAL AREA IN SQUARE MILES: Approximately 170

BACKGROUND

Grand Ronde is located in the South Fork Yamhill River valley, about 5 miles east of the crest of the Coast Range. This valley is predominantly of alluvial origin, surrounded by the mountains and hills of the Coast Range. The surrounding mountains are generally forested except where timber has been recently harvested. Tributary streams have dissected the mountains creating smaller valleys with varying degrees of slope depending on tributary size and local geology. Geologic units in the area include basalts, marine and nonmarine shales and sandstones, intrusive rocks, and alluvium. Additionally, several faults have been mapped in the area.

The primary source of water for the community is a group of springs approximately 6 miles southwest of Grand Ronde. These springs are a "sole source" supply with a reported yield of about 350 gallons per minute which is delivered through a single pipeline to the distribution system in the community. Although several domestic wells exist in the area, none are being used for municipal supply. In the Dallas-Monmouth area, just east of Grand Ronde, some wells have been reported to have water quality and yield problems.

The Tribal reservation does not have its own water-supply system, therefore it uses water from the Grand Ronde Community Water Association. The Association is a private, nonprofit organization, which owns and operates the community water system. The Tribe is a member of the Association, however, they are concerned about the vulnerability of this sole source and that it may not be capable of supplying adequate water for Tribal economic development and domestic needs that will result from Tribal members returning to the community. Additionally, the Tribe has little in the way of irrigation water rights and will likely need irrigation at several development sites.

As a result of these concerns, the Tribe has a need for information about the water resources in the Grand Ronde vicinity to determine if additional water supplies may be obtainable from shallow alluvial aquifers, deeper bedrock aquifers, streams, or other springs. Little data are currently available, so there is a need for collection of basic hydrologic and water-quality data in the area, and for collection of long-term discharge rates of the springs currently supplying the community.

This project comprised a reconnaissance hydrologic assessment conducted cooperatively between the Confederated Tribes of the Grand Ronde Community and the U.S. Geological Survey (USGS), Water Resources Division, Oregon District Office in Portland. The USGS provided 50 percent of the funds required to complete the study. The study area included most of the South Fork Yamhill River drainage basin upstream of Willamina with the emphasis of the work in the valley floor area.

OBJECTIVES

1. Compile existing water-resource data,
2. Collect field data, including flow measurements and limited water-quality data, for water resources in the study area,
3. Tabulate and analyze the data to describe the water resources, and
4. Report the findings.

APPROACH

1. Literature and data search - The USGS conducted a literature search to locate all geologic maps, ground-water studies, and consultants' reports of a hydrologic nature in the vicinity of Grand Ronde and adjacent areas. Additionally, all well, spring, and stream-discharge records were tabulated and reviewed.

2. Hydrologic reconnaissance - A survey of the surface- and ground-water resources near Grand Ronde was be made. The survey included field visits to selected stream reaches, springs, and wells. Data collected included stream and spring discharge and water levels in selected wells. The hydrogeology of the area was assessed in the field and compared to existing geologic maps. Fifteen surface- and ground-water samples were collected for analysis of major ion and nutrient concentrations in order to determine the source and general chemical characteristics of water in the area. For example, high salinity is a common problem in ground water from Coast Range marine sediments.

3. Report of findings and plans for additional study - A summary report was written that describes the findings of the study, including basic data collected during the study.

REPORTS FROM THIS STUDY

WRIR 97-4040. Assessment of the Water Resources of the Grand Ronde Area, Oregon, by Katherine A. McCarthy, John C. Risley, Rodney R. Caldwell, and William D. McFarland. (7/97) Abstract