Responses of physical, chemical, and biological indicators of water quality to a gradient of agricultural land use in the Yakima River Basin, Washington

The condition of 25 stream sites in the Yakima River Basin, Washington, wer e assessed by the U.S. Geological Survey's National Water-Quality Assessmen t Program. Multimetric condition indices were developed and used to rank si tes on the basis of physical, chemical, and biological characteristics. The se indices showed that sites in the Cascades and Eastern Cascades ecoregion s were largely unimpaired. In contrast, all but two sites in the Columbia B asin ecoregion were impaired, some severely. Agriculture (nutrients and pes ticides) was the primary factor associated with impairment and all impaired sites were characterized by multiple indicators of impairment. All indices of biological condition (fish, invertebrates, and algae) declined as agric ultural intensity increased. The response exhibited by invertebrates and al gae suggested a threshold response with conditions declining precipitously at relatively low levels of agricultural intensity and little response at m oderate to high levels of agricultural intensity. This pattern of response suggests that the success of mitigation will vary depending upon where on t he response curve the mitigation is undertaken. Because the form of the com munity condition response is critical to effective water-quality management , the National Water-Quality Assessment Program is conducting studies to ex amine the response of biota to gradients of land-use intensity and the rele vance of these responses to water-quality management These land-use gradien t pilot studies will be conducted in several urban areas starting in 1999.