USGS: Science for a Changing World

Water Quality in the Yakima River Basin


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If you have questions or comments about the Yakima NAWQA study, contact:

Greg Fuhrer
Yakima NAWQA Project Chief
U.S. Geological Survey
10615 SE Cherry Blossom Dr
Portland, OR 97216
(503) 251-3231

Aquatic Ecology


This study will examine the effects of agricultural activities on physical, chemical, and biological characteristics (algae, and benthic invertebrate communities and habitat) over a wide range (gradient) of agricultural land-use intensity. Gradient analysis is based on the observation that environmental patterns govern the structure and function of ecological systems (populations, communities, and ecosystems). Gradient studies can be applied within and among selected environmental settings to determine the management implications of the observed similarities and differences (that is, to answer the question: for maximum effectiveness, should regulations and management strategies vary across the country?). The information obtained from this investigation will help managers and regulators identify factors that can be manipulated to make cost-effective improvements in water quality on a regional and national basis.

Specific questions that will be addressed are:

  • Which physical, chemical, and biological constituents respond to the agricultural land-use gradient?
  • What is the form (e.g., increasing linear, decreasing linear, non-linear/threshold) and magnitude of the response?
  • Which landscape features are most important in differentiating physical, chemical, and biological responses?
  • Which physical and chemical factors are associated with biological responses and which are the most important factors in driving these responses,

  • At what levels of land-use intensity do physical, chemical, and biological components express the greatest change (i.e., identification of "break points" and "thresholds" where the slope of the response in community condition changes radically?
  • What are the tolerances of specific organisms to land-use changes?
  • How do physical, chemical, and biological responses to urban land-use gradients compare among environmental settings (e.g. hydroclimatic regions or ecoregions)?
  • How can the responses along urban land-use gradients be used to predict future changes (trends) in water quality?
  • What are the implications of these findings for the management and regulation of water quality?


Investigation of responses to a gradient of land-use intensity requires a very specific design in order to succeed. The gradient of interest must be precisely defined, and then sites must be selected in such a way as to control for other gradients (e.g., gradients of natural features such as climate, geography, soil characteristics, stream size, hydrologic variability) so that they do not overwhelm the land-use (anthropogenic) gradient of interest. This is particularly important for ecological studies since many natural gradients (e.g., elevation, stream size, climate) are known to have dramatic effects on the distribution of organisms even in the absence of anthropogenic influences.

The objective of gradient analysis is to identify which land-use features are associated with particular gradients and which are most important in determining water-quality conditions as defined by physical, chemical, and biological characteristics. Features that are strongly associated with changes in water-quality conditions and that can be effectively manipulated become candidates for managing and regulating the resource.

We sampled 24 intermediate-sized Yakima River tributary streams for macroinvertebrates, algae, chlorophyll a and biomass, habitat, nutrients, pesticides, suspended sediment, and basic water quality parameters. These response variables will be related to detailed landscape variables, including elevation, drainage area, slope, geology, soil types, crop type, irrigation practices, and others, to assess the effect of varying agricultural intensity in the Yakima River Basin.

U.S. Geological Survey
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Last updated: 4/4/01