Development of Biologically Based Sediment Criteria in Mountain Streams of the Western United States

Abstract

AbstractSediment has long been recognized as a leading cause of impairment of biological condition in rivers and streams of the United States. Recently, federal and state agencies have shown increased interest in developing sediment criteria to maintain or improve habitat quality for the protection of aquatic species. To develop biologically based sediment criteria, sediment amounts must be linked with aquatic vertebrate response. For our analysis, we related an aquatic vertebrate index of biotic integrity (IBI) with a measure of the areal percentage of streambed surficial fines (≤0.06 mm). The association suggested that fine sediment limits the biological potential of mountain streams. We used quantile regression to model the upper limit of IBI response; the regression equation predicted a 4.7% decline in IBI for each 10% increase in areal surficial fines. However, the limiting relationship itself did not suggest a specific sediment level above which impairment was evident. To develop more specific evidence regarding sediment impairment and to describe possible impairment thresholds along the continuous relationship, we sought additional information from (1) the range of areal percent fines at the 169 least disturbed reference sites in our sample, (2) sediment tolerance values calculated for sediment‐sensitive salmonids in the Mountains ecoregion, (3) a review of studies that describe IBI scores representing very good to excellent condition in coldwater (trout) streams of the United States, and (4) a second literature review of laboratory and field research relating the effects of sediment on the survival of salmonid eggs. Placing our initial results in the context of this additional information, we concluded that streambed areal surficial fine sediment (particle size ≤ 0.06 mm) levels of 5% or less retain habitat potential for sediment‐sensitive aquatic vertebrates in mountain streams. We offer these results as scientific guidance for the process of establishing sediment criteria.