Production of high-quality water is a vitally important ecosystem service in the largely semiarid interior Columbia River basin (ICRB). Communities, tribal governments, and various agencies are concerned about maintenance of this water supply for domestic, agricultural, industrial, recreational, and ecosystem uses. Water quantity and quality are widely recognized as important components of habitat for depleted salmonid populations in the ICRB where a large, multiagency effort is underway to restore salmon, trout, and char listed as threatened or endangered under the federal Endangered Species Act. A particularly active program of salmonid restoration is ongoing in the Entiat River subbasin, part of the ICRB that drains a portion of the eastern slope of the Cascade Mountains in central Washington state. In the Entiat River, monitoring of compliance with the 1977 federal Clean Water Act has identified water temperature and especially pH as water quality parameters of concern. For these reasons, agencies and others concerned with production of high-quality water and restoration of listed salmonids require meaningful and efficient approaches to water quality monitoring.