Purpose and Scope

The purpose of this report is to describe the groundwater and surface water data collected from February 2018 to April 2022 by the U.S. Geological Survey (USGS) on the Washington side of the WWRB study area. The topics covered in this report consist of a description of the area, the process of groundwater well selection, the initial field well inventory along with the networks established from these sites, and a description of the methods for making low-flow stream discharge measurements. A total of 46 surface water sites were measured to calculate discharge, and 191 groundwater wells were inventoried for lithology information, with 142 of the wells with a measurable depth to water.

Description of Study Area

The Walla Walla River Basin occupies approximately 1,777 square miles in Walla Walla and Columbia Counties, Washington, and Umatilla and Wallowa County, Oregon (fig. 1). The basin drains westward from the Blue Mountains, which reach approximately 5,900 feet (ft) in elevation in the eastern region of the study area. The Horse Heaven Hills form a ridgeline that bounds the basin to the south, intersecting the Columbia River at Wallula Gap just south of the confluence with the Walla Walla River. In the northern portion of the study area, the rolling Palouse Hills support extensive dryland farming. The Walla Walla and Mill Creek Valleys are gently sloping broad plains that are home to municipalities, farms, and a growing wine industry.

Geologically, the WWRB consists of the Miocene-aged Columbia River Basalt Group (CRBG) overlain by Miocene-Pliocene and Quaternary sediments. The CRBG forms a thick sequence of lavas and thin sedimentary interbeds that dip westward from the Blue Mountains. Faults define the fronts of the Blue Mountains and the Horse Heaven Hills. Pleistocene sediments drape the landscape, including thick loess (windblown silt) deposits in the Palouse hills and fine-grained slackwater deposits from the Missoula Floods. Alluvial fans have filled much of the basin with clastic sediments eroded from the Blue Mountains. The CRBG and alluvium form the two major aquifers in the region (Newcomb, 1965; Derkey and others, 2006; Kahle and others, 2011).

Streams flow westward through narrow bedrock canyons in the mountainous eastern region. Broad alluvial fans form where streams spill out from the Blue Mountains. Distributary channels and springs are found along the fan surfaces. The North Fork and South Forks of the Walla Walla River converge near the city of Milton-Freewater and are joined by Mill Creek which flows from the north. In the northern part of the study area, the Touchet River flows out of the Blue Mountains near Dayton, flowing westward before turning south and joining the Walla Walla River as a major tributary. Groundwater in the WWRB is primarily found in the gravel aquifer consisting of shallow unconsolidated sediments in the central lowlands and valley bottoms and deeper fractured basalt aquifers underlying the entire basin (Pickett, 2011).

The upper reaches of the WWRB are mountainous and forested, whereas the downstream low-lying areas are semiarid and mostly agricultural. Elevations range from 340 ft at the mouth of the Walla Walla River to 6,250 ft at the highest point at the Blue Mountains. Annual precipitation across the basin varies between areas of lowlands with those in the mountains. The 30-year climate normals data from the Walla Walla Regional airport shows the middle of the WWRB winter (November–March) to be cool, with temperature between 30–56 degrees Fahrenheit and precipitation of over 16.5 inches falling in the form of rain and snow. During spring and autumn (April–October) the temperatures range from 42 to 90 degrees Fahrenheit and total precipitation of 8.5 inches of primarily rain (National Centers for Environmental Information, 2022).

The primary land cover type in the WWRB is agricultural (54 percent of the land), primarily irrigated and dryland farming. About 25 percent of the basin areas is rangeland of shrubs and grassland and 17 percent is forested. Approximately 4 percent of the WWRB is developed (Dewitz, 2019). The main crops by area grown in the WWRB are wheat for grain, vegetables, hay, dry peas, and apples (U.S. Department of Agriculture, 2017). On the Washington side of the basin, the largest towns are Walla Walla and College Place, with a combined population of approximately 44,000 (U.S. Census Bureau, 2020). Other towns in the project area include Lowden, Gardena, Touchet, Dixie, Waitsburg, Dayton, and Prescott.

Streamflow Measurements

Stream measurements were collected at 42 non-gaged locations in August 2020 to supplement streamflow data collected at 4 active USGS gaging stations in the study area (fig. 2). Measured discharge ranged from “0.00,” or dry stream beds, at Whetstone Hollow at Prescott, WA (14017100) and Touchet River Tributary near Lowden, WA (14017490) to 47.6 cubic feet per second at Touchet River near Dayton, WA (14016810). The stream width ranged from 3.2 to 68.5 ft and mean velocity ranged from 0.11 to 2.56 ft per second at the measurement section. Eighty-nine percent of the measurements were rated fair or better with only 11 percent rated as poor because of low-water conditions.

Groundwater Network

In 2018, a preliminary field inventory began to verify the location and access of 62 wells in basalt bedrock units (basalt wells) in which water levels were previously measured by USGS staff in the 1970 and 1980s. Twenty-seven wells were successfully located and had either measurable or reported water levels. Annual water level measurements were made at these wells from 2019 through 2022 in winter months (February and March), before the irrigation season. In 2020, annual water level measurements were made at 18 additional wells. In April 2021, an additional field effort inventoried more than 145 wells in the Walla Walla River Basin, for a total of 191 wells visited between February 2018 and April 2022 providing a spatial and depth distribution across the Washington side of the WWBR.

The annual network of 28 wells visited from February 2018 to February 2022 with depths of wells ranged from 102 to 2,730 ft and the depths to water ranged from 331.0 ft below the land surface to flowing. The number of wells visited each year was dependent on accessibility during the field week. The quarterly network of 73 wells were measured in January, April, July, and October. Wells were added to the network as they were inventoried, therefore they might not have a complete year of water level data. The depths of the wells ranged from 30 to 1,301 ft below land surface and the depths to water level ranged from 5.2 to 447.7 ft below land surface. This network was reduced to 59 wells in April 2022 and will continue through July 2024.

Water levels fluctuations over the period of data collection can be seen when viewed in hydrographs (fig. 4). The seasonal changes can be seen in the hydrograph of 06N/34E-13B01 (460004118295301) with a completed well depth of 85 ft. Water level changes over the collection period and seasonally can be seen in the hydrograph of 06N/35E-18A06 (460014118282201) with a well depth of 235 ft.