We demonstrate how watershed-scale aquatic ecological risk assessments (ERAs) for multiple Stressors are conducted. Most ERAs focus on toxic chemicals, however, many Stressors may affect aquatic ecosystems within watersheds. Other watershed-scale aquatic Stressors include physical and hydrologic Stressors, and physical habitat degradation. We demonstrate a method for quantifying the risks of these other Stressors and comparing the resultant non-toxic chemical risks with toxic chemical risks, using habitat models to estimate the effects of non-toxic chemicals on habitat quality. This information is then compared to the risks from chemical toxicity, and the results used to compare the relative risks of chemical toxicity with the risks of other types of Stressors, to identify the most limiting factors. With this information, the most beneficial types of ecological restoration can be identified and resources allocated to provide the greatest ecological benefit to the watershed.

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A current regulatory challenge is defining appropriate nutrient criteria that will protect the designated uses of a waterbody consistent with ecoregional characteristics. EPA Region IX has initiated a pilot study focusing on Sub-ecoregion 6 (California oak and chaparral).Determining criteria requires information on both loads and responses. Criteria should not be less than natural background; neither should they be set at levels lower than those associated with a risk of degradation. In many cases, target nutrient levels may be "inherited" from downstream waterbodies that are likely to be more stringent than the concentration target needed to prevent impairment within the stream itself. A set of linked loading, transport, and response models were used to translate the potentially more stringent receiving water standards to upstream watershed export.The USDA Soil and Water Assessment Tool (SWAT) model (Arnold et al., 1990) was used to simulate background nutrient export from headwater streams. Modeling effort focused on developing vegetation and other geographical input for SWAT. A validation exercise was conducted with six watersheds, corresponding monitoring data, and several past modeling results. The default biomass simulations required considerable adjustment, and the adjustments did not provide reasonable biomass estimates for all vegetation types. The validation results indicate that the model must be calibrated for use at the watershed level but that SWAT can provide relative comparisons among different natural communities in Ecoregion 6.SWAT and the USGS Spatially Referenced Regressions On Watershed Attributes (SPARROW) model (Smith et al., 1997) were used to predict watershed loading and stream transport, and BATHTUB models were used for estimating lake response to nutrients (Walker, 1996). The results of the modeling analysis are estimates of natural watershed loading rates and instream nutrient concentrations that are consistent with ecoregional characteristics. The estimates provide a baseline from which to evaluate conditions where designated uses should be fully realized and allow decision-makers to discriminate water quality impacts that are due to nutrient over-enrichment.